Gas Burners
A gas burner (see Figure 2-1) is a device for supplying gas, or a mixture of gas and air, to the combustion area. Those used in residential and light commercial heating are most commonly high-pressure, gun-type burners. Only those gas burners approved by the American Gas Association (AGA) should be used in a heating system.
Gas is used as a heating fuel in both urban and rural areas. Manufactured, natural, and bottled gas are the three types used as heating fuels. Each of these gases has different combustion characteristics and will have different heat values when burned. Because of this, a gas burner must be adjusted for each gas fuel, particularly when changing from one type to another. The principal types of bottled gas used as heating fuels are propane and butane. Bottled gas is frequently called LPG (liquefied petroleum gas) and is widely used as a heating fuel in rural areas. A more detailed description of heating fuels is found in Chapter 5 of Volume 1 (Heating Fuels)’.
The gas burners used in residential heating systems are most commonly the atmospheric injection type that operates on the same principle as the Bunsen burner.
The essential features of the Bunsen burner are shown in Figure 2-2. The burner consists of a small tube or burner, which is placed inside a larger tube. The latter has holes positioned slightly below the top of the small tube. The gas escaping from the small tube draws the air in through the holes and produces what is called an induced current of air in the large tube. This air enters through the holes and is mixed with the gas in the tube. The mixture is burned at the top of the larger tube. The flame from such a burner gives hardly any light, but the heat is intense. The intensity of the heat can be illustrated by holding a metal wire over the flame for a few seconds. It will glow with heat in a very short time.
The air supply in an atmospheric injection burner is classified as either primary or secondary air and is commonly introduced and mixed with the gas in the throat of the mixing tube. The cutaway of an upshot atmospheric gas burner in Figure 2-3 illustrates this principle of operation. The gas passes through the small orifice in the mixer head, which is shaped to produce a straight-flowing jet moving
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Figure 2-3 Cutaway of the venturi, or mixing tube, of an upshot gas burner. |
At high velocity. As the gas stream enters the throat of the venturi, or mixing tube, it tends to spread and induce air in through the opening at the adjustable air shutter. The energy in the gas stream forces the mixture through the mixing tube into the burner manifold casting, from which it issues through ports where additional air must be added to the flame to complete combustion. The air coming in through the venturi is the primary air and that supplied around the flame is the secondary air.
The primary air is admitted at a ratio of about 5 parts primary air to 1 part gas for manufactured gas, and a 10 to 1 ratio for natural gas. These ratios are generally used as theoretical values of air for purposes of complete combustion. Most atmospheric injection burners operate efficiently on 40 to 60 percent of the theoretical value.
The excess air required depends on several factors, notably the following:
• Uniformity of air distribution and mixing Direction of gas travel from the gas burner The height and temperature of the combustion chamber
The secondary air is drawn into the burner by natural draft. Excess secondary air constitutes a loss and should be reduced to a proper minimum (usually not less than 25 to 35 percent). All yellow-flame gas burners depend exclusively on secondary air for combustion.
The Bunsen burner flame is bluish and practically nonluminous. A yellow flame indicates dependence entirely on secondary air for combustion. Primary air is regulated by means of an adjustable shutter. For manufactured gas, the air supply is regulated by closing the air shutter until yellow flame tips appear and then by opening the air shutter to a final position at which the yellow tips just disappear. This type of flame obtains ready ignition from port to port and favors quiet flame extinction. When burning natural gas, the air adjustment is generally made to secure as blue a flame as possible.
The division of air into primary and secondary types is a matter of burner design, the pressure of gas available, and the type of flame desired.
The gas should flow out of the burner ports fast enough so that the flame cannot travel or flash back into the burner head. The velocity must not be so high that it blows the flame away from the port. In an all-yellow flame, flame flashback cannot occur, and a much higher velocity is needed to blow off the flame.
A draft hood is used to ensure the maintenance of constant low — draft conditions in the combustion chamber with a resultant stability of air supply. A draft hood will also control backdrafts that tend to extinguish the gas burner flame and the amount of excess air. These draft hoods must conform to American Standard Requirements.
Each gas-fired appliance is wired according to the specific make and model. A wiring diagram is included in the appliance manufacturer’s installation and operation manual. All electrical connections must be made in accordance with the manufacturer’s installation instructions. Read these instructions and follow them carefully.
Caution
Only a certified HVAC technician or someone with similar qualifications and/or experience should attempt to wire a gas-fired appliance.
Note
All local codes and regulations for wiring gas-fired appliances must take precedence over the instructions in the manufacturer’s installation and operation manuals. In the absence of local codes, all electrical wiring and connections should conform with the appropriate instructions and provisions found in the latest edition of the National Electrical Code.
A description of the electrical circuits used to operate modern gas-fired heating equipment is included in Chapter 5 (Gas and Oil Controls)’.
