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I have an ancient furnace/AC combo that came with my apartment that originally came with a very simple battery powered thermostat, which I replaced with a modern Honeywell programmable wifi thermostat.

As the original thermostat did not have a c wire connection, which the new one needed, I needed to figure out where on the unlabeled low voltage panel to connect the c wire to (there was an unused wire in the existing cabling that was disconnected on both ends)

My best guess for the c connection was the common wire for the connection to the outdoor unit, which was wired thus:

R wire -> thermostat -> Y wire -> outdoor unit -> common terminal.

On powering back on the furnace, the thermostat powers on, the fan can be turned on, but the heating system seems to only turn in the fan, with no heat coming. Being winter, I didn't fully test the cooling system.

Did I break it? Or just wire it wrong?

I haven't had a chance yet to test again with the original thermostat, due to giving back the small screwdriver I borrowed before testing it.

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If your furnace and a/c are very old they may have separate power supplies. Old school heat/cool stats worked like this Rh(24v power) closes to W(heat signal) on a call for heat, that's the heating circuit. Rc(24v power) closes to G(fan) and Y(air conditioning) on a call for cooling,thats the cooling circuit. You can also close the fan switch Rc-G to operate the fan independently of the air conditioner. Newer systems just use R for both heat and cooling as all the controls are incorporated into one control board. The C(common) is just used to complete a circuit. This picture shows what happens at the furnace old systems used 2 different power sources. You need to confirm that the new stat will accommodate Rh and Rc systems. (https://i.stack.imgur.com/vhq0k.jpg)enter image description here

  • The old thermostat only had 4 wires, r, w, g, and y, so I don't think that's the case here. – Japa Illo Feb 16 at 15:05
  • I was unable to see exactly what was connected in your pictures so I gave a full description of both 5 wire system and 4 wire system. In both cases the common is not required for your application. You need to connect the R's W G and Y and that's it. It's unlikely that the thermostat is fried. But get that common C wire out of the stat. It will blow the fuse every time. – Joe Fala Feb 16 at 16:55
  • So a bit of an update, there was indeed a fried wire in the furnace, and the repair guy told me the furnace flat out won't work with any thermostat that needs a C wire. While I don't fully believe him, I don't know enough about the furnace to contradict him, and I'm not willing to try again in case I fry the same wire. The furnace doesn't even have a control board, actually. Just a tangled mess of wires. – Japa Illo Feb 18 at 15:22
  • Any furnace can work with a common on the stat. It's a matter of getting the wire there. But your stat doesn't require a common. – Joe Fala Feb 18 at 15:34

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