Thermostats and Humidistats

This chapter, and the two that immediately follow it, describe the principal components of the automatic control systems used in heating, ventilating, and air conditioning. The index should also be checked because additional information about automatic controls has been included in other chapters.

Automatic Control Systems

An automatic control system consists primarily of the following two basic components:

Controller Controlled device

A controller is any device that can detect changes in temperature, humidity, or pressure and respond to these changes by activating a controlled device.

A controlled device may be a valve or a damper, or a motor that drives either of the two. It may also be a pump, fan, electric relay, or any other device used to regulate the flow of air, steam, water, gas, or oil.

Automatic control systems can be classified as either closed loop or open loop. A closed-loop system (see Figure 4-1) is the more com­mon type and involves the following stages in the control sequence:

1. The controller measures a change in a variable condition (e. g., temperature) and actuates the controlled device.

2. The controlled device compensates for the change in the vari­able condition by regulating the flow rate of the medium (e. g., water, air, steam) carried in the system.

3. The result of the action of the controlled device is measured, and this information is fed back to the controller completing (i. e., closing) the loop.

An example of an open-loop system is one using an outdoor thermostat. It is characterized by having no means of feedback. In other words, room temperature has no effect on the operation of the controller.

CONTROLLED VARIABLE (AIR TEMPERATURE)

SOURCE OF ENERGY

CONTROLLER (THERMOSTAT)

CONTROLLED DEVICE (VALVE)

CONTROL AGENT (STEAM)

AIR FLOW

L

Figure 4-1 Closed-loop automatic control system. (Courtesy ASHRAE 1960 Guide)

Temperature Control Circuits

The thermostat is the basic controller in the electrical control circuits used to operate a heating and/or cooling system. In systems using gas-fired heating equipment, there are three basic control circuits:

Safety shutoff circuit

Fan or circulator control circuit

Temperature control circuit

There are three basic types of temperature control circuits used in heating and cooling systems. These three basic circuits are as follows:

Low-voltage control circuit Line voltage control circuit

• Millivolt control circuit

Typical wiring diagrams for these three temperature control cir­cuits are shown in Figures 4-2, 4-3, and 4-4.

Thermostats

A thermostat is an automatic device designed to maintain tem­perature control. It accomplishes this function by reacting to temperature changes with adjustments of a controlled device such as a damper or valve motor or the automatic firing equipment (gas

FAN OR FAN CONTROL CIRCULATOR OR MOTOR CIRCULATOR RELAY Figure 4-2 Low-voltage temperature control circuit. (Courtesy Honeywell Tradeline Controls)

CIRCULATOR OR MOTOR CIRCULATOR RELAY Figure 4-3 Line voltage temperature control circuit. (Courtesy Honeywell Tradeline Controls)

HIGH LIMIT CONTROL

Rdt

PILOT BURNER

THERMOCOUPLE GENERATOR

LJ U

MILLIVOLT THERMOSTAT

■OArO

Ф-VO

CONTROL VALVE OPERATOR

PILOT STAT (IN-LINE TYPE FOR COMPLETE SHUTOFF)

Figure 4-4 Millivolt temperature control circuit.

(Courtesy Honeywell Tradeline Controls)

Burner, oil burner, or coal stoker) in space-heating furnaces and boilers. Because of its specific function, a thermostat is sometimes referred to as a temperature controller.

Thermostats can be classified on the basis of how they measure (or sense) temperature changes. The following devices are most commonly used to measure temperature changes:

Bimetallic-strip sensing element

• Pressure-actuated sensing element

• Electrical resistance element

A bimetallic strip containing two dissimilar metals is probably the most widely used of these three temperature-measuring devices. Its operating principle is based on the different expansion and contrac­tion rates of dissimilar metals. When two such metals are joined together in a bimetallic strip, the differences in expansion and con­traction rates will cause a bending movement as the temperature changes. This movement is utilized to open or close an electrical circuit between the thermostat and the controlled device in the heating and/or cooling system. The bimetallic-strip sensing element is used in either snap-action switch thermostats or mercury-switch thermostats.

A thermostat that operates on the positive snap-action switching principle contains movable switch contacts. One of the contacts is
connected to a movable switch armature; the other is fixed in posi­tion. An auxiliary armature attached to a bimetal coil responds to temperatures induced by the expansion or contraction of a moving magnet (see Figure 4-5). The magnet, attached to the auxiliary armature, controls the movement of the switch armature. When the switch armature moves toward the magnet, it causes the contacts to close (see Figure 4-6).

On some thermostats, the switch contacts are hermetically sealed in a glass enclosure to protect them from dust or moisture. The ther­mostats illustrated in Figures 4-5, 4-6, and 4-7 are of this design.

Figure 4-5 Glass-enclosed contact switch in the open position. (Courtesy Robertshaw Controls Co.)

A mercury-switch thermostat contains fixed contacts sealed in a mercury-filled tube. The tube is attached to the end of a spiral bimetal element. When the temperature changes, the bimetal ele­ment tilts the tube and causes the mercury to shift its position, caus­ing a definite opening and closing of the electrical circuit.

A two-wire thermostat using the mercury tube switch method is illustrated in Figure 4-8. A drop in temperature causes the mercury switch to complete (close) the circuit. The circuit is broken (opened) on a rise in temperature.