The automatic controls are used to ensure the safe and efficient operation of a gas-fired appliance. They are mentioned only briefly here because detailed descriptions of gas system controls are found in other chapters of this volume. These controls can be roughly divided into the following six broad categories:
Room thermostats
• Ignition (lighting) devices
• Main gas valves
• Flame-sensing devices
• Pressure regulators Safety valves and switches
A gas heating system is controlled by a centrally located room thermostat. The thermostat sends a call for heat to the furnace or boiler when the temperature of the air in the room reaches the setpoint (heat setting) on the thermostat. This occurs automatically when the temperature falls below a preselected heat setting, or manually when the temperature adjustment dial or lever is moved up to a warmer heat setting. When the heat setpoint is reached, it closes an electrical circuit between the thermostat and the furnace or boiler, which, in turn, activates the furnace or boiler control circuit. (See Chapter 4, Thermostats and Humidistats, ’ in V olume 3 for a description of these thermostats.)
The main gas valve controls the flow of natural or propane gas to the burners when the thermostat calls for heat. In most heating systems, the main gas valve is combined with a pressure regulator to form a combination gas valve. Main gas valves are covered in Chapter 5 (Gas and Oil Controls)’ of this volume.
Another important control is the device used to detect whether the gas in the burners has successfully lighted after the call for heat by the room thermostat. Some systems use a thermocouple to verify that a pilot burner is lit. If it can detect the pilot flame, it will open the main gas valve, allowing gas to flow to the main burners where the pilot flame will ignite it. If it cannot detect a pilot flame, it will not open the main gas valve. The flame sensor of an electronic ignition system performs the same function as the thermocouple in standing pilot systems, but it looks for the flame in the main gas burners. See Chapter 5 (Gas and Oil Controls)’ for a description of flame-sensing devices.
The fan and limit control is another device that ensures the safe operation of a gas appliance. It controls the blower in a forced warm-air furnace. If the blower fails to operate during the heating cycle, the fan and limit control will shut off the main gas burners to prevent the furnace from overheating. Read the appropriate sections of Chapter 6 (Other Automatic Controls)’ of this volume for a more detailed description of fan and limit controls.
The pressure regulator, which is commonly combined with the main gas valve in the form of a single combination, controls the amount and pressure of the gas used by the gas-fired appliance. See Chapter 5 (Gas and Oil Controls)’.
A wide range of safety valves and switches have been created by different gas furnace and boiler manufacturers to ensure the safe and efficient operation of their appliances. Pilot safety valves, pressure switches, and other devices are described in the appropriate sections of Chapter 5 (Gas and Oil Controls)’ of this volume.
Types of residential gas burners include atmospheric injection, yellow (luminous) flame, and power burner units. Their classification is determined by the firing method used. Gas burners can also be divided into two broad classifications based on whether they are specifically designed as integral parts of gas-fired heating equipment, as in Figure 2-4, or are used to convert a furnace or boiler from one fuel to another. The latter are called conversion burners and, at least outwardly, resemble the gun-type burners used in oil — fired appliances. Gas conversion burners are commonly designed and manufactured with integral controls so that they can be installed as a unit in the existing furnace or boiler.
Note
The burner(s) producing the heat in a gas-fired appliance is sometimes called the main gas burner. Do not confuse the main gas
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Burner with the pilot gas burner. The function of the latter (where it is used) is to light the gas flowing to the main gas burner.
Gas burners may also be classified as inshot and upshot types, depending on the design of the burner tube. The burner tube of an inshot gas burner is commonly a straight, adjustable venturi that extends horizontally from the unit (see Figure 2-5). An upshot gas burner is characterized by a burner tube that extends horizontally from the unit and then bends to assume a vertical position (see Figure 2-6).
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Figure 2-5 Inshot conversion gas burner for furnaces or boilers. (CourtesyAdams Manufacturing Co.) |
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Figure 2-6 Upshot conversion gas burner for furnaces or boilers. (CourtesyAdams Manufacturing Co.) |
An integral-type gas burner assembly consists of an array of parallel burner tubes connected by a manifold pipe running at a right angle to them. The burner tubes and manifold are part of a box/drawer assembly in modern furnaces and boilers. The entire assembly can be removed from the furnace or boiler for cleaning (see Figure 2-7). Each burner tube contains a series of orifices (openings) through which the gas flows. These orifices are sized to deliver the required amount of gas flow to achieve the maximum ratings at the rated pressure listed on the appliance nameplate.
Note
Instructions for servicing burner orifices are included in Chapter
11 (“Gas Furnaces”) in Volume 1.
As shown in Figure 2-7, the burner manifold is connected at one end to the individual burner tubes and at its other end to the main gas valve. In other words, it functions as the bridge between the burner tubes and the main gas valve.
