Gas and Oil Controls
A variety of different types of controls are used in gas and oil heating systems to ensure the safe, efficient, and automatic operation of the furnace, boiler, or water heater. These controls function together as a control circuit within the heating system.
The most important functions of the control circuit are (1) to start or stop the gas or oil burner in response to a signal from the centrally located room thermostat and (2) to shut down the burner if an unsafe operating condition occurs.
A complete description of all the controls used to govern the operation of a furnace, boiler, or water heater would be too extensive to include in a single chapter. For that reason, thermostats, limit controls, and related safety and control devices are covered in other chapters. This chapter is primarily concerned with a description of those controls that directly govern the flow of gas or oil to the burner. These are primarily the safety, pressure-regulating, and flame-sensing valves and devices. It also includes descriptions of the operating (ignition) systems.
Caution
Work on gas-fired and oil-fired equipment should be performed only by qualified personnel trained in the proper application, installation, and maintenance of HVAC systems.
In addition to the room thermostat and the fan and limit control, the basic components of the control system of a gas-fired furnace, boiler, or water heater will generally consist of the following controls:
Main gas valve Pressure regulators Pilot gas cock
Automatic gas control valve Automatic pilot valve
• Pilot assembly
The pilot assembly includes the pilot burner and the thermocouple or thermopile (pilot generator) in standing-pilot systems. In more modern systems, the pilot assembly consists of the pilot burner, a spark ignition module, and a flame sensor.
A control system may also include a gas primary control, transformer, or safety pilot relay. Some controls may be eliminated, depending on the design and requirements of the system. For example, a safety pilot relay is not necessary if a nonelectric pilot safety valve is used in the gas line.
Note
The valves and other devices in a control system will vary depending on the type of ignition system.
The gas control circuits used to operate modern gas-fired heating equipment can be divided into the following three basic types:
1. Low-voltage control circuits.
2. Line voltage control circuits.
3. Millivolt control circuit.
A low-voltage temperature control circuit (see Figure 5-1) uses a step-down transformer to reduce the higher line voltage to approximately 24 to 30 volts. A 24-volt thermostat is used as the controller in most installations.
The line voltage temperature control circuit shown in Figure 5-2 is a 120-volt system. Because the voltage is not reduced, a line voltage thermostat or controller and a line voltage operator must be used in the system.
A millivolt control circuit (see Figure 5-3) operates on the thermocouple principle. A single thermocouple automatically generates approximately 30 millivolts without the aid of an outside source of electricity. A number of thermocouples used together can generate up to 750 millivolts. This combination is variously referred to as a generator, pilot generator, thermopile generator, thermopile system, or powerpile system.
Each of the three temperature control circuits described in the preceding paragraphs is also wired into a pilot safety shutoff circuit, generally via a switch-type pilot safety shutoff device. An inline pilot safety shutoff device is also located in each safety shut — off circuit, and these provide complete gas shutoff.
The primary control shown in Figure 5-4 is a solid-state electronic relay used on gas, oil, or combination gas-oil burners. It is designed
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SAFETY SHUTOFF CIRCUIT |
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TEMPERATURE CONTROL CIRCUIT |
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OR CIRCULATOR RELAY |
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Figure 5-1 Low-voltage control circuit. (Courtesy Honeywell Tradeline Controls) |
To provide operational control of the burner in response to the room thermostat and limit controls, and to instantly shut off the burner in the event of flame failure. Figure 5-5 illustrates a typical wiring diagram for a Honeywell RA890F Protectorelay primary control used in a control circuit for a gas-fired boiler.
This primary control is used with rectification-type flame detectors to sense the presence or absence of flame. The heart of the flame detector circuit in a gas-fired system is an electrode inserted in the pilot flame. In the wiring diagram, shown in Figure 5-6, the flame rod is connected to terminal F of the primary control.
In the event of pilot flame failure, the flame detector circuit responds to control the gas valve in the manifold. When there is an absence of flame, the primary control shuts off the supply of gas by closing the gas valve or by keeping it closed if it is not already open.
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TEMPERATURE CONTROL |
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SAFETY SHUTOFF CIRCUIT |
CIRCUIT |
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OR CIRCULATOR RELAY Figure 5-2 Line voltage control circuit. (Courtesy Honeywell Tradeline Controls) |
These units are designed to be fail-safe. Abnormal conditions in the flame detector circuit, such as an open circuit, short circuit, or current leakage to ground, simulate absence of flame and cause the system to shut down. Safety controls such as temperature or pressure limits or low-water cutoffs are connected ahead of the switching terminals of the relay so that shutdown of the burner occurs even in the event of a relay malfunction such as fused contacts.
The main valve circuit is deenergized 8/io of a second after flame failure occurs. On starting up or after flame failure, a trial-for-igni — tion period of approximately 45 seconds maximum occurs. During this ignition period, only pilot gas is allowed to flow to the burner. If the flame circuit is not completed within this time period, safety lockout of the relay occurs, causing a total shutdown of the system. Manual reset is then required to restart.
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TEMPERATURE CONTROL CIRCUIT
(IN-LINE TYPE FOR COMPLETE SHUTOFF) |
FAN OR CIRCULATOR CONTROL CIRCUIT
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FAN OR CIRCULATOR MOTOR |
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L 2
—- L1 (HOT)’——
POWER SUPPLY
FAN CONTROl OR CIRCULATOR R
Figure 5-3 Millivolt control circuit. (Courtesy Honeywell Tradeline Controls)
Two sets of relays are contained in a Honeywell RA890F Protectorelay primary control. The load relay (left hand) supplies current to the No. 3 terminal to control the blower motor. The flame relay (right hand) responds to the load relay but only if allowed by the flame-detecting electronic network of the relay. The flame relay can supply current to the No. 5 terminal controlling the gas valve only if the load relay has also pulled in.
The load relay is responsive to the thermostat or other operating control connected to the T/T terminal provided that safety controls, located in the 16 circuit, indicate that safe conditions exist for main burner operation.
On an interruption of power to the No. 1 and No. 2 terminals of the primary control, the relay returns to the standby position. When power is restored, normal operation is resumed except that the starting cycle is maintained longer than usual while the vacuum tube is warming up. Relay positions and their effect on the burner are listed in Table 5-1.
Relay Position Description Effect on Burner
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Standby Starting Running Abnormal Conditions |
Load and flame relays both out Load relay in; flame relay out
Load and flame relays both in Load relay out; flame relay in due to flame simulating failure
Motor and gas valve both deenergized
Motor energized; gas valve deenergized; trial-for-ignition period; safety lockout occurs after 45 seconds Motor and gas valve both energized
Motor and gas valve both deenergized; safety lockout occurs after 45 seconds
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POWER SUPPLY L 1 (HOT) A 4L 2
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For intermittent ignition, connect to terminal 3.
If line voltage controller is used connect it between the limit control and terminal 6. Jumper T1 and T2.
Hookup for standing pilot is the same as for interrupted ignition except ignition and pilot valve connections are not made.
SPDT alarm terminals optional. If line voltage alarm is used, RA890F must be mounted in suitable enclosure. Power supply-provide overload protection and disconnect means as required.
For RA890E replacement, leave power connected to terminal 1. Subbase wiring changes are not required.
Figure 5-5 Wiring diagram of the RA890F control.
(Courtesy Honeywell Tradeline Controls)
Servicing a Gas Burner Primary Control
Access to the wiring terminals of the primary control illustrated in Figure 5-4 is obtained by loosening the screws that secure the chassis to the base. When remounting the chassis, be sure to tighten all mounting screws because they also serve as electrical connections.
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POWER SOURCE
Figure 5-6 Wiring diagram showing connections between a RA890F control and other components in a gas control circuit. (Courtesy Bryan Steam Corp.) |
No attempt should be made to repair a primary control except for tube replacement. Vacuum tubes are used in Honeywell primary controls. Never replace them with radio tubes. If a primary control is defective, the entire chassis should be replaced with a good one.
Operating controls located in the T/T circuit (see Figure 5-6) should be of the low-voltage, two-wire type. A low-voltage transformer for this purpose is built into a Honeywell Protectorelay. Safety controls located in 16 terminals must be two-wire, line voltage type. With the exception of the line switch, no controls should ever be placed in the line ahead of the 13 terminals of the primary control.
Before assuming that the primary control is defective, be sure to check the pilot, pilot adjustment, flame detector circuit, and all operating and safety controls; proper operation is also dependent on these external factors. The flame circuit can be more accurately checked by the use of a microammeter to read flame current. Normal operation requires a current of 2 microamperes or more.
Never push relays in manually because it can result in accidental opening of the main diaphragm valve. Be sure to turn off the electrical power before removing the primary control chassis from the base.
The valves used to control the flow of gas through a gas-fired furnace, boiler, or water heater can be divided into two basic categories: (1) manually operated valves and (2) power-operated valves.
The two manually operated valves (gas cocks) used on gas-fired heating equipment provide a backup safety function in case the automatic gas valves fail to operate. One of these valves is located in either the main gas supply riser or the manifold. The other one is located on the pilot gas line.
The manual gas valve installed on the main gas supply line (riser) or manifold is variously referred to as the main gas shutoff valve, manual shutoff plug cock, or simply the gas cock (see Figure 5-7). This valve provides manual control of the gas flow to the main gas burners. It is not used to control the gas supply to the pilot burner, the latter being provided with its own separate shutoff valves.
The manual valve located on the pilot gas line is called the pilot shutoff cock or the pilot gas cock. It is usually the first controlling device on the pilot line (see Figure 5-7). It provides complete gas shutoff whenever it is necessary to remove and service other controls on the pilot line, such as the pilot gas regulator or the pilot solenoid valve.
Power-operated or automatic valves are actuated by some form of auxiliary power such as hydraulic pressure, pneumatic pressure, electricity, or a combination of these sources. The following are the principal types of power-operated valves used on gas-fired heating equipment:
Solenoid valves
• Direct-acting heat motor valves
• Diaphragm valves
The solenoid gas valve is commonly used on gas-fired heating equipment to provide on-off control of the flow of gas.
