Wiring a Thermostat 101

Posted by Darin DeVries on

Hello and welcome to another post of Wiring a Thermostat 101 

Wiring a thermostat can be very complicated but I will give you a brief breakdown to ease some of the stress. The thermostats come with many options and many connection points on the back of the thermostat and it seems to be overwhelming for the homeowner, here are a few pointers to help take the stress out of wiring a new thermostat. think of your thermostat as a light switch... When its on its making contact to turn on a light there are a bunch of contacts on the back of the thermostat and all of them are mini switches so the brain of the thermostat controls when to turn on heat and cool. 

You only need 2 wires for heat only and 4 wires for heat and cool simple! lets start with

Red - The red wire is 24VAC coming from the furnace transformer that is the power wire. you will always need this guy! So Red gets hooked up to R terminal on the thermostat. 

White - The white wire is heat and the white wire gets hooked up to W terminal on the thermostat. 

NOTE: So let me stop and say if you have an emergency!! The kids broke the thermostat off the wall when playing in the hallway and now you just have a pile of wires hanging out of the wall and its -30F and the house is getting cold. (Always turn the power off before doing any electrical work) If you take the Red wire and the White wire and wire nut them together and turn on the electricity your furnace will run in the heat mode FOREVER until you un-wire nut them. it's always good to remember R-W = Heat. 

Yellow- The yellow wire is AC and the yellow wire gets hooked up to Y terminal on the thermostat (So the same thing if you wire R and Y together you get cooling) Very simple! some thermostats need the G - green wire to control the fan. 

Green- The green wire gets hooked up to the G terminal to run the fan / Blower on the furnace so the same thing, if you wire the Red and Green together the fan will come on. some thermostats have a fan switch on them that says Auto or ON. Auto will bring the fan on with heat and cool and the ON will make the fan run all the time. 

That is your 4 main wires that thermostats normally use to control the heat and cool of your house. 


Now if you have extra wires these are used for extra equipment and we will talk about them below, If they are not needed in your application just wire cap and tape them off and stick them back in the wall 

RC- is the same as R and sometimes they have a jumper between the two terminals on the thermostat, the RC is for if you are running two transformers two furnaces side by side you have the extra terminal RC- not rarely used.

C-  Blue wire is Common wire and used on some thermostats and equipment

B- Black wire hooks up only if you need Reversing Valve in heating rarely used.

O- Orange wire is Reversing Valve in cooling and used rarely just on some Rheem/ Ruud equipment. 

X/AUX- Terminal is used if you have a secondary heat source like a (wood boiler) 

E- Emergency Heat some gas furnaces and heat pumps use this terminal so if there is an equipment failure your thermostat can turn on electric heat strip to keep the house from freezing up while you are gone. 

W2- Is used only if you have a secondary form of heat Stage 1 and stage 2 some of the newer furnaces have a 2 stage gas valve to save energy. It starts on low stage 1/2 the gas pressure and stage 2 is full gas pressure. 

Y2- Is used for 2 stage cooling, some compressors are built to run on low speed Stage 1 or 1/2 the freon flow to save energy and stage 2 full speed  

As the world becomes more technical we start to have smarter thermostats some thermostats have HUM - Humidifier terminals or IAQ - Indoor Air Quality for fancy air filtration systems.  

Hope this document helps ease some of the confusion of wiring your thermostat, If you found this helpful please share it and let us know how it helped you. If you want any more technical help on your equipment or need manuals or wire diagrams please email us (aberdeenrvparts@gmail.com) and we can dig into our huge library selection.

Darin DeVries


  • I always get confused with the wiring of the thermostat. This article briefed me & cleared my doubts. Keep sharing.

    Dale's AC Services on

  • Excellent article on thermostat wiring and what each wire (color) means! I really appreciate you writing this informative explanation.

    Melissa M Duplessis on

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