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125 5 Figure 2-7 Exploded view of a Thermo Pride CDX/CHX gas control System. (Courtesy Thermo Pride) |
In many furnaces, the main burner(s) can be manually adjusted. In others, no burner adjustment is required because burner aeration has been fixed at the factory. Natural gas burner flames should be well defined (but almost transparent) and should range from light to medium blue in color. Propane burner flames often have yellow — or orange-colored flame tips.
Manifold pressure adjustments, gas input adjustments, instructions for changing burner orifices, and other recommendations for gas burner maintenance and the improved operating efficiency of gas furnaces is covered in great detail in Chapter 11 (Gas Furnaces)’ in V olume 1.
A gas conversion burner is used to convert heating equipment designed for coal or oil to gas fuel use (see Figure 2-8). The boiler or furnace must be properly gastight and must have adequate heating surfaces.
A characteristic of gas conversion burners is that pressure will sometimes build up in a furnace due to puffs or backfire resulting
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Figure 2-8 Residential spark-ignition gas conversion burner. (Courtesy MIDCO International, Inc.) |
From delayed ignition and other causes. Local heating codes and regulations usually stipulate that furnace doors be held tightly closed by spring tension only (in other words, not permanently closed) in order to provide a means for relieving pressure. Figure 2-9 shows an example of a door spring that can be used for this purpose.
Figure 2-9 Door-closing springs for furnace doors.
(Courtesy Magic Servant Products Co.)
Gas Conversion Burner Combustion Chambers
The combustion chamber for a gas conversion burner is commonly located in the ashpit of a coal-fired boiler or furnace. Figure 2-10 illustrates the positioning of an upshot gas conversion burner. Note that the burner head port is located 1 inch (plus or minus Vi in) above the grate level. This is a standard measurement when installing an upshot burner in the ashpit of a furnace.
The manufacturer’s installation instructions provided with a conversion gas burner generally include specific instructions on the
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Figure 2-10 Positioning an upshot gas burner. (Courtesy Magic Servant Products Co.) |
Preparation of the combustion chamber. The main points you should remember are as follows:
All openings in the boiler must be sealed.
The combustion chamber must be thoroughly cleaned.
• Heat exchanger surfaces must be protected against concentrated heat.
All nonheat transfer surfaces must be protected.
The combustion chamber must be designed to contain combustion, to radiate heat, and to insulate the ashpit.
Additional information concerning gas conversion burners can be found in Chapter 16 (Boiler and Furnace Conversion)’ in V olume 1.
Gas Piping for Conversion Burners
Gas piping is generally wrought iron or steel with malleable iron pipe fittings. Joint compound (pipe dope) is applied sparingly to the male threads only. Make certain before applying the joint compound that it is approved for all types of gas. Never use aluminum tubing or cast — iron fittings on the main gas line. Soldered or sweated connections are also not recommended.
Tables 2-1 and 2-2 can be used to determine the size of pipe to use from the meter to the gas burner. The correct number of threads for any particular length of pipe is given in Table 2-3.
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Table 2-1 Pipe Capacity Table
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Table 2-3 Specifications for Threading Pipe
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*See AGA Requirements and Recommended Practice for House Piping and Appliance Installation. (Courtesy Magic Servant Products Co.) |
Use pipe fittings at all turns in the gas line. Never bend or lap welded pipe because it will pinch or weaken it. Support the pipe with straps, bands, pipe hooks, or hangers (never allow one pipe to rest on another or to sag).
Pitch all horizontal pipe so that it grades toward the meter without the occurrence of sags. The piping should be protected against freezing and the accumulation of condensation.
Pipes and pipe fittings that are defective should be replaced, never repaired. Every effort must be made to eliminate any possibility of gas leakage. Gas leaks on pipes and pipe fittings should be located by spreading a soap solution over the surface. Never try to locate a gas leak with a flame. The results could be extremely hazardous.
Figure 2-11 illustrates the general configuration of the main shutoff valve, pilot shutoff valve, and riser installation for a gas conversion burner. The following suggestions are worth noting:
1. Install the main manual gas shutoff valve on the riser at least 4 ft above the floor level.
2. Install the pilot valve on the inlet side of the main manual gas shutoff valve.
3. Install a tee fitting at the bottom of the riser to catch any foreign matter in the pipe. The bottom of the tee fitting should be plugged or capped.
4. Install a ground joint union in the gas line between the burner air duct box and the tee fitting in the riser.
5. Install a pilot supply line (Vi-in OD tubing) between the pilot valve and a point located on the upper right side of the air duct box on the gas burner. This line will run parallel to the riser.
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The venting system for gas heating equipment consists of the following:
1. The chimney or smoke elimination pipe.
2. The draft diverter or draft diverter hood.
Gas-burning equipment must be vented to the outside. All pipes leading from the equipment must be fitted so that the joints are tight and free of leaks.