The primary function of a solenoid valve is to provide direct valve operation. The power to operate the valve is obtained from the magnetic flux developed in a solenoid coil. A valve disc in the valve body is connected by a rod to the core of an electromagnet. When the room thermostat or power switch directs an electrical current to the solenoid, it pulls the rod (plunger) to the top of the plunger tube and lifts the attached valve disc. Gas then flows through the main valve port until the electrical circuit is interrupted by the controller. This action releases the rod, which falls and shuts off the valve. The weight of the rod and seat assembly and the gas pressure on top of the valve seat ensure a tight shutoff. The ITT
ALTERNATE LOCATION y-.i; OF MAIN GAS COCK
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PILOT VALVE Figure 5-7 Typical arrangement of gas cocks and main gas-pressure regulator. |
General Controls K3 Series gas valve shown in Figure 5-8 is an example of a direct-operated solenoid gas valve.
Some solenoid valves use a balanced diaphragm to control the flow of gas (see Figure 5-9). When the solenoid coil is energized, it lifts the rod or plunger just enough to open a bleed valve (or so — called pilot valve). Gas then bleeds from the area above the
Figure 5-8 Direct-operated solenoid gas valve.
(Courtesy ITT General Controls)
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Figure 5-9 Balanced diaphragm solenoid gas valve.
(Courtesy ITT General Controls)
Diaphragm faster than it can be replaced. This eventually results in the pressure above the diaphragm being the same as the pressure below the seat disc. This is referred to as a balanced or unloaded condition. The solenoid coil lifts the complete interior assembly to full open position. When the solenoid is deenergized, the pressure recovers above the diaphragm. The weight of the interior assembly and the gas pressure across the seat disc are sufficient to hold the valve closed. In this type of valve, the pressure of the gas is used to control its operation.
A third type of solenoid valve consists of a solenoid-operated (i. e., magnetically operated) puff bleed three-way valve and a diaphragm valve in a single unit (see Figure 5-10). The combined unit provides on-off control of the gas to the gas-fired heating equipment.
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HOT _ TO LIMIT OR
Figure 5-10 Electric diaphragm gas valve. (Courtesy ITT General Controls) |
The three-way valve (also referred to as a pilot valve), responding to electrical signals from the limit or safety controls, opens or closes the gas valve by controlling the gas pressure bleed-off above the diaphragm in the main valve body. In the normally closed position, inlet gas pressure above the diaphragm prevents the valve from opening. In the open (energized) position, the three-way or pilot valve closes off the inlet gas pressure and allows the gas pressure above the diaphragm to bleed off so that gas pressure below the diaphragm forces the diaphragm up to open the valve.
Dual-solenoid valves are designed for three-stage control (high — low-off) of the flow of gas (see Figure 5-11). Both a high-fire solenoid and a low-fire solenoid are used to accomplish this purpose. Low-fire adjustments can be made by turning the adjustment screw clockwise (to decrease low fire) or counterclockwise (to increase it).
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LOCK NUT |
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ADJUSTMENT SCREW |
Push and turn to open.
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CAP |
Turn past lock position to retract.
Figure 5-11 Dual-solenoid valve. (Courtesy ITT General Controls)
Figure 5-12 shows a schematic wiring diagram of a two-stage control containing a dual-solenoid valve. If both solenoids are to be energized at one time, the circuit requires a 40-volt transformer.
Several different solenoid coils are available from manufacturers, and the type selected for use will depend on the specific application. For example, most standard applications will require a moisture — resistant coil for normal usage of gas or fluid up to 175F. Special applications include those with especially high ambient and fluid temperatures, high voltage, or high steam pressure. A solenoid coil may also be specifically required for moisture or water applications. Under these circumstances the coil should be both waterproof and fungus proof. Some examples of solenoid coils are shown in Figure 5-13.
A principal cause of coil malfunction is excessive heat. If the valve is subjected to temperatures above the coil rating, it will probably fail. A missing part, a damaged plunger tube or tube sleeve, or improper assembly may also be a cause of excess heat. The applied voltage must be at the coil’s rated frequency and voltage.
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Figure 5-12 Dual-solenoid valve in a two-stage control circuit. (Courtesy ITT General Controls) |
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SAND W COIL HIGH TEMPERATURE APPLICATION |
Always turn off the electrical power to the solenoid valve before attempting to replace the coil. Then, having turned off the electrical power, disconnect the coil leads. Figures 5-14 and 5-15 are schematics of some typical solenoid valves. The numbers in the illustrations refer to the valve components in their order of disassembly and are identified as follows:
1. Jacket retaining nut or screw assembly.
2. Elbow for coil leads.
3. Valve O-ring.
4. Coil jacket or coil assembly.
5. Nut or screw.
6. Spring retainer.
7. Plunger tube spring.
8. Screw assembly spacer.
9. Top washer and/or sleeve assembly.
10. Solenoid coil.
11. Bottom washer and/or sleeve assembly.
12. 
Plunger tube.
Method of connection for 120 -volt operation.
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Method of connection for 240-volt operation. |
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Solder and tape this connection. |
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The solenoid coil should be reassembled in reverse order, but with the following precautions:
1. Be very careful to reassemble the top washer and/or sleeve assembly (No. 9 above) exactly as it had been assembled. Improper assembly will cause the solenoid coil to burn out.
2. Be sure to align the top washer and/or sleeve assembly so that the coil leads have an unobstructed passage out of the solenoid.
3. 
Properly align all slots in the bottom washer and/or sleeve assembly.
Direct-Acting Heat Motor Valves
A direct-acting heat motor valve depends on the heat-induced expansion and contraction of a rod-type element to provide the movement and force for its operation.
The heat is generated by the passage of an electrical current through a resistance coil wound around a metal rod. One end of the rod is secured in place. The other end rests against a flexible snap mechanism (see Figure 5-16). When the room thermostat calls for heat, the electrical current flows through the coil and heats the metal rod. The heat generated by the resistance of the coil causes the rod to expand against the snap mechanism. When enough force is applied to the rod, the snap mechanism snaps over center and opens the valve. When the rod cools and contracts, the snap mechanism returns to its original position, and the valve closes.
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ELECTRICAL TERMINALS
Figure 5-16 Operating principle of a direct-acting heat motor Valve. (Courtesy Robertshaw Controls Co.) |
Some combination gas valves utilize the direct-acting heat motor principle of operation. The Robertshaw Unitrol 1000E is an example of this type of valve (see Figure 5-17). It combines in one unit a gas cock, an automatic pilot, a pilot gas filtration device, and a heat-motor-actuated automatic valve. A main gas-pressure regulator can be added as an option. As shown in Figure 5-17, a manual opener or bypass selector is a common feature on these valves. When the bypass selector is in the on position and the room thermostat is in
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MANUAL OPENER OR BY-PASS SELECTOR
Figure 5-17 Heat-motor-actuated combination gas valve. (Courtesy Robertshaw Controls Co.) |
The off position, a bypass rate is provided to the burner for minimum input conditions.
The following are the three principal diaphragm valves, each distinguished by the kind of power used to actuate them:
Hydraulic-actuated valves Solenoid-actuated valves Heat-motor-actuated valves
A hydraulic-actuated valve utilizes a hydraulic element to provide both the thermostatic sensing means and the power for valve operation. The closed hydraulic sensing and actuating device consists of a bulb, capillary tube, and a bellows or diastat (see Figure 5-18). Temperature changes cause the liquid in the remote-bulb sensing device to expand or contract. This expansion or contraction of the liquid operates the valve by controlling the pressure exerted against a bellows in the valve body. Additional information about this type of valve is contained in the section Remote-Bulb Thermostats in Chapter 4 (Thermostats and Humidistats)’.
Both solenoid-actuated diaphragm valves and heat-motor-actuated diaphragm valves are described elsewhere in this chapter (see Solenoid Gas Valves, Oil Valves, and Direct-Acting Heat Motor Valves).
The term diaphragm valve can be confusing because valves that use a diaphragm are usually referred to by the power used to actuate them (e. g., hydraulic-actuated valves, solenoid-actuated valves) or their specific function (e. g., electric gas valves, oil burner valves).
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REMOTE HYDRAULIC TEMPERATURE DIAL
Figure 5-18 Unitrol 7000SR-I H hydraulic-actuated valve. (Courtesy Robertshaw Controls Co.) |
A diaphragm valve is any valve that contains a diaphragm; its purpose is to respond to pressure variations. Because this is an essential feature of pressure regulators, the operating principles of a diaphragm valve are described in the sections Gas-Pressure Regulators and Combination Gas Controls.
Natural gas is distributed through the city mains at pressures of 7 inches water gauge or higher. Normally this gas will be at a higher pressure than the heating equipment or appliance can properly use. Furthermore, the gas pressure in the mains (and in the building supply lines) will often fluctuate because of load demand variations. Excessively high gas pressure and gas-pressure variations are detrimental to the operating efficiency and safety of a gas-fired furnace, boiler, or water heater. Hence, they must be brought under control before the gas enters the burners.
A gas-pressure regulator (or manifold pressure regulator as it is also called) is a regulating device used to control manifold gas pressure. Gas is delivered to the burners from the outlet orifice of the regulator at a single, nonfluctuating constant pressure regardless of inlet pressure changes.
A regulator must sense all changes in gas pressure and be able to adjust the gas flow as required. The sensing device by which this is accomplished is a diaphragm and spring arrangement attached to a valve ball or disc used to restrict gas flow through the seat. These and other components are illustrated in the cutaway of the low — pressure regulator shown in Figure 5-19.