Figure 4-9 illustrates the application of the mercury tube switch method in a heating and cooling thermostat. A common terminal wire runs along the bottom of the mercury tube. Mercury can make

Figure 4-6 Glass-enclosed contact switch in the closed Position. (Courtesy Robertshaw Controls Co.)

Contact between either the heating or cooling terminals, but not both at the same time.

Sealing the contacts in a mercury-filled tube provides excellent protection against contamination; however, care must be taken to properly level the base when mounting the unit because the posi­tion of the tube determines the switching action.

On either the snap-action or mercury-switch thermostat, the tem­perature setting of the thermostat can be changed by rotating the temperature dial. This device, acting through the cam, causes the bimetal coil to rotate through its mounting post to carry the temper­ature setting (see Figure 4-10).

Another temperature-measuring device used in thermostats is the pressure-actuated sensing element. A liquid, gas, or vapor with a high coefficient of expansion is used to activate a bellows connected to a snap mechanism. A rise in temperature causes an expansion in the volume of the liquid, gas, or vapor. This expansion is transferred to the bellows, which activates the snap mechanism. See Remote Bulb Thermostats later in this chapter for additional details.

An electrical-resistance sensing element consists of a coil of wire with an electrical resistance that changes in direct proportion to temperature changes. This type of sensing element is commonly used in electronic controllers.

A thermocouple device consisting of two dissimilar electrical wires welded together at one end also serves as a temperature-sens­ing element in some thermostats. Temperature changes at the

TEMPERATURE SELECTION ON POINTER

*Жr »

]i

J0 g m

*»&/

LARGE EASY-TO-SEE DIAL-

WIRE ANTICIPATOR

BIMETAL

ROOM THERMOMETER

PERMANENT MAGNET GLASS ENCLOSED CONTACT SWITCH

I-

___________ ^_________

Figure 4-7 Thermostat with contacts in sealed glass enclosure.

(Courtesy Robertshaw Controls Co.)

Welded juncture of the two wires cause electrical changes in the control circuit, which operates a regulatory device.

Thermostat Components

A thermostat consists of two basic parts: the base and the cover. A subbase can also be added to a thermostat to provide fan and vari­ous switch functions (see Figure 4-11).

Figure 4-8 Mercury tube switch method in a two-wire

Thermostat. (Courtesy Coleman Co., Inc.)

Figure 4-9 Mercury tube switch method in a three-wire

Thermostat. (Courtesy Coleman Co., Inc.)

Figure 4-11 Thermostat cover, base, and subbase. (Courtesy ITT General Controls)

The cover protects the internal wiring of the thermostat base from dust, lint, and other possible contaminants. It also contains the temperature-setting lever, the system-switching levers, and the temperature indicator (see Figure 4-12).

The cover is secured to the base either with a positive friction snap or screws. If the former is the case, the cover can be removed by grasping the base with one hand and pulling it off with the other (see Figure 4-13). Screws may require the use of an Allen wrench (see Figure 4-14), which is usually supplied by the thermostat man­ufacturer and shipped with the unit.

The base contains the internal wiring of the thermostat. Figure 4­15 shows the back and front views of typical base wiring for the low-voltage thermostat illustrated in Figure 4-12. This is not the only possible way the base can be wired. How the base is wired will depend on the particular application. A few of the possible varia­tions are illustrated by the thermostat base wiring diagrams in Figure 4-16. The many variations in wiring will depend on which combination of the following features is required by the installation:

1. Type of switch and switching action (snap-action or mercury bulb; spst or spdt contact).

2. Number of field wires (two, three, four, or five).

3. Type of anticipator (fixed heating, adjustable heating, fixed cooling).

4.

TEMPERATURE SETTING LEVER

TEMPERATURE INDICATOR

Figure 4-12 Low-voltage thermostat for electric heating and/or Cooling system. (Courtesy Singer Controls Co., of America)

SYSTEM SWITCHING LEVERS

Use of fan and system switches.

5. Type of operating voltage (low voltage, line voltage, or milli­volt circuit).

Adding a subbase to a thermostat provides fan and other switch­ing functions. The wiring diagrams of the ITT General Controls

Figure 4-13 Removing a cover secured to the base with a positive friction snap.

(Courtesy Com-Stat Inc.)

Figure 4-14 Loosening cover screws with an Allen wrench.

(Courtesy Honeywell Tradeline Controls)

Thermostats and subbases shown in Figure 4-17 illustrate the vari­ety of different switching combinations available.

Thermostat guards can be purchased to protect the thermostat from damage. This is particularly important in warehouses, stores, and other areas where there is a greater possibility of the thermo­stat being damaged. Some examples of how these guards are used are shown in Figure 4-18. Adapter plates (wall plates) are also available from thermostat manufacturers to cover electrical utility or junction boxes (see Figure 4-19).

Thermostat Terminal Identification

The National Electrical Manufacturers Association is at present attempting to standardize thermostat markings in order to aid the installer in wiring and servicing. Thermostat terminals have been given standard identification letters that specify the function of the terminal. These identification letters are also matched, in most cases, with the color-coding of the wire. A partial list of equivalent terminal markings is given in Table 4-1.

Thermostat installation literature will generally contain at least one internal view and/or wiring diagram in which the various ter­minals are identified by a specific letter.

