I found the "Why doesn't the “fan on” mode work after I installed a new thermostat?" thread on this site and am wondering if the manual rewire steps would also work for a two wire system. The setup: older house with Trane XV80 furnace, two wire setup, thermostat/Rh to controller board/R; thermostat/W to controller board/W1; thermostat Rc to Rh jumper; controller board W1 to W2 jumper. As I understand it an additional green wire would be the best solution, but that's not an option for me. Can I get the fan to run continuously during the summer by manually moving the thermostat wire from Rh to G and the controller board end from W1 to G? Thanks.

  • There's probably a better way to set this up -- do you just want a switch somewhere you can turn the fan on during the summer with, or do you want the fan to run whenever there isn't a call for heat? – ThreePhaseEel Apr 11 '19 at 23:23
  • Well... Good questions. I run my fan 24/7 but I'm asking regarding the mother-in-law's house, so I'm looking for the best easy/option solution. As such, an on/off switch sounds good and offers both. Fan running whenever there isn't a call for heat? I think I like the other option better. – mikep Apr 13 '19 at 17:10

It's probably simpler to just wire a switch at the furnace wired R and G. In the summer you just hit the switch. You can switch the wires in your furnace and thermostat to have control at the stat. Or you could install a Fast Stat, which uses two wires to signal a full complement of wires.

  • Thanks for the response Joe. If I understand the first option correctly, you propose switching/changing only the controller board connection - in effect, this sounds like we're redirecting a call for heat from the thermostat to activate the fan. Is that correct? And option two allows for the thermostat fan on/auto switch to function as intended. Correct? And I see that the Fast Stat allows multiple signals over the same wire. That's pretty cool. – mikep Apr 13 '19 at 17:11
  • I moved the red controller wire from the red terminal to the green terminal. No fan. Just a slow blinking red light indicating no call for heat.Tried resetting the thermostat through the reset button as well as taking out the batteries. Thermostat set to heat, fan to on/auto (didn't matter which) and set the temperature to hold at 20 degrees above the current temperature being displayed. Any thoughts? Thanks. – mikep Apr 28 '19 at 23:14
  • Since the previous change had no impact, this weekend I moved the thermostat/Rh wire to G (controller board red wire still on green terminal) . Still no fan. Just a slow blinking light on the controller board. Befuddled. – mikep May 6 '19 at 16:50

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