A pressure regulator uses the available gas pressure as the primary force to open or close the valve. Outlet gas pressure presses against the diaphragm and spring. If the gas pressure is too little to overcome the force of the spring, then the attached ball or disc is pushed away from its seat. This enlarged opening allows more gas to flow. If the outlet pressure against the diaphragm is greater than the spring setting, then the valve ball or disc is brought toward its seat, narrowing the opening and restricting flow. As the gas pressure against the diaphragm equals the force exerted by the spring, the valve ball or disc is so positioned from the orifice to maintain a steady downstream pressure. This principle of operation is basic to all diaphragm valves.
The spring-loaded side of the diaphragm must be vented or the movement of the diaphragm will be restricted. The most elementary form of venting is shown in Figure 5-20. This is simply an orifice installed in a vent hole on the spring-loaded side of the
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Figure 5-19 Principal components of a low-pressure Regulator. (Coun. esy ITT General Controls) |
Diaphragm. A more complex method of venting involves connecting a tube to a tapping in the vent. This represents either the internal or external bleed system of venting gas. The principal difference between the two systems lies in how and where the vented gas is disposed.
Proper venting allows the valve diaphragm to move freely in either direction. Installing an orifice in the vent hole slows the diaphragm action, thereby providing a smoother operating control (see Figure 5-20). The use of a vent hole orifice also prevents the rapid and potentially dangerous escape of gas in the event of a diaphragm rupture.
Both internal and external bleed systems are used to vent the gas from the spring-loaded side of the diaphragm. In an internal bleed system, the bleed gas is routed to the burner or pilot where it is burned, or to the burner manifold where it is mixed with the main
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Gas supply and eventually burned. In an external bleed system, the bleed gas is vented by a tube extending to the outdoors. A variation of the external bleed system is to place the outlet end of a tube in the heat exchanger to vent the gas outside.
A simple diaphragm valve functions only to open or close the valve and is generally of the external bleed type (see Figure 5-21). The operator mechanism for a restricting internal bleed orifice diaphragm valve is basically as shown in Figure 5-22.
A gas-pressure regulator can be either an independent control on a gas manifold or a part of a combination gas control. The obvious advantage to using a combination control is the simplification of appliance assembly and the saving of space obtained when compared to the use of separate components. Less obvious are the operational advantages.
When the pressure regulator is not an integral part of the combination gas control, it either precedes or follows the location of the
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PILOT
Figure 5-21 Schematic drawing of a millivolt diaphragm bleed valve. (Courtesy Robertshaw Controls Co.) |
Latter in the manifold. If the separate pressure regulator precedes the combination control, both the main burner gas and the pilot gas are regulated by the same regulator in most installations. If this is the case, a problem will sometimes occur with pilot outage. As the main gas valve opens to provide gas to the main burners, a temporary starving of the pilot gas can occur due to regulator response delay. This condition can be avoided by installing a separate pilot gas regulator for pilot gas only or by using a regulator-equipped combination gas control with the proper sequencing of operation.
Installing a separate pressure regulator after the combination gas control (i. e., downstream from it) can sometimes result in overgassing the main burners. This occurs because the regulator remains in a wide-open position when the main gas valve remains closed. When the gas valve opens, overgassing can result from the delay of the regulator valve in resuming regulation. A combination gas control can eliminate this problem.
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(Courtesy Robertshaw Controls Co.) |
Some gas-fired water heaters use a balanced pressure regulator. This type of pressure regulator uses two internal diaphragms to control gas pressure. The operating principle is quite different from that described for the regulators used on gas-fired heating equipment. For additional information, read the section Balanced Pressure Regulators in Chapter 4 of Volume 3 (W ater Heaters and Other Appliances)’.
A pressure switch is a safety device used in positive-pressure or dif — ferential-pressure systems to sense gas — or air-pressure changes. A typical arrangement of gas-pressure switches on a gas manifold is shown in Figure 5-23. The wiring diagram in Figure 5-24 illustrates the connections between the high and low gas-pressure switches on a gas-fired boiler manifold and the primary control.
Gas-pressure switches are available in two basic types:
Falling-pressure switches
• Rising-pressure switches
In the falling-pressure switch, decreased pressure on the diaphragm actuates the device. The switch is designed to lock out when the pressure falls to the setpoint (minus differential). As the pressure
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Increases, the diaphragm rises and the switch is deactuated (except on manual reset models). An adjustable spring-loaded diaphragm determines the amount of pressure required to actuate the switch.
In a rising-pressure switch, the switch is actuated by increased pressure on the diaphragm. As the pressure falls, the diaphragm lowers and the switch is deactuated (again, except on manual reset models). An adjustable spring-loaded diaphragm determines the amount of pressure required to actuate the switch.
The pressure required to move the diaphragm in these switches is adjustable within the pressure range stamped on the switch nameplate. The pressure switch shown in Figure 5-25 can be adjusted by removing the cover and turning the adjustment screws clockwise. This action raises the actuation point of the switch. Turning the screws counterclockwise lowers the actuation point. The range scale plate in the switch is marked for four relative pressure settings. Setting A corresponds to a minimum range, D to a maximum range, and both B and C to intermediate ranges.
A typical wiring diagram for a single-pole double-throw SPDT switch is shown in Figure 5-26. An SPDT switch may be wired to open or close the circuit on pressure rise.
On switches equipped with manual reset (see Figure 5-25), the switch contacts open on pressure rise or pressure drop (depending on which model is used) and remain open regardless of pressure change. The reset button is pushed to close the switch after the pressure has returned to an acceptable level.
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RESET BUTTON |
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SWITCH |
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Figure 5-26 Wiring diagram of SPDT gas-pressure switch. (Courtesy ITT General Controls) |
The automatic pilot safety valve is a device used to shut off the gas supply when the pilot flame is extinguished or fails to light.
There are a variety of different pilot safety controls available on the market, but all are based on one of the following three operating principles:
Thermocouple
• Metal expansion
• Liquid pressure
Pilot safety controls based on the thermocouple operating principle are probably the most common. The schematic in Figure 5-27 shows the principal components of an automatic pilot in which a thermocouple is used. The constant-burning pilot of the system illustrated here provides not only burner gas ignition but also heat for the hot junction of the thermocouple. As shown in Figure 5-28, the hot junction of a thermocouple is positioned so that it is directly in the path of the pilot flame.
The thermocouple itself is actually a miniature generator that can convert heat (from the pilot flame) into millivolts of electricity. It consists of two dissimilar metals joined together at their extremities. When one of the junctions (the hot junction) remains cold, electrical energy is generated. The amount of energy generated by
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Figure 5-27 Thermocouple used in conjunction with an Automatic pilot valve. (Courtesy Robertshaw Controls Co.) |
The thermocouple is directly proportional to the temperature difference between the hot and cold junctions (see Figure 5-28). One thermocouple or junction will deliver approximately 25 millivolts. Several thermocouples or junctions can be wired in series to produce a higher voltage (see Pilot Generators). The electrical energy generated by the thermocouple energizes an electromagnet that operates the automatic pilot valve.
The automatic pilot safety valve may be an individual control but is more commonly a part of a combination control, which usually combines manual valve and thermostatic or automatic valve functions. These valves are described in this chapter (see Combination Gas Valves).
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HOT JUNCTION |
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INSULATIO |
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COPPER SHEATH |
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25-MV ; OPEN CIRCUIT |
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Figure 5-28 Common thermocouple construction. (Courtesy Trane Co.) |
Some automatic pilot valves are designed to shut off both the main gas (i. e., the gas to the main burners) and the pilot gas. This is called 100 percent safety automatic pilot or a 100 percent safety shutoff. They are recommended for use with either natural or LP gases.
A 90 percent safety automatic pilot (or 90 percent safety shutoff) shuts off the gas supply to the main burners but allows gas to continue flowing to the pilot burner. This type of gas pilot may be used safely with natural gas (and is permitted by some local codes), but it should never be used with LP gas. LP gas is heavier than air and will not vent.
The procedure for lighting the pilot in a system containing a thermocouple (or pilot generator) and an automatic pilot safety valve is fairly simple. The reset button on the automatic pilot valve must be pushed down while the pilot is being lit (see Figures 5-29 and 5-30). While the reset button is depressed, an auxiliary valve causes the main gas porting to close. At the same time, the spring — loaded automatic pilot valve is opened, allowing gas to flow only to the pilot burner, which can now be lit.
The same action that opened the spring-loaded automatic valve has also forced the keeper (see Figure 5-29) against the pole faces of the electromagnet. Eventually the heat of the pilot flame causes the thermocouple to generate an electrical current, which causes the electromagnet to become magnetized. As a result, the keeper is held against the magnet and the automatic pilot valve is maintained in an open position. The pilot should be allowed to burn for at least 60 seconds before releasing the button. This permits enough current to
be built up by the thermocouple or pilot generator to hold the valve open.
Once the pilot has been established, the reset button can be released, and the main gas and pilot gas will be free to move past the automatic pilot valve (see Figure 5-30). When the pilot flame is extinguished, the thermocouple hot junction cools and breaks the electrical current. The electromagnet is demagnetized as a result of this loss of current, and the keeper is released, causing the automatic pilot valve to close and shut off the flow of gas. Before relighting the pilot, allow at least 5 minutes for all the gas to clear the system.
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Figure 5-30 Reset button released after pilot flame is Established. (Courtesy Robertshaw Controls Co.) |
A thermopilot valve is a 100 percent safety shutoff control for gas — fired heating equipment and appliances. It operates on current supplied by a thermocouple or pilot generator. This current energizes a thermomagnet, which holds the valve open after manual reset. When there is an unstable or low pilot flame or no flame at all, the current to the thermomagnet is lost. As a result, the valve closes, shutting off the gas flow.
An example of a thermopilot valve operated by a thermocouple is the ITT General Controls MR-2G valve. Using it in conjunction with a solenoid gas valve provides 100 percent safety shutoff control (see Figure 5-31).
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T-99 THERMOSTAT |
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TO AUXILIARY LIMIT CONTROLS |
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G-251 K3A GENERATOR GAS |
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PILOT B |
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Figure 5-31 ITT control valve model MR-2G. (Courtesy ITT General Controls) |
The thermomagnet of an ITT General Controls MR-2YA valve is energized by a pilot generator. It must be used with a pilot-operated diaphragm valve to provide 100 percent safety shutoff control (see Figure 5-32).