Thermostat Anticipators

A thermostat anticipator is a device used to reduce the operating differential of the heating or cooling system. It is designed to enable the thermostat to shut off the furnace or boiler slightly in advance of the actual set temperature. As a result, the thermostat shuts off the heating equipment sooner than it would if it were affected by only the room temperature, thereby compensating for heat transfer lag. A thermostat may be equipped with a heat anticipator, a cold anticipator, or both.

A heat anticipator is a small resistor (resistive heater) connected in series with the switch inside the thermostat. Heat generated by the resistor when the switch is in the on position heats the thermo­stat bimetal actuator and causes the internal temperature of the thermostat to rise faster than the surrounding room temperature.

ADJUSTABLE
HEATING	COOLING	MERCURY	SETTING
ANTICIPATOR	ANTICIPATOR	SWITCH	LEVER

COOL-OFF-HEAT FAN SYSTEM SWITCH SWITCH

MOUNTING BACK VIEW HOLE Figure 4-15 Front and back view of Singer Thermostat model 360. (Courtesy Singer Controls Co., of America)

FRONT VIEW

FAN CONTROL (GREEN LEAD)

HEATING AND COOLING

HEAT COOL OFF

HEATING CONTROL (WHITE LEAD)

MAN ‘;AUTO FAN

ST

COOL ANTIC

COOLING CONTROL (YELLOW LEAD)
TRANSFORMER (RED LEAD)
TRANSFORMER (RED LEAD)
»R nY1
HEATING ONLY HEATING CONTROL (WHITE LEAD) COOLING CONTROL (YELLOW LEAD) TRANSFORMER	COOLING ONLY
COMPRESSOR RELAY (YELLOW LEAD) 24 V TRANSFORMER (RED LEAD)
MAN/ ‘.AUTO
CONTROL WN HEAT COOI. reversing (GREEN LEAD) OFF VALVE HEAT PUMP WITH REVERSING (WHITE LEAD) VALVE ENERGIZED ON HEAT
Figure 4-16 Thermostat base wiring. (Courtesy Singer Controls Co., of America)

Thermostats are available with fixed anticipators, plug anticipa­tors, or variable anticipators.

A fixed heat anticipator must be sized in accordance with the amperage or current draw of the operating valve in order to secure the correct degree of heat anticipation. These anticipators are generally available in the range of 0.1 to 1.5 amperes. When using a thermostat equipped with a fixed heat anticipator, check the nameplate on the valve or relay to make certain the ampere rating does not exceed the maximum amp (current) draw. For example, if the thermostat is equipped with a 0.40- to 0.60-ampere fixed heat anticipator, the valve or relay should not exceed 0.60 ampere.

SYSTEM: HEATING AND COOLING SWITCHING: SYSTEM-HEAT/OFF/COOL FAN-ON/AUTOMATIC Figure 4-17 Thermostat switching combinations. (Courtesy ITT General Controls)

The thermostat shown in Figure 4-20 is an example of one equipped with a fixed heat anticipator. The anticipator is contained in the thermostat element along with the magnetic switch and a room temperature thermometer.

Figure 4-21 illustrates a thermostat equipped with a plug-type heat anticipator. This design offers some degree of latitude in selecting a

ELECTRICAL DECORATIVE OUTLET BOX OVERLAY

Figure 4-18 Thermostat guards. (Courtesy ITT General Controls)

MOUNTING ADAPTER PLATE ON ELECTRICAL OUTLET BOX

Letter	Wire Color	Terminal Function
R	Red	Power supply; transformer
W	White	Heating control; heating relay or valve coil
Y	Yellow	Cooling control; cooling contactor coil
G	Green	Fan relay coil
O	Orange	Cooling damper
B	Brown	Heating damper
X	—	Malfunction light
P	—	Heat pump contactor coil
Z	—	Low-voltage fan switch

Note: The letters Rh (heating) and Rc (cooling) are used on thermostats with isolated circuits.

Proper anticipator. The current (amps) drawn by the primary control or valves is first determined, and an anticipator having the proper value is selected from among those listed in Table 4-2. The heating cycle can be lengthened by selecting an anticipator one step above the proper value. Shorter cycles can be obtained by selecting an anticipa­tor one step below the proper value.

Thermostats with adjustable (variable resistance) heat anticipa­tors can be adjusted over a range of approximately 0.1 to 1.5 amperes (see Figure 4-22). Before making any adjustments, you should first read the equipment manufacturer’s instructions for selecting proper anticipator values.

Heat anticipator adjustments are made on a thermostat with an indicator or adjustment lever that moves along a scale (see Figure 4-23). Adjustments are made in accordance with the values marked along this scale.

As with the plug-type heat anticipator (see Figure 4-21), the cur­rent (amps) drawn by the primary control or valve must be deter­mined first. In a gas-fired heating system, the heat anticipator should be set to correspond to the secondary (thermostat) current of the valve or relay. In an oil-fired heating system, the heat antici­pation indicator should be set 0.15 ampere higher than the rated secondary current of the relay.

The anticipator adjustment lever should be moved only Vi to V/2 scale division at a time. Never move or set the lever more than 1V/2 scale divisions under the valve or relay current ratings. Longer on periods can be obtained by setting the adjustment lever at a slightly

SWITCH ACTION ON SUPPLY SUBBASE

THERMOSTAT

BIMETAL MAGNET

THERMOMETER ADJUSTMENT

MOUNTING HOLE

WIRING TERMINALS

Connect wires to subbase terminals use # 19 (or larger) color-coder solid conductor copper wire. Do not strip lead wire insulation below bottom of terminal. Push access wire back through slot in subbase. Wire in accordance with applicable codes.

