Problem: 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?
Answer: Please always take a picture or mark the wires with tape or labels as to which terminal they were connected to on the old thermostat before thermostat removal. I would be able to help you easily if I had a picture to see where the wires were hooked up to the old thermostat. 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 thermostat, a Yellow wire that energizes the outdoor unit’s contactor, (if you have air conditioning) and a white wire that energizes the gas valve, electric heat relay or oil burner when you need heat. The white wire is usually connected to the “W” terminal. The thermostat acts as a switch, like a light switch. The 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 does your air conditioner turn on? One of the wires has to be the hot wire coming from the power side of your low voltage transformer. This power wire is usually the Red 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. Below we have a very informative Youtube Video made by grayfurnaceman which explains thermostat wire color codes. We also have another really good video made by David Jones which shows how to wire up a thermostat. These universal wire color designations have been used by heating and air conditioning professionals for many years. You should be able to use this wire color code for installing Nest, Honeywell, Ecobee, and just about any thermostat. If you have a heat pump thermostat and if you have questions about wiring a heat pump thermostat then please click here where we have a post with YouTube videos that show how to wire a heat pump thermostat. If you have any other questions please feel free to email us: firstname.lastname@example.org or comment below. We would love to help you out and have your business!