7.3.3
Identifying the Components & Operating
Characteristics of Wall Thermostats
(Heat Only)
For most heating systems, the call for heat signal to start gas
appliance operations is initiated by a wall thermostat. In this
module, you will learn to:
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
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 1
In this module, you will learn to:
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
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 1
Identifying the purpose of the wall thermostat
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)
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 1
Identifying the operating characteristics of electromechanical wall thermostats
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
7.3.3 Student Book  © 2005 Propane Education & Research Council
Pages 1 & 2
Figure 2. Magnetic Contact
Wall Thermostat
Figure 3. Mechanical Thermostat
with U-Shaped Bimetal
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 3
Bimetal Sensing Devices
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.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 2
Temperature Adjustment— 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
7.3.3 Student Book  © 2005 Propane Education & Research Council
Pages 4 & 5
Heat Anticipator
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.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 6
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)
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 7
The heat anticipator illustrated in
Figure 7 is constructed of one short
strand of high resistance wire.
Figure 7.
Heat Anticipator
(Top of Thermostat)
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.
Systems which operate properly in well-insulated dwellings should have
an operating differential of about 1° F to 2° F.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 8
Mounting Base
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.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 9
Selecting a proper thermostat location
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).
7.3.3 Student Book  © 2005 Propane Education & Research Council
Pages 9 & 10
Identifying characteristics of thermostat circuits
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
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 10
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
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 10
Figure 9. Wall Thermostat with Fan Selector Circuit
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 11
Identifying variations in electro-mechanical wall
thermostats
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.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Pages 13 & 14
Identifying possible malfunctions of electro-mechanical wall
thermostats
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.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Pages 14 &15
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.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 15
Installing a millivolt thermostat
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.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 15
Identifying elements of an installation checklist for thermostats
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.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 16
Identifying operating characteristics of electronic thermostats
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
7.3.3 Student Book  © 2005 Propane Education & Research Council
Pages 16 & 17
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
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 18
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.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 18
Figure 12. Electronic Thermostat Wiring Diagram for Furnace
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 19
Technical
Tip 
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.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Page 19
Time to See If You Got the Key Points of
This Module…
• Complete the Review on pages 20 - 22.
• See if you are ready for the Certification
Exam by checking off the
performance criteria on page 23.
7.3.3 Student Book  © 2005 Propane Education & Research Council
Pages 20 - 23
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