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Honeywell Thermostat Wiring and Repair Question

Posted by rak710 (My Page) on
Wed, Sep 18, 13 at 16:31

I have an old Honeywell CT3400A1007 thermostat. Every winter when I first go to turn it on, the heat won�t work. We have found that sometimes opening the furnace door and then replacing it and waiting a while works, along with various other inconsistently successful tricks. Once it gets going it is usually fine except for once or twice all winter. A few months ago our tech finally decided it was one of the channels, I believe the W, in the thermostat. He said it just needs to be replaced. For a variety of reasons, I prefer not to.

I did some research and here is what I find. There are 4 wires, white, green, yellow and red, connected to W, G, R and Y, of course. I used my digital multimeter to check voltage. If I touch the red probe to R and then the black to the other terminals, I get readings of 27.5, except for the W which reads 12.5 or so. So I assume this confirms a wiring problem.

Just to be clear, when putting on the heat, neither the fan nor the furnace comes on. The AC and fan work fine otherwise. The furnace is not the issue, and we even had a new mother board installed recently just in case. The W terminal reads the same no matter if the thermostat is set to Off or to Heat. And for right now, it is not working. I can touch any of the white wire that is exposed, not just the screw, and still get 12.5 V.

Do I just take the thermostat off the wall and maybe clean the wiring and replace it? Could the problem be in the thermostat itself? If so, can it be fixed? I assume the latter is the case. I don�t want to start pulling it off the wall or apart until I have some idea what I am looking for in there. If I remember correctly, I think the tech was able to bypass the W terminal to get it on, but I am uncertain of this or the details. Again, I am not trying to save money. I just don�t want a new thermostat.

Thanks for any suggestions.

Follow-Up Postings:

RE: Honeywell Thermostat Wiring and Repair Question

A lower voltage by itself would not indicate a wiring problem. It think it depends more on the brand and the thermostat. You might get a better response over in the "Heating and Air Conditioning" forum (but this one is more active).

RE: Honeywell Thermostat Wiring and Repair Question

One simple troubleshooting method would be to swap the yellow and the white wire connections on both ends (furnace and thermostat). If that solves the problem then you have verified that its just the white wire that is causing a problem with the heat function. If that does not resolve the problem, then it may be the thermostat. You could troubleshoot that by borrowing a working thermostat, or buying an inexpensive to try and return.

Thermostat wire is fairly small and light, and seems to be often installed by the HVAC company. They don't take much time to tack it down, so you might be able to use the existing wire to pull in a new one. It just depends on how many turns it takes and whether you have access. Even if you have to run a new wire using another route, the wire itself is quite inexpensive.


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