The following suggestions should be carefully observed when installing ITT thermopilot valves in order to ensure efficient valve operation:
• Locate the valve so that it is easily accessible and where the ambient temperature is below 200F.
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FAN AND LIMIT CONTROL |
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102465AT JUMPER |
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PRESSURE Figure 5-32 ITT control valve model MR-2YA. (Courtesy ITT General Controls) |
Make sure the piping is clean (blow out all particles and other impurities).
Apply thread seal sparingly to male threads only.
• Install valve with gas flow in the same direction as the arrow in the valve body is pointing.
Use the pipe wrench on the valve body flats at the end being connected.
Avoid stress on the valve body by aligning the inlet and outlet pipe connections.
Check all pipe connections for gas leaks with a soap and water solution. Never use a flame.
• Check the valve with a millivolt meter (see next paragraph). All wiring connections should be clean and tight.
• Pilot burner should be installed on main burner so that the ignition flame will light the main burner with the pilot turned down low.
Testing an ITT General Controls MR-2G thermopilot valve with a millivolt meter requires the use of a special adapter. The adapter and thermocouple are connected to the valve as shown in Figure 5-33. Attach the meter clips as shown. Figure 5-34 illustrates the testing of an ITT MR-2YA valve. In either case, the valve should be replaced if the meter reading is above that shown on the applicable scale and the valve fails to open after following the lighting instructions.
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0-50 SCALE ‘-/ ‘"(G) BUSHING |
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Figure 5-34 Testing control |
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Of valve MR-2YA. (Courtesy ITT |
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The function of the thermocouple is to detect (prove) the existence of the pilot flame. If the pilot light is lit and the thermocouple detects the pilot flame, it opens the main gas valve and allows natural gas to flow to the burner(s). If there is no pilot flame, the thermocouple shuts off the gas to the pilot and closes the main gas valve so that no gas can flow to the burner(s).
Thermocouples are made from flame-resistant metal alloys and are available in standard lengths of 13.5 to 48 inches (see Figure 5-35). Industry-standard G bushing or R bushing threads are used for connecting the thermocouple to the valve. Thermocouples from the various manufacturers can also be interchanged. The thermocouples manufactured by ITT General Controls, Robertshaw-Grayson, Baso, Honeywell and others are designed for universal installation when used with the proper adapter and retainer (where required) (see Figures 5-36 and 5-37).
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Figure 5-35 Typical thermocouple. (Courtesy ITT General Controls) |
Always test the thermocouple before replacing it. The problem with the pilot flame or safety control may be caused by something other than a malfunctioning thermocouple.
Manufacturers of thermocouples generally provide a special adapter for testing thermomagnet valves. The adapter is screwed into the gas valve, the thermocouple (or generator) leads into the adapter, and the test is run as follows:
1. Attach the meter clips as shown in Figure 5-38. Reverse the meter clips if the needle moves to the left of zero on the millivoltmeter.
2. A meter reading of less than 7 millivolts for thermocouples or less than 140 millivolts for pilot generators indicates the orifice and primary air holes on the pilot burner need to be cleaned.
Figure 5-36 Thermocouple adapters and retainers. (Courtesy
Robertshaw Controls Co.)
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RETAINER
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Figure 5-37 Retainer used to hold thermocouple in burner bracket.
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VALVE
PROBES Figure 5-39 Checking a terminal block connected to a high-energy cutoff switch. (Courtesy ITT General Controls) |
Sometimes a thermocouple will have a terminal block connected to a high-energy cutoff switch. The millivoltage should be checked with meter probes as shown in Figure 5-39. The various readings suggest the following courses of action:
Replace the high-energy cutoff switch if the reading exceeds 4 millivolts.
Replace the thermocouple if the reading is still less than 7 millivolts.
• Replace (or repair) the valve if it fails to hold open with a meter reading of more than 7 millivolts.
Table 5-2 lists possible remedies for a number of different operating problems associated with thermocouples.
Table 5-2 Troubleshooting Thermocouples
Symptom and Possible Cause Possible Remedy
Pilot flame lit but safety control fails to function.
(a) Thermocouple not hot (a) Wait at least 1 minute for the-
Enough to generate current. rmocouple to become hot enough.
(b) Drafts deflecting flame (b) Eliminate source of draft. away from thermocouple.
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Table 5-2 |
(continued) |
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Symptom and Possible Cause |
Possible Remedy |
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(c) Pilot flame too small or |
(c) Disconnect, clean |
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Yellow in color due to restricted |
Thoroughly, and reconnect. |
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Pilot line or dirt in primary air |
Change orifice if necessary. |
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Opening or burner head. |
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(d) Loose or dirty electrical |
(d)Disconnect, clean, |
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Connections. |
Reconnect, and tighten. |
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(e) Thermocouple tip too |
(e) Check installation to make |
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Low in pilot flame |
Sure thermocouple is properly. |
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Mounted in bracket. |
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Safety control operates but fails when main burner has |
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Been on a short time. |
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(a) Restriction in pilot or |
(a) Eliminate restriction. |
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Main gas tubing. |
Provide normal pressure. |
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(b)Draft-deflecting flame couple. |
(b) Eliminate draft or baffle. |
Thermopiles (Pilot Generators)
The amount of electrical energy generated by a thermocouple that has a single hot junction and a single cold junction (i. e., approximately 25 millivolts) is considered adequate for most residential heating equipment. However, a few residential furnaces and boilers and most commercial types require a higher voltage. An increase in voltage can be obtained by using a number of thermocouples wired in series (see Figure 5-40). A series of thermocouples in one unit is called a thermopile, pilot generator, or thermopile generator. A thermopile forms a part of a millivolt or self-energizing control circuit.
An example of a thermopile used in gas-fired heating equipment and appliances is shown in Figure 5-41. This particular unit produces
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360O ADJUSTABLE IGNITION |
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PORT |
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STRAIGHT — BASE FITTING |
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LADDER TYPE |
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ANGLE-BASE FITTING |
Figure 5-41 ITT pilot generator model PG-1.
(Courtesy ITT General Controls)
Approximately 320 millivolts in an open circuit for gas valve control. It is available with an adjustable ignition port and interchangeable orifices for use with any gas.
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Figure 5-42 ITT Pilot generator model PG-9A with cartridge. (Courtesy ITT General Controls) |
A thermopile that produces an even higher voltage is the ITT General Controls PG-9A (see Figure 5-42). This thermopile provides a pilot flame for gas burner ignition and generates electricity from the heat of the pilot flame to operate millivolt gas valves and relays. The replaceable cartridge in the thermopile contains many thermocouples connected in series. The top % to 1i2 inch of the cartridge is heated by the pilot flame, which produces approximately 500 to 750 millivolts (V2 to % of 1 volt) open circuit.
A millivoltmeter must be used to test a thermopile. The meter leads are attached to the valve or relay terminals to which the wires of the pilot generator are also attached. The thermostat must be calling for heat and the pilot burning during a millivoltmeter test.
Pilot-Operated Diaphragm Valves
A pilot-operated diaphragm valve is used to provide automatic shut — off of main line gas when there is an unstable pilot flame or no flame at all. These valves are operated by electrical energy (millivoltage) produced by the pilot generator. Their operation is controlled by the room thermostat, limit devices, and other operating controls.
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LIMIT CONTROL |
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Figure 5-43 Pilot-operated diaphragm valve. (Courtesy ITT General Controls) |
The ITT General Controls B60 gas valve (see Figure 5-43) is an example of a pilot-operated diaphragm valve commonly used with gas-fired heating equipment. Where 100 percent shutoff is required, it should be used in conjunction with a thermopilot valve (see Figure 5-44).
A combination gas valve (or combination gas control) combines in a single unit all manual and automatic control functions required for the operation of gas-fired heating equipment. In other words, a single valve replaces the various individual pilot line and main line gas controls. A gas-pressure regulator is usually optional.
Many manufacturers of gas controls offer a complete line of combination gas valves; each valve is designed for a different kind of installation or application. Usually these valves will differ on the basis of the controller voltage or voltage source, valve application or function, required Btu capacity for the installation, and type of gas used.
Honeywell manufactures a line of standardized and interchangeable gas control components. A complete preassembled combination gas control can be ordered from the factory or one can be assembled in the field from a variety of different standardized components in order to meet the needs of a particular installation. This add-on feature also allows field replacement of a defective component without removing the complete valve from the installation. A number of possible combinations are illustrated in Figures 5-45 and 5-46.
Standing Pilot Combination Gas Valves
A typical Honeywell combination gas control used with a standing pilot consists of the following basic components (see Figure 5-47):
1. Main valve body.
2. Valve operator.
3. Pressure regulator.
As shown in the schematic diagram of the control (see Figure 5-48), the main valve body (or manifold control) contains a valve diaphragm (5) and disc (3). This portion of the combination gas control operates as a conventional diaphragm valve. The valve opens and closes in response to the presence or absence of gas in the pressure chamber (4). This gas is called the working gas because it provides the lifting force necessary to raise the valve disc off its seat.
The valve operator controls the flow of working gas by means of an electrically actuated lever (1). This lever is opened or closed by the temperature control circuit. When the burner is off and there is no call for heat from the room thermostat, the lever is in the position shown in Figure 5-48. Note that while the lever is in this position, it blocks the admission of gas into the valve. Furthermore,
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LP-NATURAL GAS CHANGE OVER V5309A |
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STEP-OPENING V5307A |
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Locating tab recess. |
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Figure 5-45 Various gas-pressure regulators available for use on a combination gas control equipped with a valve Operator. (Courtesy Honeywell Tradeline Controls) |
The working gas can escape through the working channel (2), resulting in a reduction of the gas pressure in the pressure chamber. The main gas valve closes with this loss of working gas pressure.
