How Do I Wire an AC Thermostat?


You just bought a new thermostat but which wire goes where?

First, before you even attempt to change out a thermostat from the Air Conditioner, shut off the power to the air handler. The transformer that supplies the 24 volts needed for the HVAC control system gets its voltage here. Although 24 volts won't kill you, It is very easy to create a short that will either blow the control fuse or trip the breaker on the transformer itself.

When removing the original thermostat either take a picture of how it currently wired or mark the wiring and terminal designation for future reference if needed. Also below is a basic reference. Different manufacturers can designate terminals differently.

Please read the manual that is provided with the new thermostat. Make sure there is enough bare thermostat wire exposed to get a good connection on each terminal & that it is snuggly secure when tightening.There may be a small switch on the back of the thermostat ( or may have to program ) to indicate the type of heat your system uses. Confirm what you have and adjust accordingly.

Standard thermostat wiring color code

1. ( R ) terminal - Red wire

The red wire is the power coming in from the transformer. Some t-stats require a jumper between " RC " which means red cool, to " RH " which means red heat. Sometimes you have to make this jumper, or often there will be one from the factory already installed.

2.( Y ) terminal - Yellow wire

The yellow wire provides power to the compressor contractor. This turns on the condenser.

3. ( G ) terminal - Green wire

The green wire provides power to energize the indoor blower motor.

4. ( W ) or ( W / Aux ) terminal - White wire

The white wire provides power to the electric heat strip located in the air handler. This may be the only heat source if you have a straight cool AC or it provided supplemental heat if you have a heat pump.

5. ( C ) terminal - Blue wire.

This is the common wire. It completes the electrical circuit to provide power to a digital thermostat. If the is no blue wire for you to connect, it is likely the thermostat needs batteries installed to power up the display.

6. ( O / B ) terminal - Orange wire

The orange wire is used only if the system is a heat pump. It provides power to energize the reversing valve in the condensing unit outside. Also in the thermostat programming or on the back of the thermostat, there may be a switch O/ B that determines how the t-stat controls this voltage. Rheem and Rudd units, for example, require this to be set on " B " to operate correctly. Most other brands will be set to "O ." If the thermostat is installed and the thermostat is set to cool, but the heat comes on. There is a good chance the setting is incorrect.

This wiring is for a single stage air conditioning or heat pump system only. If you have a higher end unit with multiple stages or if it is a variable speed/inverter style unit, a specialized thermostat may be needed, and the wiring is completely different.

Need a thermostat installed for your AC?

Air Zero provides all styles of thermostats from basic non-digital, programmable digital thermostats, wifi thermostats and more. Air Zero services all of Pinellas County including Largo, St. Petersburg Fl., The Clearwater area, and Seminole. Please give us a call if we can help. 727-392-6111