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Blog Title: how to install a programmable thermostat

Main Que: I purchased a new programmable thermostat. My thermostat wiring only has only three wires. The thermostat instructions call for a red power wire, a yellow, green and white wire. My three wire colors are White, Yellow and Green. How do I wire this thermostat up?

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  • Query By: ANNA SANCHEZ (Caroga Lake, GA)
  • Date: 06/19/2015

Answer: Please always mark the wires with tape or labels as to which terminal they were connected to on the old thermostat. I will be able to help you better. Do you have both heating and cooling? Most thermostats have a Red wire that is the power wire that usually connects to R & RC terminals of the thermostat, a Green wire that energizes the Fan relay, connected to the “G” terminal on the stat, a Yellow wire that energizes the outdoor unit’s contactor, if you have air conditioning using the “Y” terminal, and a white wire that energizes the gas valve or oil burner when you need heat connected to the “W” terminal. The thermostat acts as a switch, like a light switch. Red to white switch turns on the heat. Red to Green turns on the fan, Red to Yellow turns on the air conditioner. The main question would be do you have air conditioning? One of the wires almost has to be the hot wire. The color might not be red, but maybe they, the installer, used another color for the hot wire. You would need to look at your low voltage transformer, trace the wire down that is coming out of the transformer to determine the color.

  • Replied By: JUANITA AUSTIN (Van Orin, NC)
  • Replied Date: 07/06/2015