A call for heat from the room thermostat energizes the valve operator and causes the lever to open the inlet port (see Figure 5-49). The working gas then flows into the pressure chamber (4) and pushes the diaphragm (5) up against the valve disc assembly (3) to allow the flow of gas through the valve to the burner. At the same time, gas also flows into the pressure regulator chamber (8) and through the evacuation gas channel (6) into the combination gas control outlet.
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Figure 5-46 Standard gas-pressure regulators installed on manual manifold control not equipped With valve operator. (Courtesy Honeywell Tradeline Controls) |
Changes in the outlet pressure of the gas cause changes in the position of the regulator diaphragm (9). If the outlet pressure rises, the regulator valve (7) opens slightly to allow more working gas into the evacuation gas channel (6). This discharge of working gas causes the main valve diaphragm (5) to drop and allows the main valve disc (3) to move downward on its seat. This action reduces the flow of main burner gas through the control to correct the rise in outlet pressure.
A drop in the outlet pressure has the opposite effect. The regulator valve (7) closes slightly and reduces the amount of working gas
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BODY Figure 5-47 Basic components of a typical Honeywell combination gas control. (Courtesy Honeywell Tradeline Controls) |
Entering the evacuation gas channel (6). This action increases the gas pressure in the main valve pressure chamber (4) and forces the main valve disc upward away from its seat. As a result, the flow of gas through the control is increased enough to counter the fall in outlet pressure.
Manufacturers provide detailed instructions for lighting a pilot in an installation equipped with a combination gas valve. These instructions should be read carefully before attempting to light the pilot burner. If there are no instructions with the equipment, the following procedure is suggested:
1. Turn the wall thermostat to off or its lowest setting.
2. Depress the gas cock and rotate it to the off position (see Figure 5-50).
3. Allow 5 minutes for any gas in the burner compartment to escape.
4. Turn the gas cock dial to the pilot position.
5. Depress and hold the gas cock dial down while lighting the gas pilot.
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Iniet gas pressure |
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INLET GAS PRESSURE |
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ON-OFF LEVER WORKING GAS CHANNEL MAIN VALVE BODY |
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0) |
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6. Allow the pilot to burn 60 seconds before releasing the gas cock dial.
7. Turn the gas cock dial to on position after the pilot burner flame has been established.
8. Set the room thermostat to the desired temperature position.
The pilot position on a combination gas valve is used for temporary or seasonal shutdown. The off position is used when complete shutdown is necessary.
Continuous Pilot Dual Automatic Gas Valve
The Honeywell VR4205 direct-spark ignition (DSI) combination gas valve shown in Figure 5-51 contains a safety shutoff, a manual valve, two automatic operators, a pressure regulator, and a pilot adjustment device. Some models use a single electrode for spark ignition and flame sensing; others use separate electrodes for spark ignition and flame sensing. The Honeywell VR4205 gas control is
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EVACUATION GAS CHANNEL |
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(«) |
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SERVO REGULATOR PRESSURE CHAMBER— W SERVO REGULATOR VALVE ‘ |
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Also designed to provide on-off manual control of gas flow. In the off position, gas flow to the main burner is mechanically blocked. In the on position, gas flows to the main burner under control of the thermostat, the direct-spark ignition (DSI) module, and the two automatic main valves.
The operation of a Honeywell VR4205 valve gas control is illustrated in Figures 5-52 and 5-53. When the thermostat calls for heat, the DSI module is energized. The module activates the first and second automatic valves of the gas control, which allow gas to flow to the main burner. At the same time, the DSI module generates a spark at the igniter-sensor to light the main burner. The second automatic valve diaphragm, controlled by the servo pressure regulator, opens and adjusts gas flow as long as the system is powered. The servo pressure regulator monitors outlet pressure to provide an even flow of gas to the main burner.
Loss of power (thermostat satisfied) deenergizes the DSI module and closes the automatic valves. The first automatic valve and the
1. Turn to PILOT. Press dial in and light pilot. Hold for 60 seconds and release.
2. 
Turn dial counter-clockwise to ON. Use this position for thermostat control. Set thermostat for desired room temperature.
3.
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LIGHTING/RESET Figure 5-50 Lighting procedure for a combination gas control. (Courtesy ITT General Controls) |
Press dial in and turn clockwise to OFF. Use this position when complete shutdown is necessary. (Use PILOT position for temporary or seasonal shutdown.)
Second automatic valve operator close, bypassing the regulator(s) and shutting off the main burner and the pilot. As pressure inside the gas control and underneath the automatic valve diaphragm equalizes, spring pressure closes the second automatic valve to provide a second barrier to gas flow. The system is now ready to return to normal service when the thermostat again calls for heat and power is restored.
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Figure 5-51 Honeywell VR4205 direct ignition dual automatic combination gas control. (Courtesy Honeywell, Inc.) |
Some gas control modules are offered with slow-opening or step-opening regulation of the gas flow. Slow-opening gas controls function the same as standard models except that when the thermostat calls for heat, the second automatic valve opens gradually. Step-opening gas controls actually combine two pressure regulators, one for the low pressure and one for the full-rate pressure.
Universal Electronic Ignition Combination Gas Valve
The Honeywell universal electronic ignition combination gas valve shown in Figure 5-54 is designed for use with intermittent-spark ignition, direct-spark ignition, and hot-surface ignition systems in 24 VAC gas furnaces and boilers. This combination gas valve includes a manual valve, two automatic operators, pressure regulator, pilot adjustment, pilot plug, and ignition adapter.
A pilot burner is a device used in a gas-fired appliance to light the main gas burners and generate sufficient millivoltage to operate a thermocouple or thermopile pilot safety shutoff device (see Automatic Pilot Valves in this chapter). Modern pilot burners are designed to burn continually in order to provide an automatic ignition of the burners when the main gas supply is turned on.
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SECOND
Figure 5-52 Continuous pilot dual automatic gas valve operation during burner on cycle. (Courtesy Honeywell, Inc.) |
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SECUND
Figure 5-53 Continuous pilot dual automatic gas valve operation during burner off cycle. (Courtesy Honeywell, Inc.) |
Figure 5-54 Honeywell universal ignition combination
Valve. (Courtesy Honeywell, Inc.)
The pilot burners in common use today can be divided into aerated and nonaerated types.
An aerated pilot (see Figure 5-55) is one that injects primary air through an air intake opening into the gas stream. The air and gas are mixed before burning.
An aerated pilot burner produces a very stable flame. For this reason, these pilots are often used where the pilot location is particularly inaccessible. Although the flame produced by an aerated pilot burner is more stable than one produced by a nonaerated type, an aerated pilot burner does have some disadvantages. An important one to remember is the tendency for the small primary air openings to clog with lint and dirt. Frequent cleaning is required, particularly when these pilots are used in areas having a large amount of foreign material in the air.
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A nonaerated pilot (see Figure 5-56) does not inject primary air. As a result, the air and gas are not premixed, and the combustion process must be completed with secondary air only. This results in a less-stable flame than the one produced by an aerated pilot. On the credit side, a nonaerated pilot requires less maintenance than an aerated pilot. For this reason, the nonaerated pilot burner is usually preferred by most utility companies.
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COMBUSTION COMPLETED WITH
Figure 5-56 Nonaerated pilot. (Courtesy Trane Co.) |
A pilot burner assembly consists of the pilot bracket, pilot orifice, primary air intake, lint screen, mixing chamber, pilot ports, and pilot hood.
The pilot bracket is a device used to mount the pilot in a fixed relationship to the burner. Some pilot brackets also contain means for mounting the thermocouple or pilot generator so that the hot junction is located directly in the path of the pilot flame (see Figure 5-57).
The pilot ports are the openings through which the gas (in nonaerated pilot burners) or the gas and air mixture (in aerated pilot burners) passes before burning. The gas and air are premixed in the mixing chamber of an aerated pilot. The air is injected into the mixing chamber of an aerated pilot through a hole or opening called the primary air intake. The amount of primary air can be controlled by adjusting an air shutter that covers the primary air intake opening.
A lint screen is generally used to remove lint, dirt, and other contaminants from the primary air before it enters the primary air opening. In some pilots, the lint and other particles are burned
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FLAME
Figure 5-57 Pilot bracket with means for mounting thermocouple. (Courtesy Robertshaw Controls Co.) |
Before entering the primary air intake. An incinerated pilot is an aerated pilot in which the primary air passes adjacent to the flame area where the particles are burned out of the air before it enters the primary air intake.
Impurities are also found in the pilot gas. These impurities can be removed by installing a pilot gas filter in the line upstream from the pilot adjustment means in the control. The pilot gas filter is expected to operate several years without service. Clogging of the filter will be indicated by shortened pilot flames, which will result in improper pilot operation. Shortened pilot flames can also be caused by pilot tube stoppage or a dirty pilot orifice. These possibilities should be checked before removing the filter. If the filter should become clogged, replace the entire filter rather than just the filtering medium.
The pilot orifice is a removable component in the pilot that contains precisely sized openings that control the admission of gas to the pilot. Pilot orifices are either of the spud or insert type. A spud orifice screws into the bottom of the pilot burner. It is both an orifice and a fitting combined into a single unit with threads at either end (see Figure 5-58). An insert orifice must be held in position by a separate fitting (see Figure 5-59). The pilot gas line (tubing) is connected to the bottom of the pilot orifice.
Always follow the manufacturer’s instructions when installing a new pilot burner. Read these instructions very carefully before beginning any work.
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Figure 5-58 Spud orifice. (Courtesy Honeywell Tradeline Controls) |
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PILOT BRACKET |
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PILOT BURNER |
THERMOCOUPLE
INSERT-ORIFICE 1/4 INCH COMP FITTING *"
1/4 INCH:O. D. TUBING ""
Figure 5-59 Insert orifice. (Courtesy Honeywell Tradeline Controls)
If no installation instructions are available, the following location and mounting requirements should be carefully observed:
1. Choose a location for the pilot burner that provides easy access, observation, and lighting.
2. Rigidly affix the pilot burner to the main burner. Other mounting surfaces should not be used (see Figure 5-60).
3. Mount the pilot burner so that the flame is properly positioned with respect to the main burner flame (see Figure 5-61).