Figure 4-21 Thermostat with plug heat anticipator. (Courtesy ITT General Controls)

SNAP-ON COVER

Higher amp value. For shorter on periods, set the lever at a slightly lower amp value.

The cold anticipator for the thermostat shown in Figure 4-24 is not adjustable and should not be changed. The same is true of the other thermostats. Only fixed cold anticipation is used. Furthermore,

	Table 4-2	Anticipator Values
Amperes	Color	Amperes	Color
0.830.72	Brown-red	0.330.29	Orange-yellow
0.720.68	Brown-blue	0.290.25	Orange-green
0.680.55	Orange	0.250.22	Red
0.550.48	Blue	0.220.19	Green-blue
0.480.41	Blue-orange	0.110.10	Orange-red
0.410.36	Blue-yellow	0.100.09	Red-yellow
0.360.33	Green	0.090.08	Green-yellow

Figure 4-23 Heat anticipator adjustment lever and scale. (Courtesy Honeywell Tradeline Controls)

SNAP-ON COVER

Connect wires to subbase terminals use # 19 (or larger) color-coder solid conductor copper wire. Do not strip lead wire insulation below bottom of terminal. Push access wire back through slot in subbase. Wire in accordance with applicable codes.

TIGHTEN ALL CIRCUIT SCREWS

MOUNT SUBBASE LEVEL

Figure 4-22 Thermostat with adjustable heat anticipator. (Courtesy ITT General Controls)

CALIBRATION SCREWS

THERMOSTAT ELEMENT

SUBBASE

BIMETAL MAGNET

MOUNTING HOLE

THERMOMETER ADJUSTMENT "’i

WIRING TERMINALS COLD ANTICIPATOR

HEAT ANTICIPATOR

SWITCH ACTION ON SUPPLY Figure 4-24 Thermostat with fixed cool and adjustable heat Anticipator. (Courtesy ITT General Controls)

On cooling thermostats, the cold anticipation is in parallel with the cooling contacts so that anticipation heating occurs while the cooling unit is off.

Types of Thermostats

Many different thermostats are manufactured for use in heating and cooling systems. The design differences depend largely on the type of application.

The most common thermostat is the wall-mounted room thermo­stat used to control a heating and/or cooling system. The measuring element is contained in the thermostat unit itself. This distinguishes it from the remote-bulb-type thermostat used to measure tempera­tures in spaces separate from the location of the thermostat.

An insertion-type thermostat (or duct thermostat) is used to measure temperatures inside an air duct. The temperature-measur­ing element is contained in an insertion device that extends into the duct. The immersion-type thermostat is similar in design but is used to measure the temperature of fluids inside a pipe or tank. These thermostats are commonly used on water heaters.

A heating-cooling thermostat (also referred to as a summer-winter thermostat) is designed to be switched to either a heating or cooling application. The day-night thermostat (or electric clock thermostat) operates on a similar working principle except that it is designed to automatically switch from day to night operation and back again.

A multistage thermostat is designed to operate two or more cir­cuits in sequence. These thermostats are used for line voltage or low-voltage temperature control of heating and cooling equipment. They are commonly used in heating and/or cooling systems where zone control is necessary.

A thermostat and humidistat can also be combined in the same control unit. These combined units sometimes also include the elec­tronic air-cleaner control.

Room Thermostats

The room thermostat (see Figure 4-25) is regarded as the nerve cen­ter of the heating and cooling system because it controls the opera­tion of the furnace, boiler, or air conditioner. Ideally, it should be mounted in an area of the living or working spaces where it is not subjected to temperature or moisture extremes (see the following section, Location of Room Thermostats).

Low-voltage room thermostats are recommended over the line voltage types for residential heating and/or cooling systems. The low-voltage thermostats respond more quickly to temperature

Figure 4-25 Room heating and cooling thermostat. (Courtesy Com-Stat Inc.)

Changes and will maintain the temperature and humidity more closely than the line voltage types. A low-voltage thermostat requires the use of a transformer to reduce the line voltage for the control cir­cuit, but the cost of the transformer is more than offset by the lower installation cost of this thermostat.

Location of Room Thermostats

The location of the room thermostat is very important to the efficient operation of the heating and/or cooling system. If the room thermo­stat is improperly located, it will often call for heat or cool air when

Neither is necessary. It is therefore important to locate the thermostat where it will measure the actual temperature conditions of the space.

The following recommendations are offered as a guide for the proper location of a room thermostat:

1. Never locate the thermostat on the interior surface of an out­side wall. Outside walls are subject to temperature extremes caused by weather changes. Always locate the thermostat on a suitable inside partition.

2. Never locate the thermostat in the path of warm or cold air drafts. The thermostat should not be placed opposite warm air outlets or near a window or an outside door.

3. Never locate the thermostat where it will be subjected to the direct rays of the sun or other forms of heat radiation. Fireplaces, table lamps, and floor lamps are common sources of this type of heat.

