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Page 34
Chapter 2: Circuits and Their Components
Electrical Theory & Applications for HVACR
A heaƟng thermostat opens on temperature rise and closes on temperature drop. Conversely, air
condiƟoning thermostats close on temperature rise and open on temperature drop.
One common type of temperature sensing thermostat uses a bimetal switch. Two dissimilar types of
metal are bonded together to form a bimetal. Because the metals expand at different rates, a change
in temperature will cause the bimetal strip to bend. A contact switch at the end of the strip can then
be either opened or closed, depending on the applicaƟon. Most oŌen, the bimetal strip is bent into a
helix to save space.
Another type of switch that has been used with bimetal is a mercury contact. A small pool of mercury
is sealed inside a glass container that also contains a set of contacts. Most mercury switches are
constructed as single‐pole double‐throw, which means there is a common terminal, a normally closed
terminal, and a normally open terminal. This design allows the thermostat to be used for both
heaƟng and cooling applicaƟons. Mercury has been phased out due to its toxicity.
Newer electronic programmable thermostats have many more features. An advantage of
programmable thermostats is the ability to raise or lower a structure’s temperature at pre‐specified
Ɵmes on each day of the week.
Electronic thermostats are very versaƟle, providing mulƟple opƟons for heaƟng and cooling control.
The same thermostat can be programmed to operate a fossil fuel system as well as a heat pump
system with mulƟple stages of heaƟng and cooling. Electronic thermostats do not use an heat
anƟcipator. Most have an opƟon to set cycle Ɵme per hour. The thermostat will adjust run Ɵme
based on a biorhythm developed by the micro processor.
Some heaƟng and cooling thermostats using a single transformer have a jumper connecƟon on the
sub‐base of the thermostat to Ɵe the circuits together. The thermostat may have the leƩers RH or
the number 4 to indicate the feed for the heaƟng circuit and R or RC for the cooling circuit feed.
For proper operaƟon a residenƟal heaƟng/air condiƟoning system thermostat should be located at
the height that local codes or ADA regulaƟons mandate, in a place where draŌs or sunlight will not
affect its temperature sensor.
Thermostats must be mounted level to operate effecƟvely. Failure to mount a thermostat correctly
will result in incorrect temperatures in the condiƟoned space. The small hole in the wall behind the
thermostat for the entrance of thermostat low‐voltage wiring MUST be sealed or poor temperature
control will result and room temperature will not be maintained to the thermostat set point.
HEAT ANTICIPATORS
Mechanical bimetal thermostats have a fixed anƟcipator for cooling and an adjustable anƟcipator for
heaƟng. The heaƟng anƟcipator is an adjustable resistance heater located near the bimetal coil in a
thermostat. It heats the bimetal strip slightly to prevent overshoot of the system. For example, a
forced‐air system operates by heaƟng a metal heat exchanger. When the heat exchanger reaches the
operaƟng temperature, a thermostaƟc or electronic switch closes, turning on the blower motor to
move heated air to the living area. When the temperature set point is reached, the thermostat