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Figure 5-61 Proper positioning of the pilot burner flame. (Courtesy Honeywell Tradeline Controls) |
A pilot flame should never be exposed to falling scale, which could impair the ignition of the main burners. Furthermore, the flame should not be exposed to draft conditions or to sudden puffs of air sometimes caused by igniting or extinguishing the main burner. Always provide an ample air supply free of contaminating products of combustion.
Replacing the Pilot Burner Orifice
The procedure for replacing a pilot burner orifice depends on whether it is a spud or an insert orifice. For a Honeywell spud orifice, the procedure is as follows:
1. Disconnect the gas supply tubing from the pilot burner.
2. Unscrew the spud orifice and throw it away.
3. Remove any burrs from the tubing and square off the ends.
4. Remove the one-piece nut and ferrule from the new assembly and slip them over the tubing.
5. Install the new assembly in the pilot burner and tighten securely.
6. Push the tubing (along with the nut and ferrule) into the burner as far as it will go and engage the nut.
7. Tighten the nut by hand until it will turn no further. Use a wrench to make one final turn.
The connection in step 7 should be tight enough to prevent any gas leakage. Do not tighten it too much or you will run the risk of stripping the threads. Try not to bend the tubing near the fitting after the nut has been tightened.
As shown in Figure 5-59 the procedure for replacing an insert orifice presents no serious problems. The gas supply tubing must first be disconnected from the pilot burner by unscrewing the compression fitting. The small insert orifice can then be removed. Sometimes a light tap on the pilot burner bracket will be required to dislodge the orifice.
Place the new orifice on the end of the gas tubing and insert both the orifice and tubing into the pilot burner. Tighten the compression nut until it is secure. Use the same procedure described for tightening a spud orifice.
Read the appliance manufacturer’s lighting instructions before attempting to light the pilot burner. The basic procedure for lighting a pilot is as follows:
1. Turn the room thermostat to its lowest setting.
2. Shut off the main gas supply to the main burner and the pilot burner.
3. Allow at least 5 minutes for the unburned gas to vent.
4. Light the pilot burner in accordance with the appliance manufacturer’s lighting instructions.
Venting the unburned gas (step 3) is very important. This is especially true for LP gas because it is heavier than air and will not vent upward naturally. Every precaution should be taken to ensure that the appliance is properly venting any unburned gas.
Appliance and pilot burner manufacturers provide very detailed lighting instructions for their equipment. Moreover, the development of various types of combination gas controls has simplified the lighting procedure and increased the safety factor (see Combination Gas Valves in this chapter).
The pilot flame must be adjusted for proper color and size. Appliance manufacturers refer to this procedure as pilot flame adjustment or pilot gas adjustment.
The appliance manufacturer will provide instructions for making pilot flame adjustments. On combination gas valves, this involves the removal of a pilot adjustment cap and turning the adjustment screw until the desired flame characteristics are obtained. The best pilot flame is a steady, nonblowing blue flame that envelops the upper 3/8 to V2 inch of the thermocouple or generator (see Figure 5-62).
The pilot burner should ignite the main burner quietly and reliably under all operating conditions, including low gas supply pressure. The ignition of the main burner gas should occur within 4 seconds from the time that gas is admitted to the main burner. This should occur when the pilot gas supply is reduced to an amount just above the point of pilot flame extinction.
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Figure 5-62 Pilot flame should generally envelop 3/s to ‘/2 inch of the thermocouple or thermopile tip. (Courtesy Honeywell Tradeline Controls) |
A main burner ignition test should be performed after the main burner gas input and primary air adjustments have been made. The type of test used to check main burner ignition will depend on the type of pilot used in the gas-fired appliance. For example, in a pilot
generator system, main burner ignition is checked with the pilot flame adjusted to the minimum millivoltage required to open the main valve. The manufacturer’s installation literature for the appliance will probably include instructions for testing main burner ignition. If no literature is available, check with the local gas company.
A pilot-pressure switch operates on the same principle as the gas — pressure switch used in the manifold of the gas-fired furnace or boiler. Its function is to prevent the premature failure of the spark igniter or glow coil should a prolonged gas interruption occur while the thermostat is calling for heat.
The pilot-pressure switch is installed in the pilot gas line between the pilot and pilot regulator. Installing a pilot switch requires that the pilot gas line be disconnected downstream from the pilot gas regulator. The gas line is then connected to the pilot-pressure switch by cutting the existing tubing to the regulator and connecting into the tee provided for switch mounting. A typical wiring of a pilot — pressure switch is shown in Figure 5-63.
PILOT FLAME SWITCH
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TO LIMIT SWITCH TO GLOW COIL TRANSFORMER |
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PILOT- PRESSURE SWITCH |
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RE H <■* |
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Figure 5-63 Typical wiring of a pilot-pressure switch. (Courtesy janitrol)
Electronic (solid-state) ignition systems have been developed to improve the energy efficiency of gas furnaces, boilers, and water heaters. These electronic ignition systems replace the less energy-efficient standing pilot used in traditional gas-fired appliances. There are three different types of controls used in electronic ignition systems:
• Intermittent pilot ignition module
• Direct ignition gas module
• Hot-surface ignition module
Note
Ignition modules do not have replaceable parts. If defective, the entire unit must be replaced. A replacement module must be of the exact same model and type as the defective one.
Manufacturers of electronic ignition modules offer a wide range of models designed to fit a variety of different applications (see Figure 5-64). Always make certain that the make, model, and operating specifications of the original gas valve control module match those of the replacement one. Specification sheets and installation instructions for electronic ignition modules are available online at the manufacturer’s Web site or by writing directly to the manufacturer (see Appendix B for contact information).
The following three sections briefly describe the three types of electronic ignition modules used in gas-fired furnaces, boilers, and water heaters. For more detailed and model-specific information, consult the owner’s manual, installation manual, and/or the specification/data sheets provided by the manufacturer.
Intermittent Pilot Ignition Module
An intermittent pilot ignition module is a solid-state ignition device used to automatically light a pilot burner and simultaneously energize (operate) the main gas valve of the heating system when the room
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Thermostat calls for heat (see Figure 5-65). Figure 5-66 illustrates the wiring connections between an intermittent pilot module, the dualvalve combination valve, and the combined pilot burner and igniter-
Sensor unit.
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AVENT 24 V TH-W DAMPER |
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CVD 24 V (DPT) PLUG (OPT) SENSE |
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The operating sequence for a gas burner operated by an intermittent pilot ignition control is as follows:
1. Room thermostat calls for heat, and the intermittent pilot ignition module simultaneously opens the pilot valve and supplies a continuous spark to the electrode in the pilot burner.
2. Pilot burner gas ignites and produces a flame.
3. Pilot flame sensor detects the pilot flame and signals the intermittent pilot ignition control to discontinue the spark and energize (open) the main gas valve.
The main gas valve will not be energized until the flame sensor detects the presence of the pilot flame. As long as the main gas valve remains closed, no gas from the supply line can flow through the burners. Should a loss of flame occur, the main gas valve closes and the spark recurs within 0.5 second.
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GND MV MV/PV PV (BURNER) |
A VENT 24 HV TH-W DAMPER GND 24 HV(DPT) PLUG (DPT) |
SPARK |
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PILOT |
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TCOM |
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DUAL VALVE COMBINATION GAS CONTROL |
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MAIN |
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I I/-I I <) I K. .J |
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LIMIT CONTROLLER |
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L2 |
L1 (HOT)
A
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PLOT GAS SUPPLY
Power supply, provide disconnect means and overload protection as required.
Alternate limit controller location.
/3 Maximum cable length 3 ft (0.9 m).
4 Controls in 24 HV circuit must not be in ground leg to transformer.
For module with TH-W terminal and vent damper plug,
/5 connect thermostat to TH-W. Leave 24 HV open. Do not remove vent damper plug.
Figure 5-66 Intermittent pilot module with a combination pilot burner and igniter-flame sensor in a heating system with an atmospheric burner. (Courtesy Honeywell, Inc.)
4. Gas from the gas supply line flows through the now-open main gas valve to the burners and ignites. This is the burn or on cycle. When the heat has reached the level required by the thermostat setting, the main gas closes and the burner or burners shut down. This is the off cycle in an intermittent pilot ignition control system.
• Run—The period during which the main gas valve remains energized and the spark is turned off after the successful ignition.
• Trial for ignition—The period during which the pilot valve and spark are activated, attempting to ignite gas at the main gas burner.
• Flameout—The loss of proven flame.
• Proven flame—A pilot flame detected by a flame sensory device.
• On cycle—Period of time during which the main gas valve is open and the burners are operating.
• Off cycle—Period of time during which the main gas valve is closed and the burners are not operating.
If the pilot flame is extinguished, even though the room thermostat is still calling for heat, the intermittent pilot ignition control immediately deenergizes the main gas valve, causing it to close its open supply port and stop the flow of gas to the burners. A spark at the pilot burner electrode will recur within 0.8 second.
As soon as the pilot flame is reignited and detected by the pilot flame sensor, the main gas valve is energized, the valve port is opened, and the spark is extinguished. The intermittent pilot ignition control then deenergizes the pilot gas and main burner gas valve when the thermostat stops calling for heat.
The direct-spark ignition (DSI) module illustrated in Figure 5-67 is a low-voltage, solid-state unit that controls the gas valve, monitors the burner flame, and generates a high-voltage spark for ignition. DSI modules are available with or without a purge timer and with separate or combined igniters and flame sensors. Typical wiring connections for a direct-spark ignition system are illustrated in Figure 5-68.