When you select a location for the room thermostat on an inside wall, make certain that you are not placing it over a warm air duct, steam pipe, or hot-water pipe. The warmth from these ducts or pipes will interfere with the operation of the thermostat.

For the most satisfactory operation, locate the room thermostat about 5 feet above the floor on an inside wall where there is good natural air circulation and where the thermostat will be exposed to average room temperatures.

Installing a Room Thermostat

Before attempting to install the thermostat, read the manufacturer’s installation instructions. These instructions should be followed as closely as possible to ensure efficient thermostat operation.

Always handle the thermostat carefully. Rough handling may decrease its accuracy. Inspect the thermostat carefully when you unpack it. Report any damage to the shipper if the damage was caused after it left the factory or distribution center. Their insurance should pay for any replacement. Any other damage or malfunction should be reported to the thermostat manufacturer or his field rep­resentative. The thermostat is the basic controller in any heating and/or cooling system. It must operate accurately and efficiently.

Make sure the thermostat selected for the heating and cooling system is the appropriate one for the job. Heat pumps require two — stage thermostats. Furnaces and boilers used with air-conditioning systems commonly require thermostats with extra terminals. Thermostats with changeover terminals are required for many

Zoned systems. Check the manufacturer’s specification sheet before making a final selection.

The installation instructions differ from one thermostat manu­facturer to another, usually on the basis of design and construction; however, the following points are common to all:

1. Mercury-switch thermostats are usually shipped with some form of protection around the mercury tube to prevent break­age. This must be removed before operating the thermostat.

2. Disconnect the power supply before connecting the wiring to the thermostat in order to prevent electrical shock and/or equip­ment damage. Low-voltage thermostats are used in heating and cooling systems. These low voltages are not fatal, but they can deliver a nasty shock and may damage the system controls.

3. All wiring must be done in accordance with local electrical codes and ordinances. Be sure to follow the thermostat manu­facturer’s wiring diagram when making the connections.

4. Use a plumb line or spirit level to accurately level the subbase or wall plate when mounting it on the wall (see Figure 4-26). Thermostat control deviations are often caused by inaccurate leveling.

5. Follow the manufacturer’s instructions for making any inter­nal wiring connections (e. g., connecting lead wires to terminal screws), connecting a heating thermostat to a cooling base, and so on.

IS PLUGGED Figure 4-26 Using a plumb line and level. (Coun. esy Honeywell Tradeline Controls)

6. Check out the installation to make sure the thermostat is operating correctly.

Thermostat Calibration

Thermostats are accurately calibrated at the factory under con­trolled conditions and should not require recalibration. Sometimes a thermostat will appear to require recalibration when the problem is actually a quite different one. For example, a thermostat that is not level or one subjected to a high degree of radiant heat from the sun, radiators, convectors, or other heat sources often fails to func­tion properly. Before jumping to the conclusion that it needs to be recalibrated, check out the possibility that some external cause may be the source of the difficulty. If you are certain the problem is in the thermostat, then you should call a trained serviceman to recali­brate it. Do not attempt to recalibrate it yourself unless you have the necessary experience.

A new thermostat is generally shipped with complete installation literature. This literature also usually contains instructions for recalibrating the thermostat. By way of example, the instructions for recalibrating a Honeywell T87 Room Thermostat include the following steps:

1. Remove the thermostat cover ring (see Figure 4-27). The lock­ing cover will require the use of an Allen wrench to loosen the screws securing the cover.

2. Set the thermostat below the room temperature, and allow it to remain in an off position for approximately 10 minutes.

3. Slowly raise the setting until the switch just makes contact. If the thermostat pointer and setting indicator do not read the same the instant the switch makes contact, then the thermo­stat requires recalibration.

4. Turn the setting dial a few degrees above room temperature.

5. Slip the calibration wrench onto the calibration (hex) nut under the bimetal coil (see Figure 4-28).

6. If the thermostat has a stationary pointer, hold the dial firmly and turn the calibration nut counterclockwise until the mer­cury breaks contact. If the thermostat has a movable pointer, turn the calibration nut clockwise.

7. Turn the thermostat dial to a low setting, and wait approxi­mately 5 minutes.

8. Slowly turn the dial until the pointers read the same.

THERMOSTAT CABLE OPENING

T187 C OR F

THERMOSTAT COVER RING

104456B WALLPLATE

MOUNTING HOLES (4)

CAPTIVE SCREWS (3)

Figure 4-27 Removing the cover ring. {Courtesy HoneyvellTrjdeline Controls)

ADJUSTABLE SCALE STOPS FOR LOCKING Figure 4-28 Calibration nut under bimetal coil. (Courtesy Honeywell Tradeline Controls)

9. Hold the dial firmly, and turn the calibration in the opposite direction from the one in step 6 until the mercury switch slips to the heating contact end of the tube.

10. Recheck the calibration, select the desired temperature, and replace the cover.

Programmable Thermostats

Sometimes a room thermostat is combined with a time switch. A time switch is an electrical switching device operated by a clock to provide one or more on periods for the space heating or domestic (hot water) heating system. These are called programmable room thermostats. There are three basic types of programmable room thermostats: (a) the standard programmer, (b) the full programmer, and (c) the mini-programmer.

