7.3.3
(Heat Only)
1.
Identify the purpose of the wall thermostat
2.
Identify the operating characteristics of electro-mechanical wall thermostats
3.
Select a proper thermostat location
4.
Identify characteristics of thermostat circuits
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5.
Identify variations in electro-mechanical wall thermostats
6.
Identify possible malfunctions of electro-mechanical wall thermostats
7.
Install a millivolt thermostat
8.
Identify elements of an installation checklist for thermostats
9.
Identify operating characteristics of electronic thermostats
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The purpose of a wall thermostat is to monitor and automatically maintain a desired set temperature within a specific area.
Wall thermostats control the on/off action of the heat source and the amount of heat in a given space by allowing or preventing an electrical current to flow through the contacts, initiating the call for heat that results in gas appliance burner operation. The thermostat may be rated for millivolts, 24 VAC or 120 VAC operation.
Two main types of wall thermostats are used with gas-fired equipment:
• Electro-mechanical thermostats
• Electronic thermostats (programmable or non-programmable)
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The major components of the basic electro-mechanical wall thermostat are:
• Bimetal temperature sensing device
•
Single pole/single throw
(SP/ST) electrical switch
• Temperature adjusting knob or lever
• Heat anticipator
• Thermostat cover
• Mounting base
Figure 1. Basic Components of a
Mercury Contact Thermostat Control
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Figure 2. Magnetic Contact
Wall Thermostat
Figure 3. Mechanical Thermostat with U-Shaped Bimetal
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The temperature-sensing device in most residential wall thermostats is bimetal. The bimetal configuration may vary from one design to another; however, the function of the bimetal is the same.
The bimetal activates the single pole/single throw (SP/ST) switch in the wall thermostat. Since the bimetal is a good conductor of electrical current, it is sometimes used as part of the electrical circuit. (Figure 2.)
The bimetal in Figure 1 is used for activating a mercury capsule type
SP/ST switch.
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Wall thermostats are designed so they may be manually adjusted to maintain various temperatures. The range of many wall thermostats is from 40 ° F to 90 ° F.
Figure 5.
Temperature Adjustment on a Mercury Thermostat
Figure 4.
Temperature Adjustment on a Wall Thermostat
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The heat anticipator is a small length of high resistance wire which is connected "in series" with the thermostat circuit. The function of the heat anticipator is to generate a small amount of heat near the area of the thermostat bimetal.
The small additional heat generated by the heat anticipator causes the thermostat (bimetal) to open its contacts several degrees before the actual temperature of the thermostat is reached. The thermostat breaks the circuit to the gas valve blocking gas from flowing to the main burner.
Heat distributed throughout the dwelling, after the thermostat has cycled off, will add the additional heat required to meet the selected temperature setting of the thermostat .
The heat anticipator must be adjusted so it generates the proper amount of heat for the particular system. The adjustment of the heat anticipator must be calculated to the current draw of the components in the thermostat circuit. For example, if the gas valve (solenoid) is rated at .2 amps and the time delay relay is rated at .3 amps, the total current draw in the thermostat circuit is .5 amps.
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When an ammeter is used to determine the current draw the thermostat contacts must be closed. The ammeter is attached "in series" to one terminal of the thermostat and to the wire normally attached to that terminal. NOTE: While determining the current draw the heat anticipator must be positioned on the lowest setting and the heated air blower fan must be running.
Figure 6.
Heat Anticipator Adjustment
(Bottom-Right)
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The heat anticipator illustrated in
Figure 7 is constructed of one short strand of high resistance wire.
The primary objective of the wall thermostat is to provide as small a change as possible from the set point temperature. Variance from set point temperature is called operating differential. The heat anticipator can correct a wide variance in temperature operating differential.
Figure 7.
Heat Anticipator
(Top of Thermostat)
Systems which operate properly in well-insulated dwellings should have an operating differential of about 1 ° F to 2 ° F.
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The base should be positioned on the wall according to the vertical and horizontal lines embossed on the base. The bimetal is calibrated to warp a certain amount at a certain temperature, and if the thermostat is mounted improperly, it will not maintain the correct temperature setting.
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For proper temperature control the wall thermostat should be located on an inside wall of the dwelling, approximately five feet from the floor. It is extremely important to avoid mounting the thermostat in an area where air does not naturally circulate (dead air), or an area where air circulates too readily (from blower duct).
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The connection diagram of the basic wall thermostat circuit illustrated in Figure 8 represents a 24 VAC gas heating circuit. The basic circuit includes a 24 VAC wall thermostat, a 24 VAC gas valve, and a 24 VAC transformer.
Figure 8. Basic 120/24 Volt AC
Wall Thermostat Circuit
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The 120 VAC supply enters the primary side of the transformer and is reduced to a 24 VAC supply on the secondary.
Line one (L1) of the 24 VAC supply is attached to the wall thermostat.
