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I have force air (propane furnace). I would like to upgrade to a new "smart" thermostat, but they didn't run a C (common) wire which is required for most of these units to run properly.

I do however, notice that there is a Y wire, which from my understanding is used to switch on an air conditioning compressor (which I don't have).

Since I don't have one (nor do I plan on getting one anytime in the foreseeable future), is there any reason why I couldn't just move Y over to C? Seems like I should be able too, but I just wanted to make sure that it won't somehow screw with the furnace control to not have it hooked in.

Here's a picture of the furnace board:

enter image description here

(on a side note, does anyone have any idea why there are also stranded wires going into R & W?!)

  • Do you have a humidifier or some other sort of device? – ThreePhaseEel Aug 14 '18 at 11:38
  • I do not believe so... If it is it's not exposed in the utility closet – NSjonas Aug 14 '18 at 22:14
  • Where do the extra wires lead then? – ThreePhaseEel Aug 14 '18 at 22:16
  • @ThreePhaseEel I just tracked them down. It would appear there is a second thermostat in the master bedroom... I guess the previous owner wanted the ability to override the programmed thermostat? Doesn't seem like a good idea in my mind – NSjonas Aug 14 '18 at 23:45
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You'll want to pull a new cable

The installer who put the existing programmable thermostat in didn't pull the correct kind of cable -- they grabbed a spool of network cable and used that instead. Problem is, network cables use much thinner gauge wire than thermostat cables (22-24AWG for network cabling, 18AWG for thermostat wire), which means that the voltage drops over the cable become excessive due to the improper substitution, even with the doubled-up wires.

I would suggest pulling an 8-wire, CL2 rated, 18AWG thermostat cable at a minimum -- if you can get 10-wire, 12-wire, or 13-wire stuff, all the better to avoid having to put someone else through this chore again. The unused wires can simply be wrapped back around the cable jacket at each end.

  • ahhhhh.... Curse whoever came before me – NSjonas Aug 16 '18 at 3:16
  • whats the impact of the voltage drop? Thermostat malfunction? Furnace malfunction? Either/Or/Both? If it was run efficiently, it would only be about 30 feet of cable. – NSjonas Aug 16 '18 at 3:20
  • @NSjonas -- Either/Or/Both -- the thermostat itself may not boot up properly (more common with power-hungry IoT 'stats like the Nest and Ecobee), or the furnace may have issues due to the gas valve getting browned out at full current -- a 24VAC gas valve coil can pull up to 500mA, and I've seen a bad relay contact in a thermostat cause the valve to brown itself out on that, causing the furnace to fail to heat (many furnace control boards switch W to the gas valve to enable gas, instead of having separate power from R to the valve and using W merely as a request-for-heat input). – ThreePhaseEel Aug 16 '18 at 3:28
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I will second the thought of putting in 18 awg wire to the thermostat. From your picture, it is difficult to correctly identify the terminals. The best option is to download the furnace manual off the internet for the model you have. Just to verify exactly what each terminal is.

  • ya, sry that just the angle of the photo. It's easy to tell in real life – NSjonas Aug 16 '18 at 3:16

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