Thermostat Wiring (Common Wire)


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Old 07-30-14, 04:45 PM
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Thermostat Wiring (Common Wire)

Hey

Just purchased two Honeywell RTH7600D thermostats to replace old White-Rogers units. I am starting with my downstairs unit and have a question. The old thermostat had 4 wires (W, RH, G, Y), these are self explanatory on the new one. There is also a fifth wire in the wall that's not being used (Blue). I went down to the furnace board and saw that the blue wire is not being used on the furnace side as well. There are two groups of wires coming out of the furnace board (W, RH, G, Y, Blue) going to thermostat, and (White, Red) going to AC. The White going to the AC is connected to COM on furnace, the Red is connected to "Y" on furnace.

My question is can i also hook the blue wire into the "COM" and furnace and use this as a "C" wire on the new thermostat?

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Old 07-30-14, 04:50 PM
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FYI I approved your post..

Welcome to the forums.....

New members need so many posts before they approve automatically. This is because of links or pictures that our spam system thinks is spam..

A pro will be with you shortly to answer your questions...

 
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Old 07-30-14, 04:51 PM
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Thank You, I may have accidentally tried to post this twice. Please delete the second one.

Thanks
 
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Old 07-30-14, 04:56 PM
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My question is can i also hook the blue wire into the "COM" and furnace and use this as a "C" wire on the new thermostat?

Im pretty sure you can, as that is the only way really you will get 24v to the T stat.... But let one of the HVAC pros confirm this...

( I would have probably tried it already... But I know some of those stats are $$$$)
 
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Old 07-30-14, 05:07 PM
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My question is can i also hook the blue wire into the "COM" and furnace and use this as a "C" wire on the new thermostat?
You certainly can. There will be two wires on the C terminal which is fine.
 
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Old 07-30-14, 06:55 PM
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Thanks Guys, worked like a charm. I will do the upstairs tomorrow. Its a heat pump thermostat so I'll investigate how its wired and ask any questions if I have any.
 
 

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