Line two (L2) of the low voltage supply is attached to one terminal of the gas valve.
An additional line connects the wall thermostat with the other terminal of the gas valve to complete the simple series circuit.
Figure 8. Basic 120/24 Volt AC
Wall Thermostat Circuit
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Figure 9. Wall Thermostat with Fan Selector Circuit
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The varieties of heating equipment require a variety of different voltages.
The most common voltages used in residential heating equipment are millivolt and 24 VAC. These two popular styles of systems basically utilize the same style wall thermostat switch.
As a general rule each voltage supply utilizes a wall thermostat specifically designed for that voltage.
NOTE: A millivolt thermostat does not have a heat anticipator and will not replace a 24 VAC thermostat.
The other major variations in wall thermostats are the additional application the thermostat may perform. For example, many wall thermostats are both heating and cooling models. The circuits of the heating/cooling models are more complex, and the many variations prohibit inclusion in this section.
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The major causes for thermostat malfunctions are:
• Poor wire or terminal connections to the thermostat
• gas valve failure
• relay or transformer failure
In order to properly test the electrical circuit of the wall thermostat, make the following simple tests:
1) The simplest method of testing the wall thermostat from the wall thermostat location is to simply by-pass the thermostat contacts (low or millivolt systems only) .
2) With the voltage source disconnected , perform a continuity test on the thermostat switch for making or breaking the circuit.
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When checking out a malfunctioning system the service technician may improperly jump-out (short) the terminals of the gas valve.
(Only do this if one thermostat wire is disconnected from the gas valve.)
All manufacturers of gas valves have a warning label located on the gas valve. The warning states "Do not short (jump-out) gas valve terminals. This will result in damage of wall thermostat and void warranty."
Essentially, this statement warns against jumping out the gas valve to determine if the wall thermostat is malfunctioning.
When incorrectly jumping out the gas valve (with the thermostat still connected to the circuit) the heat anticipator of the wall thermostat is shorted, which causes it to burn out destroying the wall thermostat.
The gas valve will be destroyed if high voltage is used to jump out the gas valve.
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Millivolt systems do not have a voltage source external to the appliance system to power the control system; rather, they use a powerpile (pilot generator) to convert pilot burner heat to a millivolt power supply.
Because of the very low voltage, the thermostat must not have any measurable resistance at the contact point or the system will not operate. Wire size is also critical when installing thermostats some distance away from the gas valves they control.
With a voltmeter measure the powerpile output, then progress through the controls and finally check the voltage at the thermostat. Too much voltage drop will cause a malfunction.
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With the exception of item 5 which will not apply to electronic thermostats, the following checklist should be used when installing thermostats.
1.
Read and follow manufacturer’s installation instructions.
2.
Be sure sub-base plate is level and secure.
3.
Use wire gauge and type specified by manufacturer.
4.
Avoid locations that subject the thermostats to other heating or cooling devices or place the thermostat in air drafts that would result in temperatures at the thermostat that are not typical of the space that is being heated or cooled .
5.
Set the heat anticipator according to manufacturer’s directions, and based on multimeter measurements made between the gas control valve and the thermostat.
6.
Apply an approved sealant to the wall opening behind the sub-base plate.
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Electronic thermostats are actually small but powerful computers. As with most computers, electronic thermostats have a built in clock and daily, weekly (and in some models) annual calendars. The system operator (home owner or building maintenance technician) can program the thermostat to vary temperature settings throughout the day, week, and with some thermostats, seasons.
Figure 10a.
Electronic Thermostat
Figure 10b. Circuit Board, Backup
Batteries and Programming Controls
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Unlike electro-mechanical thermostats which simply turn heating and air conditioning systems on and off based on a current temperature sensing element, electronic thermostats typically increase or decrease temperatures gradually, turning the heating and cooling components on and off several times to save energy by avoiding “overshooting” the comfort temperature desired.
Figure 11. A Programmed Heating Control Cycle
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Electronic thermostats do not require heat anticipator adjustment as is the case with electro-mechanical thermostats. The function of the heat anticipator is to electrically heat the bimetal heat sensor so that it will tend not to overshoot the temperature selection. The computer programs of electronic thermostats perform this function.
With some models of electronic thermostats, the computer also “learns” how the building and heating/air conditioning components interact by memorizing how long it takes to reach programmed temperatures under varying conditions throughout the program cycles. Some newer boilers and furnaces also incorporate indoor and outdoor temperature sensors to “fine tune” the heating cycle program, and maximize appliance efficiency.
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Figure 12. Electronic Thermostat Wiring Diagram for Furnace
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
Programming instructions for each make and model of electronic thermostat will vary. Remember that customers may have difficulty understanding the instructions and operating characteristics at first.
To avoid a frustrated and unhappy customer, be sure that you understand how to program and operate the thermostat, and then take the time to explain the thermostat to the customer in a simple but complete manner.
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