The standard programmer type controls both the space heating and domestic hot-water heating with the same time settings. A full programmer thermostat, on the other hand, provides independent time settings for space heating and domestic hot-water heating. This allows the two to operate independently of one another. Finally, the mini-programmer permits the domestic hot-water heating to be on alone (without space heating) or to be on together with the space heating. It does not allow the space heating to be on alone.

Some room thermostats have a night setback feature, which reduces energy use by lowering temperatures at night when the occupants are sleeping. These are called day-night (or twin-type) thermostats. They comprise an assembly of two thermostats mounted on a single base operating in conjunction with a timer or clock. The electric clock can be set to throw the temperature con­trols from one thermostat for the daytime onto the other for the night (or vice versa) at a predetermined time setting on the clock. This conveniently permits a low temperature at night and normal temperature during the day. Figure 4-29 shows a wiring diagram of a typical twin-type thermostat and illustrates the connections between the clock and primary control.

Day-night thermostats designed to provide automatic temperature — switching control for only a heating system can be modified to pro­vide system and fan switching. The Honeywell T882 Chronotherm clock thermostat provides these functions for a heating and cooling installation when used with the Honeywell Q611A thermostat sub­base (see Figures 4-30 and 4-31).

Insertion Thermostats

An insertion thermostat is used primarily to measure the air tem­perature in a duct. The thermostat is mounted on the outside of the

TWIN THERMOSTAT I———————- 1 Figure 4-29 Twin-type thermostat wiring diagram.

Air duct with the sensing element extending inside. Because of this application, it is sometimes referred to as a duct thermostat. Remote-bulb-type thermostats are also used to measure the air tem­perature in ducts.

Immersion Thermostats

An immersion thermostat is commonly used for water temperature control in an automatic gas-fired water heater (see Figure 4-32). This is usually a direct, snap-action, bimetallic thermostat in which contraction of the thermal element immersed in the stored hot water causes the main gas valve to open. This occurs when there is a drop in the temperature of the water in the storage tank. Expansion of this element serves to close the main gas valve when the tank water attains the selected predetermined temperature.

——- — TIME INDICATOR

CLOCK SET WHEEL

(ON BOTTOM) TIMER DIAL

Figure 4-30 Honeywell T882 Chronotherm clock thermostat.

(Courtesy Honeywell Tradeline Controls)

These thermostats normally operate at a temperature differential of approximately 12F. In other words, if the thermostat is set to shut off the gas to the main burner when the tank water tempera­ture reaches 140F, it will react to open the valve when the temper­ature drops to 128F.

Quite often, an immersion water heater thermostat will be included in a combination control. These combination controls used in water heaters are described in Chapter 4 of Volume 3 (W ater Heaters and Other Appliances)’.

Cylinder Thermostats

A cylinder thermostat is used to control the temperature of the domestic (potable) hot-water tank and to turn on and off the

O

Provide overload protection and disconnection means as required.

® Filter malfunction light (optional).

©

Clock terminal connect clock to AT75 transformer only — no other power source.

0 Use heating transformer if adequate. Otherwise replace.

Figure 4-31 Wiring diagram of the T882 thermostat and Q61 IA

Subbase. (Courtesy Honeywell Tradeline Controls)

O		DIAL
N
J 0		PILOT
	STARTER
	BUTTON
	TEMPERATURE
-A	SETTING DIAL

STARTER

TO PILOT BURNER

THERMOSTAT

MAIN GAS OUTLET TO BURNER

Figure 4-32 Immersion thermostat.

Water heater. A single target temperature may be set on this thermostat.

Boiler Thermostats

A boiler thermostat is a safety control device installed inside a hydronic boiler. Its function is to limit the temperature of the hot water produced by the boiler. When the water temperature reaches a preset maximum limit, the thermostat switches off the burner and boiler. The target temperature can be permanently fixed or adjusted by the user.

Remote-Bulb Thermostats

A remote-bulb thermostat is distinguished from other thermostats by having a sensing element (usually enclosed in a bulb-type device) located at some distance from the thermostat controller body.

In residential heating and/or cooling applications, remote-bulb thermostats are used for space temperature control of room and vented-recess heaters, room air-conditioning units, or radiator valves. Outdoor thermostats also operate on the remote-bulb principle.

Sometimes these thermostats are combined with other controls to provide a number of different control functions in one package. Two examples of this thermostatic combination control are the Robertshaw Unitrol 110SR and 7000SR controls.

GAS INLET

"BURNER Figure 4-33 Unitrol I I0SR combination control on a gas-fired Room heater. (Courtesy Robertshaw Controls Co.)

SENSOR BULB

The sensing bulb is located in the return air stream at the bottom of the heater. The temperature of the return air is sensed and the valve is actuated to open and close by the hydraulic system in the control. The gas cock and automatic pilot mechanism provide safe lighting of the heater. If the pilot light should go out, the main gas and pilot gas supplies are shut off by the automatic pilot valve. Main burner gas pressure regulation is provided by the pressure regulator incorporated in the Unitrol 110SR control. Figure 4-35 illustrates the principal components of a Unitrol 7000SR control. This is a diaphragm valve that operates through the center of a thermostatic bleed valve in an internal bleed line. The Unitrol 7000SR combines into a single package a diaphragm valve, thermostat valve, temperature (thermostat) dial, gas cock, automatic pilot valve, and a pressure regulator.

