Heat Pump Thermostat 101

    The thermostats on HVAC system is the most visible and in many cases most misunderstood components on a system. On a "conventional" system such as oil, gas or electric with or without added cooling, the thermostat will control the fan, heat call or cooling call and nothing else. Some systems may have 2 stage heat or cool which means 2 compressors or a 2 stage compressor.

    On a heat pump that uses electric back up or gas or oil backup the thermostat needs to control the compressor, fan, reversing valve, electric heat *, indicator lamps, emergency heat function *. Some electronic thermostats will also control dehumidification, electric heat usage based on outside temperature, fossil fuel changeover based on outside temperature, display outside temperature and do nothing with it, 2 stage operation of compressor if so equipped, delay functions, auto changeover functions and even remote monitoring. * see definitions.

    The wiring for thermostats, conventional and heat pump are standardized, however there are exceptions and non conventions.

    WIRING conventional (gas, oil or electric forced air):

Colors: red, green, white, yellow, blue or black.

Name R, G, W, Y, C.

Function Hot side of 24 volt transformer, Fan signal, heat call (gas, oil or electric) compressor contactor cooling only, common to power digital thermostat or operate indicator lamps..

RH and RC are used separately if the system consists of one cooling system and a forced air or hydronic (or steam) system that has separate control transformers. The RH will feed the W terminal and the RC will feed the rest. On a system where the functions are combined or the same control transformer is used the RH and RC terminals will be jumpered together.

WIRING generic heat pump:

Colors: red, green, white, yellow, orange, blue or black.

Name R, G, W (can be W1 or W2), Y, O, C.

Function Hot side of 24 volt transformer, Fan signal, Electric heat call, compressor contactor cooling and heating, reversing valve energized to cool (and defrost), common to power digital thermostat or operate indicator lamps.

WIRING GE, Trane American Standard heat pump:

Colors: red, green, white, yellow, orange, blue .

Name R, G, W (can be W1 or W2 on thermostat), X2 (second electric heat control) Y, O, B.

Function Hot side of 24 volt transformer, Fan signal, Electric heat call, second electric heat call, compressor contactor cooling and heating, reversing valve energized to cool (and defrost), common to power digital thermostat or operate indicator lamps. Note: York (and York brand families) use B for common too. On these heat pumps the T terminal is used for outdoor reset with a thermistor and resistor combination. I have seen a terminate designated T on a packaged system used as system common and ground.

WIRING Rheem (Ruud, Weatherking) heat pump with reversing valve high for heat:

Colors: red, green, white, yellow, blue, black or brown.

Name R, G, W (can be W1 or W2), Y, B, C.

Function Hot side of 24 volt transformer, Fan signal, Electric heat call, compressor contactor cooling and heating, reversing valve control line energized to heat, common to power digital thermostat or operate indicator lamps.

WIRING York, Luxair, Fraser Johnston heat pump:

Colors: red, green, white, yellow, orange, blue, black .

Name R, G, W (can be W1 or W2 on thermostat), E, Y, O, B. (thermostat)

Function Hot side of 24 volt transformer, Fan signal, Electric heat call, emergency heat call, compressor contactor cooling and heating, reversing valve energized to cool (and defrost), common to power digital thermostat or operate indicator lamps.

York description: York heat pumps can use a simple defrost scheme or they can employ the Yorkguard® module that has been in use for decades. The Yorkguard® is similar to the computer in a car but does a lot less functions, as far as heat pump controls go it is one of the more complicated systems. The module monitors at least 3 temperature inputs: compressor discharge, outdoor coil, ambient and in the case of a fossil fuel setup; indoor plenum temperature. Auxiliary and Emergency heat functions are sent through the Yorkguard® and sent back to the electric furnace. The usage of electric heat will be based on outdoor temperature and settings of the module. In a fossil fuel setup the module will control the function of the fossil fuel usage based on outdoor temperature.

(MOVE THIS UP?)

Standard wire colors
Red
Green
Yellow
White
Orange

Brand dependent
Blue
Black

Non Standard wire colors
Brown
Pink
Purple
Tan
Grey

On thermostats with an E terminal (Emergency Heat) function the terminal may be configured in 3 different ways.
1: The terminal is active on a call for heat with the thermostat in the emergency heat position and no call from any other terminals.
2: The terminal is active whenever the thermostat is in the emergency heat position and the call comes from the W or W2 terminal. Relays are needed to manage the stages of electric heat usage.
3: The terminal is active on a call for heat with the thermostat in the emergency heat position and a call comes from the W or W2 terminal.
Some thermostats such as Ritetemp do not have an E terminal but deselect the Y terminal and only activate the W2 terminal to provide electric heat only. Relays are needed to manage the stages of electric heat usage.