I have an all electric air handler/furnace connected to a separate AC unit and it is also a heat pump. I've purchased the latest and greatest Honeywell WiFi "Smart" Thermostat and it needs a C wire. Because the furnace is all electric (not gas, etc.), Honeywell told me it's either not compatible or I'd need to hire a technician to determine.

Following are pictures which show the current thermostate wiring, furnace model #, terminal wiring and wiring diagram (it's a furnace from 1989, so please forgive any lack of clarity on the diagram).

I've read the other posts about the C wire, but am unable to determine from those if the B would work in my situation as a replacement for the C. I already have an extra Blue wire wired to B and am hoping I can use that for the C wire, but I don't want to damage anything and was concerned from some other reading that it may be for the heat pump.

Higher resolution picture of wiring diagram: http://i.imgur.com/68bhMjn.jpg

Any help would be greatly appreciated!

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Wiring Diagram

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1 Answer 1


According to the schematic, the B terminal does appear to be connected to the transformer.

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I'd actually trace the wiring in the unit just to confirm, but based on the schematic it looks good.

You'll want to make sure the transformer has enough power to supply the additional load of the new thermostat, but typically it isn't a problem. If you hook up the new thermostat, and the fuse inside the unit blows. That would be an indicator that the thermostat is drawing too much juice.


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