The Unitrol 110SR is used as a combination control for small gas-fired room heaters (see Figure 4-33). The control contains a thermostatic valve, a sensing bulb, temperature (thermostat) dial, gas cock, automatic pilot valve, and a pressure regulator (see Figure 4-34).

GAS COCK FLEXIBLE HYDRAULIC CONTROL OUTLET Figure 4-34 Principal components of a Unitrol 110SR combination Gas valve. (Courtesy Robertshaw Controls Co.)

Application of a Unitrol 7000SR on a gas-fired vented-recess heater is illustrated in Figure 4-36. The sensing bulb is placed in the return air opening at the bottom of the furnace.

Both the Unitrol 110SR and Unitrol 7000SR thermostatic space heater controls use a closed hydraulic sensing and actuating device consisting of a bulb, capillary tube, and a bellows or diastat.

The cross section of a typical hydraulic sensor and actuator is illustrated in Figure 4-37. The bulb, capillary tube, and actuator are filled with a liquid that has a high coefficient of expansion. When the bulb senses a rise in temperature, it causes the volume of the liquid to expand. This expansion is transferred through the capillary tube to expand the hydraulic bellows. The bellows is spring-loaded to oper­ate a snap mechanism to a valve open condition (see Figure 4-38). As the temperature rises, the expansion of the liquid opposes the spring-loading to secure a valve closed (i. e., off) condition when the required temperature is reached.

SENSING BULB

TEMPERATURE DIAL

GAS COCK DIAL

PILOT CONNECTION

PILOT ADJUSTMENT

INLET

; w*

THERMOCOUPLE CONNECTION OUTLET

Figure 4-35 Principal components of a Unitrol 7000SR combination Gas valve. (Courtesy Robertshaw Controls Co.)

Proportional Thermostats

Some thermostats are designed to provide proportional control for valve and damper motors in heating or cooling systems. This type of controller is generally referred to as a proportional thermostat.

The Honeywell T92 proportional thermostat, shown in Figure 4-39, contains a bellows that adjusts one or two potentiometers in proportion to temperature changes. These potentiometer adjust­ments regulate the power supplied to the controlled device. This particular thermostat is designed to provide 24- to 30-volt propor­tional control for valve and damper motors in the system. Those models equipped with two potentiometers are capable of unison or sequence control.

Outdoor Thermostats

An outdoor thermostat is designed to maintain the proper balance between the temperature of the heating medium inside the structure and the outdoor temperature.

Vented-recess heater.

The outdoor thermostat, illustrated in Figure 4-40, can be used as an operating controller for a hot-water or warm-air heating sys­tem. This is a remote-bulb thermostat suitable for line voltage, low — voltage, or millivolt switching. It is designed to automatically raise the heating medium control point as the outdoor temperature falls.

Outdoor thermostats are frequently used as controllers in hot — water heating systems. A simple on-off control is possible, but this usually involves stopping the circulation of the water in the system during those periods when there is no call for heat. Anticipating

Figure 4-37 Cross section of a typical hydraulic sensor and activator. (Courtesy Robertshaw Controls Co.)

Control systems are preferred to the simple on-off types because there is no noticeable lag time between cold temperatures and a call for heat. An anticipating control system provides continuous circu­lation of the water temperature in direct ratio to changes in out­door temperature.

Troubleshooting Thermostats

Thermostat problems are sometimes the result of poor wiring connec­tions. Check the wiring first. You should also make certain that the fan and system switches and the temperature setpoint are properly set.

The troubleshooting chart in Table 4-3 includes many of the more common symptoms and possible causes of operating prob­lems associated with thermostats.

Humidistats

A humidistat is a switching device used to control the level of humidity in a confined space. Standard applications include the basic on-off humidity control of a heating/cooling system or the high-limit safety interlock of a humidifier.

TEMPERATURE ADJUSTMENT DIAL

LOADING SPRING Figure 4-38 Schematic of a hydraulic sensor and actuating system used for snap-action control. (Coun. esy Robertshaw Controls Co.)

The earliest humidistats used a hygroscopic material in the con­struction of the sensing or control element (see Figure 4-41). Hygroscopic materials, such as animal hair or certain types of plas­tics, are those affected by the moisture content of the air. When the material absorbs moisture from the air, it expands. When the sur­rounding air becomes drier, the hygroscopic material gives up mois­ture to the air and contracts. This expansion and contraction of the control element in the humidistat opens or closes the electrical cir­cuit controlling the humidifier. These early humidistats, many of which are still in use today, are commonly called electric humidis­tats, mechanical humidistats, electromechanical humidistats, humidity controllers, or hygrostats—a lot of different names, but all operating on the same principle of using a hygroscopic material to mechanically switch an electrical current on and off.

Electronic humidistats are being used in most new heating/ cooling systems today instead of the older electromechanical types.

They are also being used to replace the older units in existing sys­tems. An electronic humidistat uses electronic switching circuitry to create the switching action. In some electronic humidistats, a thin film capacitance is used to sense the moisture content of the sur­rounding air. Others use various polymer-resistance analog humidity — sensing technologies.

