Question about Honeywell Programmable Thermostat Heater

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Blower stays on with new themostat

My old programmable thermostat got fried on Jan 1 with no stores openned. Managed to find a UPM THM303M thermostat.

We AC and gas furnace with blower. 4 wires coming out of thermo: red, green, white, black.

Prior thermo had red to R, green to G, white to W and I believe black to Y but not 100 % certain.

Installed same layout to new thermo but now blower stays on indefinitely when fan set to auto. When I switch the Cool / Heat switch to the off position, the blower will stop but I don't want the blower/fan on one the furnace stops.


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  • jesterbryanc Jan 04, 2009

    Thanks, machtc1

    Old fried t-stat issue not related. Two prongs (wife) got pushed together on the t-stat itself melting a part of the circuit board.

    Don't notice wire touching on new t-stat install. Tried black on Y and circuit breaker at the panel tripped.

    Black is currently back on B on the new t-stat not on Y. All works but the fan/blower stays on indefinately. Green wire still in G.

    Will be re-tightening the wires around the screws just in case this is the problem.

    Also bought a different brand (Honeywell) on the off chance that the UPM is defective.

    We have a gas furnace and external AC. Red wire is tape marked RC. Can you identify which wire runs which function? This might help me figure out the right combination.

    Thanks for all whom contribute to helping out!


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Check to see if there is a short between the green wire and the red wire. Green is the fan control wire for AC. Sounds like some wires may be touching. How did the old t-stat get fried?

Posted on Jan 02, 2009

  • Matt McElhenny
    Matt McElhenny Jan 05, 2009

    Red R-24v ac from furnace(R) terminal board to T-stat Rh/Rc

    White W-24v from T-stat to furnace (W)termnal board(during call for heat)

    Green G-24v from T-stat to furnace(G) terminal board(during call for AC or FAN ON)

    Yellow Y-24v from T-stat to furnace (Y)terminal board or out door condensing unit/or both.

    The furnace will also have a C terminal (common)

    Look at the furnace circuit board and see what black is hooked up to.


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