I have an old system and want to update the thermostat

however, the white-rodgers thermostat has no letters such as R,G,B,Y,W,C

So I checked out the furnace terminate wires and the colors of the wires do not match the colors of the wires on the thermostat.

Thus, i can't follow the colors of the wires up from the furnace.

  • Do you have just one thermostat? A system with multiple zones will have a zone controller between the thermostat and the furnace. – TomG Sep 1 '13 at 0:30

If the wires from the old thermostat to the furnace are not the same colors at each end it seems to imply that there is a cable splice somewhere along the run where someone did a hack job of the splice work.

All is not lost however. You can figure this out fairly easily though. Start by disconnecting all the wires at both ends. (You may want to document the colors to terminals at the furnace end before you go and remove them all). Then get a temporary wire that you can run loose through the house between the two end points. Now temporarily connect the loose wire to each of the thermostat wires in turn and at the furnace end use a multimeter in "ohms mode" or a continuity tester to find which one of the furnace connection wires shows a connection back through to the end of the temporary wire. Write down the color relationship for a permanent record and for use in aiding you in connecting up the new thermostat.

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    Or, with everything disconnected, short two wires on one end and test for continuity on the other end. Keep using one of the same wires for your tests so after the first test you can determine which of the two wires is which. – BMitch Aug 31 '13 at 15:01
  • @BMitch - Yeah, that works too but has some drawbacks. First off it complicates the procedure for a DIY person that may be doing a continuity check for the first time in their lives. Secondly if there are some strange connections at the splice or any open/broken wires it can be rather confusing to someone with limited experience to try to sort out what is going on using the scheme you propose. – Michael Karas Aug 31 '13 at 15:14
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    But the advantage is that you don't have to find another wire to run across your home. If you've got open/broken wires, then you won't get any reading. And if there is a strange short, then it may be time to run a new thermostat wire. – BMitch Aug 31 '13 at 15:51

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