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Thermostat Wiring

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  • Dec 15th, 2020 4:02 pm
Deal Addict
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Aug 1, 2007
1495 posts

Thermostat Wiring

I'm thinking of replacing our house's old thermostat. Rookie question but why are there two wires connected to R here and how should the orange wire be labelled?
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6 replies
Deal Addict
May 23, 2009
3681 posts
Don't get caught up on the colours, the HVAC installer probably used what was available during install. The labels on the thermostat panel is the important bit.

Assuming you have a furnace and AC unit. The R supplies 24v power to run the HVAC.
One R is probably paired with the black wire connected to the Y terminal for your A/C. The 2nd R probably goes towards for furnace. Hard to confirm as the wires are bunched together in your picture.
Deal Addict
Oct 19, 2020
1047 posts
Your system may have two transformers - a separate one for cooling. it takes having a old furnace or separate heating system from cooling to have that.

You have to see what the wires are connected to on the other end.
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Deal Expert
Aug 2, 2004
38204 posts
East Gwillimbury
It’s hard to tell from your picture, but if you have 5 wires in that jacket, then you are good to go. I would go down to the furnace and see how the wires are connected to the furnace and line it up with the new thermostat. The fifth wire is the C wire that everyone wants
Deal Fanatic
Jan 21, 2018
9652 posts
cmackie wrote: I'm thinking of replacing our house's old thermostat. Rookie question but why are there two wires connected to R here and how should the orange wire be labelled?
It's very common to wire Rh (heat return line) and Rc (cooling return line) together if they go to a common transformer terminal. Some thermostats have a "bridge Rh/Rc" setting or a bridge wire to enable this when you have the two lines connected to separate terminals on the thermostat. In your case you appear to be lacking an Rc label on your orange wire, but that's what it would normally be unless your installer has done something strange at the furnace end.

In any case, 5 wires are probably sufficient, but you may have to do a little repurposing since you don't have a C wire identified at the thermostat right now. Smart thermostats require a C wire so they can draw power from the transformer independently from the heating and cooling circuits (W - Rh and Y - Rc). The G wire is supposed to be an independent fan control line, but you should check if that's what it's actually doing. Some people use that as a C wire.

Whatever you change, remember to label it clearly for the next person who has to figure out what you did!
Deal Fanatic
Jun 24, 2015
8389 posts
The Rh (R) and Rc terminals can be combined at the furnace terminals instead of at the the thermostat terminals, that way you can use the unused wire as a C wire and use more modern thermostats like a nest, honeywell or echo B
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