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Heating & Air Conditioning

Thermostat Wiring Question

13 years ago

First off if this is a stupid question I'm sorry, I'm kind of being brain dead today so it's a possibility. I've got a single stage AC, gas furnace system. It works perfectly fine but I'd like to change the thermostat to the Ecobee Smart Thermostat and I'm running into a small wiring question.

The existing stat has C,R,Y1,W1,G all wired, the Ecobee has all those terminals except for C. To power the Ecobee you can either use a 120V -> 24V adapter (not included of course) or you can hard wire the 24V from the system. Now for the brain dead part, the 24V terminal on the stat (actually they have a separate equipment interface module) has two connections. One gets the C line from the system, what the heck would the other get to complete the circuit? Their wiring diagrams are quite detailed regarding all the other terminals and as good as non-existent for the 24V terminal block.

Thanks in advance

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Comments (4)

weedmeister
13 years ago
Usually R (red) is the 24v hot. C (common) is usually the 'ground'. The instructions should tell you where 24v and ground should go.
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jgmiller
Original Author
13 years ago
Common isn't the same as ground, a ground connection in any circuit should never carry voltage unless something is wrong. You are correct the instructions should show it but in this case they don't, R (red) goes to Rh & Rc just like any thermostat. There is no specific terminal for common but there are TWO terminals for the 24V supply. So my initial assumption was common goes to one and a splice from red goes to the other. However I have a gut feeling that is a bad assumption.
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weedmeister
13 years ago
"Common isn't the same as ground"

That's why I put it in 'quotes'.

The power to the thermostat comes from the ungrounded side of a 120v-24v transformer and is AC. You can confirm this with a voltmeter. (That is to say, neither of the two leads from the low voltage side of the transformer are connected to an earth ground.)

From looking at this new thermostat, your assumption is not a bad one. I believe you will find 24VAC between R and C at all times. 'C' being the common side of the relays and 'R' being the, well, 'return' side. Keep the Rh/Rc jumper in place.

This seems like an interesting device. I've worked on some communicating thermostats from Aprilaire and others. Do you have the Zigbee interface installed? Does the web s/w communicate with the 'stat via wifi?
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jgmiller
Original Author
13 years ago
Thanks I'll give it a try and run common to one terminal and splice red off to the other, after of course checking it with a multimeter. I don't have the Zibee interface on it but yes it is going to communicate on Wifi so we'll see how all this works. It's a pretty cool little device.
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