How to wire a home AC thermostat

Many homeowners are handy, but tend to get confused when it comes to their home AC system. In this article I will go over how to wire a home AC thermostat. First you will have to choose a AC thermostat to satisfy your personal needs. Whether you decide on choosing a Programmable or Non Programmable thermostat. They are wired the same way. Some of the newer Smart thermostats have Wifi compatibility so you can control the units with your smartphone or computer while away from the house. These thermostats usually require a Common Wire. So it is smart to look at the existing thermostat wiring if you don’t want to rewire a thermostat wire from the unit to the thermostat. Common wire is usually black, but I have seen different colors used. Common wire is attached to the common side of the 24v transformer that the red wire is attached to.

Thermostat wiring colors

Red – 24v POWER from transformer
Green – Fan
Yellow – Cooling (compressor/outdoor fan)
White – Heating (Gas Furnace or Electrical Heat Strips, Optional)
Orange – Heating (Heat Pump with a Reversing Valve, Optional)
Blue – Heating (Heating on Rheem and Ruud units)
Black – 24v Common from transformer

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When removing the cover of your old home AC thermostat you will see wires coming from the wall. Try to avoid breaking the wires when removing the old thermostat. REMEMBER THAT THE RED WIRE IS HOT!  I recommend disconnecting the power of the unit if you are a novice electrician. You could possibly burn out a small fuse or pop a micro circuit breaker on the transformer.  I tend to replace the thermostats really quickly without disconnecting power, but I have changed hundreds of thermostats and deal with different electrical wires daily. You just have to be careful with where the metal ends of the wire touch. I recommend installing the red wire first to get it out of the way. Going from one thermostat base directly into the other one. Green wire controls the indoor fan. Yellow wire controls the outdoor unit compressor and outdoor fan motor. White wire is usually used for gas furnaces or electrical heating. Orange wire is used for heat pump units, and it activates the reversing valve. Reversing valve changes the flow of the refrigerant. Blue wire is used on some units for heating. Black wire is almost always common. It is attached to the common side of the 24v transformer that the red wire is attached to.

After attaching the wires and reattaching the face plate. It is important to finish the steps in the instructions that came with the home AC thermostat. The newer Smart thermostats will have a series of questions to answer. It is important to answer the questions to the best of your ability. After you are done with the set up questions. I recommend testing the unit in the different modes to ensure that the wiring is correct. I will always test in cooling and heating to prevent a call back.  If you have any questions about your home AC thermostat. Feel free to send us questions at dollarmech@gmail.com

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