Asked on Jun 01, 2013

2 thermostats into 1??

I have 2 separate thermostats, one for heat and one for central air. I'm trying to combine them into one. Off the furnace one of each of these colors, red, green, yellow, white. If I attach all these to the new thermostat, where do I attach the ones off the central air unit?
  5 answers
  • SDair SDair on Jun 02, 2013
    We need more information about the equipment you have. Does your air conditioning use the fan motor of your furnace to recirculate air during the cooling cycle, or does it have its' own fan? If they have seperate fans, you will not be able to consolidate to one thermostat easily. The typical wiring for a thermostat with 1 heat and 1 cooling stage are as follows red = 24 volt green = fan white = heat yellow = air conditioning blue = common
  • If you have two totally separate systems then you normally have two sources of 24 volts. There are thermostats that are designed to incorperate both in to one stat. There are many variations but basically the terminal designatios are as follows. RH Heating R (examples might be forced air furnace, boiler, electric baseboard with a contactor) W Heating this would go to the W terminal on the heating equipment. RC Cooling R (examples might be attic A/C Outdoor self contained unit) Y Cooling Y connection G G connection on cooling equipment If you use this type of thermostat for two systems make sure you remove the jumper that is between RH and RW. This jumper would stay in if you only had one source of 24 volts. If you have an A coil above your furnace then you are typically using the furnace fan for cooling. There may be more terminals and if you have a heat pump there would be more connections. If you have electric baseboard heat the thermostats may be high voltage, be sure that your systems have a low voltage control system before attempting to wire.
  • Miles Enterprises Inc. Miles Enterprises Inc. on Jun 02, 2013
    most moden t-stats are capable of handling seperate systems .all that is required is taking out the jumper between R and RC. yYou need to put the wires from the heating system on R or RH and W. the 3 wires from the cooling system go on RC Y and G.
  • A1tec service A1tec service on Jun 04, 2013
    r and w is your heat on your stat remove a jumper wire were it has r and rc then you will have yellow green red put the red on rc yellow on y green on g be carefull you on buying a stat that doesnt have this .or you will short both system if you move help call 8642483119 A1 tec service and we trie to talk you more on wiring 24 volts noteDONT buy from lowes or homedepot tstat before you learn more on the wiring your system and what can heating an cooling unit you have
  • I am assuming you have both stats on the same wall. The issue is if the heating system and the cooling system shares the same transformer for the 24 volts or if it does not. The furnace thermostat uses only two wires. (W)hite and (R)ed. The red wire comes from the transformer in the furnace up to the thermostat. Once the thermostat calls for heat it makes a connection to the (W)hite wire which is then connected to the gas valve and it powers it up and turns it on. In traditional heating and cooling systems the (R)ed wire comes up to the thermostat which like before is the power from the transformer. The switch somewhere on the thermostat selects what wires will be powered up when the thermostat calls for heat or cooling. In the case of heating the same thing as above takes place, the power is connected from the (R)ed wire through the thermostat then back to the (W)hite wire and the heat runs. In the cooling mode the power goes from the (R)ed wire to the (Y)ellow wire, which in turn powers up the outside part of the cooling system. Also at the same time the (G)reen wire which is connected to the relay in the furnace is powered up in conjunction with the (Y)ellow wire control and the fan starts at the same time the AC turns on. Lastly of course the (G)reen wire is turned on by itself should you turn the switch that says fan on/auto to the on position. It is then the fan runs regardless if the heat or cooling system is running on its own. So to recap. R is transformer, Rw is 2nd transformer should two exist. W is heat, W1, W2 is additional stages of heating should your furnace require this type of connection, Y is cooling and Y1, Y2 etc are different stages of cooling again dependent upon the type of system you have. Lastly G is the fan control. Ideally the wires are connected based upon their colors that match the screw letters. However this is not always the case, so beware, do not try to connect these colored wires to the color letter. It may not always be the same. On some older systems the AC system contained its own transformer. When that was the case, the power that comes from the AC system is connected to the RC post and the (Y)ellow wire is connected to the Y post as before. IN your case even if its not age related, if you have a 2nd independent AC system perhaps located in the attic and the furnace or boiler is located elsewhere. Get a pro to do this wiring. It can be done, but its tricky assuring your wires are connected to the correct side of each transformer as this makes a big difference depending upon how each system is wired. As A1tec service stated, I would suggest that you hire a pro to do this wiring for you. Let them purchase the correct thermostat that has the features you want in conjunction with the proper sub-base needed to wire the system(s) you have.