Pneumatic humidistats are also designed to sense changes in ambient relative humidity (see Figures 4-42 and 4-43). As shown in Figure 4-43, the sensing device is a nylon element near the set­point adjustment wheel at the bottom of the humidistat. When the nylon sensing element responds to changes in the ambient relative

Figure 4-40 Outdoor reset control for hot-water or warm-air Heating systems. (Courtesy Honeywell Tradeline Controls)

Symptom and Possible Cause	Possible Remedy
Room temperature overshoots thermostat setting (too cold).
(a) Thermostat not mounted level (mercury-switch types). (b) Thermostat not properly calibrated. (c) Thermostat exposed to heat source. (d) Thermostat setpoint too low. (e) System sized improperly.	(a) Remount thermostat in level position. (b) Recalibrate or replace. (c) Move thermostat to better location. (d) Reset. (e) Determine correct sizing and make system adjustments.
Room thermostat does not reach setting (too warm).
(a) Thermostat subject to draft. (b) Thermostat not mounted level (mercury-switch types). (c) Thermostat not properly calibrated. (d) Thermostat setpoint too high. (e) System sized improperly. (f) Thermostat damaged.	(a) Wiring hole may not be plugged. Move thermostat to better position. (b) Remount thermostat in level position. (c) Recalibrate or replace. (d) Reset. (e) Determine correct sizing and make system adjustments. (f) Replace thermostat.
System cycles too often.
(a) Thermostat exposed to heat source. (b) Thermostat differential too small. (c) Thermostat heating element improperly set. (d) Thermostat subject to vibrations. (e) Thermostat exposed to cold draft.	(a) Relocate thermostat. (b) Reset or replace thermostat. (c) Reset or replace thermostat. (d) Remount thermostat in location free from vibrations. (e) Remount in better location. (continued)

System does not cycle often enough (burner operates too long). (a) Thermostat not exposed to (a) Remount in better location. Circulating air. (b) Contacts dirty. (b) Clean or replace. (c) System sized improperly. (c) Determine correct sizing and make system adjustments. (d) Thermostat differential (d) Reset or replace thermostat. Too great. (e) Thermostat heating element (e) Reset or replace thermostat. Improperly set. (f) Thermostat set too high. (f) Reset.

Room temperature swings excessively.

(a) Thermostat not exposed (a) Remount in better position.

To circulating air.

(b) Thermostat exposed to (b) Remount in better position.

Heat source.

(c) System sized improperly. (c) Determine correct sizing and

Make system adjustments.

Thermostat jumpered (system works).

(a) Thermostat contacts dirty. (a) Clean or replace.

(b) Thermostat setpoint too high. (b) Reset or replace thermostat.

(c) Thermostat damaged. (c) Replace thermostat.

(d) Break in thermostat circuit. (d) Locate and correct.

Burner fails to stop.

(a) Thermostat in cold location. (a) Relocate in better location.

(b) Thermostat set too high. (b) Reset.

(c) Defective thermostat. (c) Replace thermostat.

(d) Thermostat out of adjustment. (d) Recalibrate or replace

Thermostat.

(e)

(e) Correct.

Thermostat contacts stuck.

Figure 4-41 Components of a typical humidistat. (Courtesy Honeywell Tradeline Controls)

Humidity, it produces a proportional change in the branch line pressure and changes the control actuator position by the same proportion.

Location of Room Humidistats

The proper location of the room humidistat is important to its effi­cient operation. The following recommendations are offered as a guide to locating the humidistat:

(Courtesy Honeywell, Inc.)

• Never locate the humidistat in an area where there are heavy concentrations of moisture. A kitchen, bathroom, or laundry room will frequently have high levels of moisture in the air.

• Never install the humidistat on the inside surface of an out­side wall. Exterior walls are subject to temperature extremes caused by weather changes.

Never locate the humidistat where the air circulation is restricted.

Never locate the humidistat where it can be affected by a nearby heat source. Heat sources such as sunlight, lamps, tele­vision sets, fireplaces, warm-air outlets, or heat-producing appliances will interfere with the efficient operation of the humidistat.

GAGE TAP

THROTTLING RANGE SCALE (MINIMUM UP)

Figure 4-43 Honeywell pneumatic humidistat with cover removed. (Courtesy Honeywell, Inc.)

Troubleshooting Humidistats

Read the humidistat manufacturer’s installation and operating manual for troubleshooting problems, their possible causes, and their suggested remedies. In most cases, they will be specific to the type of humidistat (electromechanical, electronic, or pneumatic) and the model. Some very basic troubleshooting problems that apply uniformly to all humidistats are listed in Table 4-4.

Problem and Possible Cause	Suggested Remedy
Slow response.
(a) Humidifier installed in a dead	(a) Relocate to appropriate
Air space.	Location.
(b) Inadequate airflow caused by an incorrect cover.	(b) Install a correct cover.
Inaccurate reading.
(a) Backplate too tight.	(a) Retighten.
(b) Humidistat installed on inside surface of an outside wall.	(b) Move to appropriate location.
(c) Humidistat installed near	(c) Move nonpermanent heat
Heat source.	Source (lamp, TV set, etc.) away from humidistat or move humidistat away from permanent heat source (constant sunlight, fireplace, stove, etc.).
Constant readings.
(a) Defective humidistat.	(a) Replace humidistat.
(b) Incorrectly calibrated humidistat.	(b) Recalibrate humidistat.
(c) Humidistat is undersized.	(c) Replace the humidistat with a correctly sized one.

Posted in Audel HVAC Fundamentals Volume 2 Heating System Components, Gas and Oil Burners, and Automatic Controls