Handyman Dan Blog


NOTE: This article was created from notes from an A/C installer who recently installed a new control unit for one of the owners.  It has not been double-checked for the various types of A/C units and installs in our complex.

One of the simplest and most cost-effective upgrades that you can do to improve the energy efficiency of your home is to upgrade your thermostat to an electronic control.  A modern thermostat will allow you to automatically set the temperature based upon time of day and day of week, and could save hundreds of dollars each year in heating and cooling expense.

It is quite simple to install almost any electronic thermostat with our the fairly simple heat pump A/C units used at Concord Village.  Before fully disconnecting your old control unit from the wall, remove the face plate and mark the main control wires, which are indicated as O-G-RH (and possibly Y+C) letter markings on your existing thermostat.  If you find that your colors and wires differ from the list below, use the colors that are actually connected to your old thermostat, and follow the instructions carefully on your new thermostat.

  • (O)range - Changeover valve control (switches between heating and cooling)
  • (G)reen - Fan control relay 
  • (R)ed/RH - Power for Heat Pump - usually the XXX wire (some control units have both an R and RH connection, and usually include a jumper to link them)
  • (C)ontrol Wire (optional) this is usually the BLUE wire if connected, but it could also be the WHITE wire. 
  • (Y)ellow - 1st stage compressor contactor (controls air conditioning system and 1st stage of heat pump) - we have included this because some control unit in the complex have this wire installed -- need to do more research here...
The control wire is only needed on some very advanced thermostats which need extra power to transmit data wirelessly to a computer, etc.  Because these features were uncommon when our units were built, this control wire may not be connected properly to the A/C unit on the roof, and if connected later, the installer may have used either the extra blue or white wire (usually blue).  Since control wires may not exist or may not be hooked up, most digital thermostats also have batteries in them to power the display, so if you are not using a control wire and your A/C stops working and the display goes blank, you probably just need to change the batteries.