Help with Wiring a Vintage 4-Wire Honeywell Thermostat

Old 10-18-12, 01:49 PM
Smargus's Avatar
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Help with Wiring a Vintage 4-Wire Honeywell Thermostat

Hello, my name is Allison and am an incorrigible nerd when it comes to historic home restoration and am hoping a forum member may be able to assist me via the boards in installing a vintage 1931 Honeywell Mercury Switch thermostat in place of the existing digital programmable thermostat.

Our home has a central air conditioning unit located outside and a forced air furnace located in the basement.

As such the existing digital programmable thermostat has five wires attached,
(Y,G, C, W, Rh, Rc*)



Whereas the vintage Honeywell is a one-stage heating/one-stage cooling with four wire attachment screws.


(W, G, R, B)

Could someone please advise a proper wire hook-up schematic, and thank you in advance for any help someone may provide.
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Old 10-18-12, 05:29 PM
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Personally, I think you're carrying the historic restoration theme a bit too far. Why not keep the energy efficient programmable thermo in service and move it to a hidden location and just screw the antique to the wall for looks?
Old 10-18-12, 07:18 PM
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Connect the existing wires to the new (old) thermostat using the same colors and initials as the old, e.g. red to R, yellow to Y, etc. insulate and unused wires (C).

For reference here is the standard color code for HVAC thermostat wiring.

R(ed)= supply power (usually also jumped to Rc on thermostat.)
W(hite)= heating signal (R to W) when in heating mode.
Y(ellow)= cooling signal (R to Y) when in cooling mode.
G(reen) = manual fan (blower) operation. Normally leave Auto-Fan-On switch on thermostat in the Auto position.
C(ommon) may be almost any color other than the four already listed. Blue is commonly used. This is for power to the thermostat for indicator lights and such. Not always installed.
Old 10-19-12, 02:11 AM
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What is the model number of your vintage stat? Is it made for cooling?

I would suggest as toolmon did, that you install the stat only as a dummy and install a thermostat with a remote indoor sensor to a location that is hidden.

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