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25 — VOLT GROUND |
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CONNECTIONS TO COMBINATION GAS |
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25 — VOLT CONNECTION TO TEMPERATURE CONTROLLER |
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Figure 5-67 Typical direct-spark ignition module. (Courtesy Honeywell, Inc.) |
The principal components of a hot-surface ignition system are the hot-surface ignition module, a line voltage silicon carbide igniter (also sometimes called a glow stick or glow plug), a remote flame sensor, a 24-volt AC ignition-detection control, and a 24-volt (AC) redundant gas valve (see Figure 5-69). The flame sensor is designed to detect the presence of a flame. It can be mounted remotely on multiple burners or next to the gas burner.
The hot-surface ignition module, similar to the one shown in Figure 5-70, is a microprocessor-based gas ignition control designed for direct ignition gas-fired appliances. It provides direct
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LIMIT
Protection as required. A Alternate limit controller location. A Maximum ingniter-sensor cable length 3 ft (0.9 m) or less. A Factory-installed wire on VR-type controls. Do not remove. A 3 A Replaceable fuse. Figure 5-68 Typical wiring connections for a direct-spark ignition System. (Courtesy Honeywell, Inc.) |
Main gas burner ignition, remote sensing, and prepurging. It will retry for ignition and has a fixed time for flame lockout.
Some hot-surface ignition modules have self-diagnostic capabilities. A diagnostic light on the HSI module provides the following information:
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GND (BURNER) VENT
DAMPER PLUG
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Figure 5-69 Typical hot-surface ignition
Module. (Courtesy Honeywell, Inc.)
• If the diagnostic light on the module flashes on and off one time at initial startup, the unit is functioning properly.
• If the diagnostic light is lit continuously, there is most likely an internal problem with the module. Check for an internal problem by interrupting the line power or 25-volt thermostatic power for a few seconds and then restore it. If the burner still fails to ignite, replace the module.
• If the diagnostic light continues to flash, the problem is in the external components or wiring.
For HIS modules without self-diagnostic capabilities, a qualified HVAC technician or electrician should troubleshoot the system with the appropriate test equipment. The test equipment should include the following:
• A volt-ohm meter for checking both the voltage and the resistance.
• A precision microammeter for checking the flame sensor output and location.
• A pressure gauge (low scale) for checking gas pressure.
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BURNER GROUND Figure 5-70 Honeywell hot-surface ignition module with wiring connections to the flame sensor and the hot-surface igniter. (Courtesy Honeywell, Inc.) |
Warning
Extreme caution must be taken when working on a hot-surface ignition system. Because of the high voltage present, there is always the potential for serious electrical shock.
If the unit is not equipped with a self-diagnostic light, closely follow the troubleshooting suggestions provided by the manufacturer. These will be specific to the make and model. Some simple things to look for include the following:
Checking to make certain the manual knob on the gas valve is in the on position and gas is available at the inlet piping Checking the outlet gas pressure to make sure it matches the nameplate rating
Checking the wire leads to the gas valve for proper connection or damage
An igniter produces the spark for direct ignition of the main gas burner in various heating applications (gas furnaces, gas boilers,
Gas water heaters, etc.). The Honeywell Q347 igniter shown in Figure 5-71 consists of an internal electrode with a ceramic insulator, bracket, and ground strap.
Note
The flame-sensing rod is separate from the hot-surface igniter in most hot-surface ignition systems.
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An igniter is used to provide the spark to ignite the main burner flame. Some igniters have an integral flame sensor. When this is the case, the igniter both ignites and senses (proves) the main gas burner flame.
The operating sequence of a system in which an igniter is used may be summarized as follows:
1. Room thermostat calls for heat.
2. Gas valve opens and gas flows to the burner(s).
3. Burner ignites when the gas reaches the main burner.
4. Spark igniter shuts off.
The duration of the spark operation must be within the igniter manufacturer’s specified lockout timing period. The igniter manufacturer will provide a chart of the ignition control lockout times in the service literature for the igniter. The example shown in Table 5-3 is for Honeywell’s Q347 igniter.
Table 5-3 Ignition Control Lockout Times
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Maximum Safety Lockout Time |
Specified Lockout Time (Stamped on Ignition Module)
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(Courtesy Honeywell, Inc.)
The electrode spark gap in the igniter must be within the specified maximum (see Figure 5-72). If the gap is not within specifications, it will have to be adjusted for optimum performance.
The flame rod of a combined igniterflame sensor unit must be immersed 1 inch in the burner flame to produce the best flame signal
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BEND GROUND |
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D. 1.64 IN. (4.17 MM) SPARK GAP |
SPARK
IGNITER
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Figure 5-73 Flame rod immersed 1 inch in burner flame. (Courtesy Honeywell, Inc.) |
(see Figure 5-73). Examples of poor flame conditions and their probable causes are illustrated in Figure 5-74.
The flame signal can also be adversely affected by a bent bracket, bent rod, or cracked ceramic insulator. Sometimes the bracket can be bent back into shape. If the rod is bent or the ceramic insulator is cracked, the igniter should be replaced.
Note
Always check the specifications of the replacement hot-surface igniter before installing it. Not all igniters have the same voltage or warm-up time as the original design.
The igniter used in a hot-surface ignition operating system differs in design from the type used in intermittent pilot or direct-spark ignition systems described in the preceding paragraphs (see Figure 5-75).
Be careful when replacing a hot-surface igniter because they are fragile and easily damaged. Sometimes a crack in the igniter surface is so small that it is not visible. A cracked hot-surface igniter may still work, but it will have a much shorter service life. After it is installed, check the hot-surface igniter for any inconsistencies in its glow pattern.
In electronic ignition systems, a flame sensor is used in conjunction with the igniter to control the burner flame. The ignition module provides AC power to the flame sensor, which the burner flame rectifies (changes) to direct current (DC). The level of flame current is measured by the flame sensor to ensure flame presence. The flame current must be the specified minimum for the ignition module. For example, the flame current for a Honeywell S87C direct-spark
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• HIGH VELOCITY OF SECONDARY AIR
INSTALL SHIELD IF NECESSARY
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Typical hot-surface
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Figure 5-75 Igniter. |
Ignition module must be at least 1.5 ^A, whereas for a Honeywell S89 hot-surface ignition module it must be at least 0.8 ^A.
Note
In electronics, rectification is the process of converting alternating current (AC) to direct current (DC). Flame rectification indicates that a flame (e. g., a gas burner flame) is used to convert from AC to DC current.
Many people frequently confuse mercury flame sensors (MFS) with thermocouples because each has a similar sensor that extends into the pilot flame and a tube connecting the device to the gas valve. Moreover, they have the same function in the control system of a gas-fired appliance, but there are significant differences.
The principal components of a mercury flame sensor (MFS) device are (1) a pilot flame sensor (the portion of the MFS device extending into the pilot flame), (2) a diaphragm/SPDT switch assembly located at the main gas valve, and (3) a hollow capillary tube connecting the flame sensor to the diaphragm/switch assembly.
The operation of a mercury flame sensor device depends on the evaporation of mercury. The sensor end, capillary tube, and the SPDT switch are filled with mercury. When there is enough heat produced by the pilot flame at the sensor end of the capillary tube, it vaporizes and pushes the remaining nonvaporized (liquid) mercury down the capillary tube to the bellows-type diaphragm/switch assembly located at the main gas valve. Movement of the bellows diaphragm presses against a nonadjustable, calculated spring tension with enough force to snap the SPDT switch from one set of contacts to another. This action causes the switch contacts to move from one position to another. In an MFS device switch assembly, the normally closed contact opens and the normally open contact closes. This action deactivates the igniter (after the pilot flame is proven) and opens the main gas valve to allow raw gas to flow to the burners.
Note
Mercury flame sensors are no longer used in gas-fired furnaces and boilers, especially those equipped with solid-state control modules. However, manufacturers still produce replacement MFS units along with their compatible main gas valves.
The principal functions of the oil controls are (1) to turn the oil burner on and off in response to temperature changes in the space or spaces being heated and (2) to stop the system if an unsafe condition develops. The following controls are necessary to perform these functions:
Thermostat
• Limit controls
• Primary control
• Oil valves Time-delay controls
• Circulator or fan control
• Other auxiliary controls
This chapter is concerned with a description of the oil burner primary control, oil valves, and time-delay controls. The remaining controls found in an oil burner control system are described in Chapter 4 (Thermostats and Humidistats)’ and Chapter 6 (Other Automatic Controls)’.
Oil valves are used to provide on-off control of the flow of oil to the oil burner. These are normally closed solenoid valves that open when energized and close immediately when deenergized. They are variously referred to as solenoid oil valves, magnetic oil valves, or oil burner valves and are available in either immediate-discharge or delayed-discharge models.
An immediate-discharge oil valve discharges oil as soon as it is energized. A delayed-discharge valve is equipped with an integral thermistor to delay the valve opening for about 3 to 15 seconds (the length of time will vary depending on the manufacturer). This delay allows the burner fan to reach operating speed and establish sufficient draft before the oil is discharged.
A solenoid oil valve will make an audible click when it is opening and closing properly. If the valve fails to open after the room thermostat calls for heat, the following conditions may be responsible:
Inadequate fuel pressure available at the valve An obstructed bleed line
• No voltage indicated at valve
Check the voltage at the coil lead terminals against the voltage shown in the nameplate. Also check the inlet pressure against the rating on the nameplate. If none of these conditions is causing the problem, the failure of the valve to open is probably due to a malfunctioning solenoid coil. The position of the coil is shown in the exploded view of the valve in Figure 5-76. The steps for replacing the solenoid are as follows:
1. Remove the nut on top of the valve by turning it counterclockwise.
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Nut —…………..
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2. Remove the powerhead assembly from the spindle.
