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My home currently has a central a/c unit which is controlled from an 3-wire old mercury switch thermostat. I want to replace it with a newer Honeywell that requires 4 wires.

The existing thermostat a Green, White and Red wire. The new thermostat requires and additional Yellow wire.

I tried lots of combinations, but nothing worked properly, yet with the old thermostat, I can turn on the a/c, or just the circulating fan.

Any suggestions?

  • What model of thermostat are you trying to install, and where do the wires lead on the old A/C unit? – ThreePhaseEel Jun 13 '16 at 23:36
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  1. Replace the old thermostat wire bundle with a new bundle with 4+ wires, or

  2. Return the new thermostat you have and obtain a wireless version instead. (Examples: Honeywell YTH 6320, Venstar T1100—or the Honeywell RTH 7600 needs only two wires.)

The first suggestion might be very difficult. You may have to tear off sheetrock, or do many tricky bits of drilling and coaxing wire through. Or, if there is a short distance between thermostat and heat plant, it could be as easy as pie if the wire is not tacked inside the wall.

  • Thanks, I greatly appreciate your response. For suggestion 1, I can definitely reach the source (I've located it up in the attic) but I wouldn't be sure how to replace the harness. For suggestion 2, could you recommend a specific device that would work with the current 3-wire set-up? – Jamie Aug 31 '14 at 18:59
  • @JamieNYC: I have added some examples which might work for you. – wallyk Aug 31 '14 at 20:39
  • Wow, the Honeywell RTH 7600 is definitely a contender -- but it it only needs 2 wires, what do you do with the third? – Jamie Sep 1 '14 at 2:04
  • @JamieNYC: Ignore the wire you don't need. It will sit beside the wires in use just fine. If, in the future, a third wire is needed for something, it is ready for use. – wallyk Sep 1 '14 at 5:12
  • ok, so I guess the ultimate question is... which 1 of the 3 wires gets ignored? Thanks again. – Jamie Sep 1 '14 at 22:17

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