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Discussion Starter · #1 ·
A friend of mine just purchased a new condo in Denver. I'm guessing the building was constructed in the 70's or 80's, by the appearance of it. The condo unit has hot water baseboard heating, and has a stand-alone pneumatic thermostat for that.

The AC unit is also stand-alone and has its own, separate thermostat. The outdoor unit an old Westinghouse wall-mount, similar to a Bard unit. I want to replace the old thermostat with a new one. The old one still has a mercury switch and doesn't work too well after decades of service.

I pulled off the cover of the existing AC thermostat and there are only three wires; red, white and black. These differ from the modern wiring of thermostats that control heating and cooling (red, yellow, green, blue, and white). I tried searching the Westinghouse unit model number from the nameplate, hoping to find a wiring diagram, but nothing comes up.

Has anyone seen this before? Are these 3 wires just hot, neutral, and ground? How should a new thermostat be wired to properly control the outdoor AC unit?
 

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On the existing thermostat you have terminals V, C and F. I think these would correspond to a new thermostat as follows:


Old-------- New
V---------- Rc/Rh
C ----------- Y
F------------ G


To test this, (old thermostat) jump V and F wires, fan should turn on.
Jump V and C wires, compressor should come on. If that works, the old to new connections above should work. You won't hurt anything (or yourself) jumping these wires out. If you are thinking of a programmable stat make sure it's battery operated and doesn't require a Common wire.
 

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I see the existing terminal markings as being F(fan)...R(24vac)...Y(cool) which would be equal to the terminals G...Rc...Y on a new stat. You will have other terminals on the new stat... ignore them. You don't need the Rh terminal or any jumper between it and the Rc terminal, but if it's there, you can leave it, won't affect anything.

Old-------- New
F------------ G
R ----------- Rc
Y------------ Y

Be sure and heed Tinkerman's advice "If you are thinking of a programmable stat make sure it's battery operated and doesn't require a Common wire."

The old reliable mercury switch stats last forever although they do tend to have a wider differential than most people like. I always hate to retire one when it's still doing what it always did. However, new is better... be sure to dispose of it properly.
 

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Discussion Starter · #5 ·
I finally got a chance to replace the tstat yesterday. The jump test worked as described and I was able to match up the wires with the proper terminals on the new tstat, and now the old AC unit works a lot better. The old mercury switch stat would run the AC for 5 min and be off for 10 min; too much cycling. Now it runs for about 10-15 min and off for 45 min. Much better with the new electronic stat.

Thanks for your help guys! :smile:
 
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