A draft diverter or draft hood is a wind deflector placed in the chimney to prevent downdrafts of air (i. e., air moving down the chimney from the outside) from blowing out the pilot light. Many draft diverters are designed and positioned so that the downdraft is deflected into the room containing the heating unit.
Any room containing gas heating equipment must have adequate ventilation. Provision for incoming air (i. e., air necessary for combustion) is especially important in rooms or buildings of tight construction. The minimum area requirements (in relation to each 1000 Btu/h input) for both ventilating air openings and air inlet openings can be obtained from the manufacturers of the gas-fired appliance.
The annual cleaning and inspection of gas heating equipment is important not only because it contributes to its efficient operation but also because it provides an additional safety factor.
Electrical controls should be connected on a separate switch. This enables the circuit to be broken should the equipment malfunction.
Because fuel gas is extremely volatile, it should be handled and stored with the utmost care. Propane is especially dangerous when it leaks. Because it is heavier than air, propane will accumulate at low points in a room and present an explosion hazard.
Be sure to observe the following basic safety rules when working with heating gases and gas controls:
• Always shut off the gas supply to the device when installing, modifying, or repairing it. Allow at least 5 minutes for any unburned gas to leave the area before beginning work. Remember that LPG is heavier than air and does not vent upward naturally.
• Always conduct a gas leak test after completing the installation, modification, or repair. To test for a gas leak, coat the pipe joint, pilot gas tubing connections, and valve gasket lines with a soap-and-water solution. Then, with the main burner in operation, watch for bubbles at those points. The bubbles will indicate a gas leak, which can normally be eliminated simply by tightening joints or screws or by replacing the gasket.
Never use a flame to check for a gas leak.
Always disconnect the power supply to prevent electrical shock or equipment damage before connecting or disconnecting any wiring.
• Change the main burner and pilot orifice(s) to meet the appliance manufacturer’s instructions when converting a gas system from one type of gas to another.
• Always read and carefully follow the installation and operating instructions supplied with the appliance or component. Failure to follow them could result in damage or cause a hazardous condition.
Make certain that the appliance or component is designed for your application. Check the ratings given in the instructions and on the appliance or component.
Check the operation of the appliance or component with the manufacturer’s instructions after installation is completed.
Do not bend the pilot tubing at the control after the compression nut has been tightened. This could cause a gas leak at the connection.
• Never jump (or short) the valve coil terminals on 24-volt controls. Doing so could short out the valve coil or burn out the heat anticipator in the thermostat.
Never connect millivoltage controls to line voltage or to a transformer, because doing so will burn out the valve operator or the thermostat anticipator.
Do not remove the seals covering control inlets or outlets until you are ready to connect the piping. The seals are there to prevent dirt and other materials from getting into the gas control and interfering with its operation.
As is the case with all mechanical and electrical equipment, it is recognized that occasional repair and adjustment may be necessary on any burner. Table 2-4 represents a very general list of troubles and causes that can occur with gas burners and gives some suggested remedies.
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Table 2-4 Troubleshooting Gas Burners
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Table 2-4 (continued)
Pilot goes out frequently during standby or safety switch needs frequent resetting.
(a) Restriction in pilot gas line.
(b) Low gas pressure.
(c) Blocked pilot orifice.
(d) Loose thermocouple connection on 100 percent shutoff.
(e) Defective thermocouple or pilot safety switch.
(f) Poor draft connection.
(g) Draft tube set into or flush with inner wall of combustion chamber.
Pilot goes out when motor starts.
(a) Restriction in pilot gas line.
(b) High or low gas pressure.
(c) Excessive pressure drop when main gas valve opens.
Burner motor does not run.
(a) Burned-out fuse or tripped circuit breaker.
(b) Thermostat or limit defective or improperly set.
(c) Relay or transformer defective.
(d) Motor burned out.
(e) Tight motor bearings from lack of oil.
(f) Improper wiring.
(a) Clear or replace line.
(b) Check for possible gas supply problem first with local gas company; Replace defective burner.
(c) Clear blockage or replace.
(d) Secure connection or replace defective thermocouple.
(e) Replace thermocouple or pilot safety switch.
(f) Correct.
(g) Move tip of draft tube to proper location.
(a) Remove restriction or replace line.
(b) Check for possible gas supply problem with local gas company; replace defective burner.
(c) Check for possible gas supply problem with local gas company; test main gas valve and replace if defective.
(a) Replace fuse or reset circuit breaker. If problem continues, call an electrician.
(b) Reset thermostat or limit, or replace if defective.
(c) Replace relay or transformer.
(d) Replace motor or burner.
(e) Lubricate bearings; repair or replace damaged bearings.
(f) Check wiring diagram for burner and rewire correctly.
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Table 2-4 (continued)
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