3. Disconnect and remove the solenoid coil.
4. Connect the replacement coil and reassemble.
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Figure 5-77 Magnetic valve used in controlling oil flow to the oil burner. (Courtesy Honeywell Tradeline Controls) |
Examples of delayed-discharge valves are shown in Figures 5-77 and 5-78. In both valves, the timing delay is governed by a thermistor attached to the solenoid coil. In these valves, the timing delay will vary with ambient temperature, voltage level, and other factors during normal operation. If the timing is significantly off, it may be necessary to replace the thermistor. Because the thermistor is attached to the solenoid coil, the coil must also be replaced in order to replace the thermistor.
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V Figure 5-78 Magnetic oil valve. (Courtesy Honeywell Tradeline Controls) |
Delayed valve opening can also be obtained by using an electronic time delay wired in series with the oil valve (see Figure 5-79). Unlike the thermistor, the timing of this device is not affected by ambient temperature. On a call for heat, the valve opening is delayed for approximately 5 seconds.
The oil burner primary control is an automatic safety device designed to turn off the oil burner motor should ignition or flame failure occur.
Each primary control operates in conjunction with a sensor by which the burner flame is monitored throughout the burner on cycle. The method used to sense the burner flame determines the type of primary control used and its location in the heating system.
The two types of primary controls commonly used in oil burner control systems are (1) the cadmium cell primary control and (2) the stack detector primary control. The cadmium cell primary control is burner mounted and uses a light-sensitive cad cell flame detector (sensor). The stack detector primary control relies on a thermal sensor to detect flame or ignition failure. This type of primary control assembly is available with the thermal sensor mounted in the stack and the
Figure 5-79 Electronic time delay wired in series with the oil valve.
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Power supply. Provide overload protection and disconnect means as required. |
(Courtesy Honeywell Tradeline Controls)
Primary control mounted on the burner, or with both the primary control and thermal sensor mounted in the stack as a single unit.
A cadmium cell primary control consists of a primary control assembly operating in conjunction with a cadmium detection cell.
The cadmium detection cell is considered the most effective sensor used to monitor the burner flame. It consists of a light-sensitive photocell, a holder, and a cord assembly (see Figure 5-80). The surface of the detection cell is coated with cadmium sulfide and overlaid with a conductive grid. Electrodes attached to the detection cell are used to transmit an electrical signal to the primary control.
Figure 5-80 Cadmium detector
Cell . (Courtesy Honeywell Tradeline Controls)
The variable resistance of this surface to the presence of light (i. e., the burner flame) is used to actuate the flame detection circuit. When light is present (in the form of the burner flame), the resistance of the cadmium sulfide surface to the passage of electrical current is very low. Consequently, as long as the burner flame lasts, an electrical current will pass between the cadmium detection cell and the primary control unit, and the burner motor on cycle will continue operating. If the burner flame should fail or if ignition should fail to occur, the resistance of the cadmium sulfide surface to the passage of electrical current will be very high. This will interrupt the passage of the electrical current to the primary control and will cause the latter to shut off the burner motor.
The detection cell is mounted inside the burner air tube so that it faces the burner flame (see Figure 5-81). The exact location of
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Figure 5-81 Cadmium detection cell mounted inside burner air tube and facing burner flame. |
The detection cell is determined by the oil burner manufacturer and dictated by the design of the oil burner. In any event, the detection cell must be placed so that it views the burner flame directly. The fact that the detection cell responds to any light source means that it must be located where its surface will be shielded from any form of direct or reflected external light. Moreover, the ambient temperature should be kept below 140F because excessive temperatures can also cause the detection cell to malfunction.
Sometimes a malfunctioning oil burner will cause a heavy layer of soot to accumulate on the cell surface. The cell surface should be carefully wiped to remove the soot and restore full view of the oil flame. A damaged detection cell should be replaced.
The type of primary control used with a cadmium detection cell will depend on the type of controller voltage, the type of ignition system, and the length of safety switch timing required by the installation.
The Honeywell R8184G Protectorelay primary control shown in Figure 5-82 has a transformer included in the unit to supply 24-volt power to the control circuit. This is a low-voltage primary, and it requires a 24-volt thermostat. Other models are available that require a line voltage controller (see Figure 5-83).
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1KRELAY
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FLAME-DETECTOR SAFETY-SWITCH LINE-VOLTAGE
Figure 5-83 Primary control used with a line voltage thermostat. (Courtesy Honeywell Tradeline Controls) |
The primaries illustrated in Figures 5-82 and 5-83 are designed for use with nonrecycling constant-ignition oil burners. Automatic recycling control of an intermittent-ignition oil burner can be obtained by using the primary shown in Figure 5-84. The basic differences between the constant-ignition and intermittent-ignition systems can best be illustrated by the wiring diagrams shown in Figures 5-85 and 5-86. An intermittent-ignition system contains the same components as a constant-ignition system plus the following:
1. A interlock contact in the ignition circuit (T1).
2. An ignition timer heater (T).
3. An interlock contact (T2) in the circuit between the safety switch heater (SS) and the ignition timer heater (T).
Safety switch timing can be 15, 30, 45, or 80 seconds depending on the manufacturer and model.
Stack Detector Primary Control
Stack-mounted oil burner primary controls employ thermal sensors to detect ignition or flame failure. A typical stack detector thermal sensor (see Figure 5-87) consists of a bimetal element
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IGNITION TIMER |
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LOW-VOLTAGE TERMINAL STRIP / / |
SAFETY
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TRANSFORMER
Figure 5-84 Honeywell R8I85E Protectorelay primary control.
(Courtesy Honeywell Tradeline Controls)
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A Power supply. Provide disconnect means and overload protection as required. A. Switch opens when flame is sighted. |
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Figure 5-86 Schematic diagram of Honeywell model R8I85E Primary control. (Courtesy Honeywell Tradeline Controls) |
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(2) Drill holes and fasten flange with screws furnished. |
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Inserted into the stack (see Figure 5-88). The thermal sensor (combustion thermostat) is usually located on the stack where the element will be exposed to the most rapid temperature changes. The thermal sensor should always be mounted ahead of any draft regulator. If installed on an elbow, it should be mounted on the outside curve of the elbow.
The stack-mounted primary control illustrated in Figure 5-89 combines a Honeywell RA117A Protectorelay control for burner
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HOLDER Figure 5-88 Bimetal element inserted into the stack. (Courtesy Honeywell Tradeline Controls) |
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Cycling control and a thermal detector for sensing temperature changes of the flue gases (as high as 1000F maximum temperature). The safety switch shown on the center-left of the unit is designed to lock out if the flame is not properly established. If the flame goes out during the burner on cycle, the primary control will make one attempt to restart. If the attempt is unsuccessful, the safety switch will lock out. A manual reset is then required in order to restart the burner. The primary control shown in Figure 5-89 is used with a two-wire or three-wire primary controller.
A stack-mounted combination line voltage primary control and flame detector is shown in Figure 5-90. This type of primary control is used with constant-ignition oil burners and is designed for flange-mounting on a stack, flue pipe, or combustion chamber door of a furnace or boiler. It must be used with a line voltage thermostat or controller.
Combination Primary Control and Aquastat
The combination primary control and aquastat is designed for use with a constant-ignition oil burner in a hydronic heating system. The purpose of this unit is to supervise the operation of the oil burner and provide both water temperature and circulator control. A remote sensor (cadmium detection cell) is used to detect any irregularities in the oil burner flame.
Figures 5-91 and 5-92 illustrate a number of different combination primary control and aquastat units used in hydronic heating systems. In operation, the high-limit switch (SPST) will automatically turn off the burner if the boiler overheats. The low-limit circulator switch (SPDT) is used to maintain water temperature for the domestic hot-water supply. It will also prevent the circulator from operating if the water temperature is too low (i. e., below the setpoint).
On the units shown in Figures 5-91 and 5-92, a call for heat from the room thermostat pulls in relays 1K and 2K to turn on the oil burner and start heating the safety switch. Under normal operating conditions, the burner should ignite within safety-switch timing. If such is the case, the cadmium cell detects the flame, and relay 3K pulls in to deenergize the safety-switch heater. The oil burner then continues to operate until the call for heat is satisfied.
The circulator (pump) in the heating system operates when relay 1K pulls in only if the R to W contact on the aquastat control is made (see Figure 5-92). A drop in water temperature will cause the R to B (low limit) contact to be made. This acts as a call for heat, pulling in relay 2K to turn on the oil burner.
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^1 Power supply. Provide disconnect means and over load protection as required.
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A Control wires can be run with line-voltage wires in conduit but then must have NEC class 1 insulation.
Figure 5-91 Model R8182H Protectorelay primary control. High-limit/ low-limit aquastat switching with remote-bulb sensor.
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• LOW VOLTAGE A Power supply, 120 volts AC. Provide disconnect means and overload protection as required. |
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Combination primary control and aquastat units are also used on water heaters (see Chapter 4 of Volume 3, W ater Heaters and Other Appliances)’.
Troubleshooting the Oil Burner Primary Control
Table 5-4 lists possible remedies to a number of different operating problems associated with oil burner primary controls. Before checking the primary control, examine the following parts of the oil burner and ignition systems:
• Main power supply and burner motor fuse
• Ignition transformer
• Electrode gap and position
• Contacts between ignition transformer and electrode
Other system components that should be checked before examining the primary control are the oil piping to the tank, the oil filter, oil pump pressure, oil nozzle, and oil supply.
Table 5-4 Troubleshooting Oil Burner Primary Control
Symptom and Possible Cause Possible Remedy
Repeated safety shutdown.
Slow combustion thermostat response.
(b) Low line voltage.
(c) High resistance in combustion thermostat circuit.
(d) High resistance in thermostat or operating control circuit.
(e) Short cycling of burner.
(a) Move combustion thermostat to better location. Adjust for more — efficient burner flame. Clean surface of cad cell.
(b) Check wiring and rewire if necessary. Contact local power company.
(c) Replace combustion thermostat.
(d) Check circuit and correct cause.
(e) Clean filters. Reset or replace differential of auxiliary controls. Repair or replace faulty auxiliary control. Set thermostat heat anticipation at higher amp value. Clean holding circuit contacts.
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Table 5-4 (continued)
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