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Old 10-09-2019, 02:14 PM   #1
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Combining Thermostat Wires


When the original homeowner installed their central air they already had boiler baseboard heat that had it's own thermostat. Instead of taking a couple extra hours to combine the thermostats into one when installing the AC he just had separate thermostats about 50 feet apart in the house. I find this irritating and would also like to have a smart thermostat for free from my utility company so I combined them into one location that is also not by the door leading in from the garage or near a vent and is generally more centrally located in the house. That said, for some reason it isn't working. The central AC has one yellow and one red wire that were being used (there is a tan, orange, blue, and green not being used that run's down to the blower control in the basement). Touch the red and yellow together and it kicks on. I had this hooked up to a digital thermostat already and it worked fine. The boiler heat only has two wires: red and white. Same thing, touch them together and it kicks on the heat. Both were originally hooked up to old mercury thermostats. I took the heat thermostat out during a remodel and just now ran the wires to the new spot. The AC I bought a temporary digital thermostat that was programmable. I put in the red and yellow and it worked fine. But now when I combine them, neither work...

I put the red and yellow from the AC into RC and Y into the new thermostat. I then put the red and white from the heat into RH and W1 from the heat. There is also a W2, but that didn't work either. I did not hook up the blue, green, orange, AC white, or brown wires. Is that causing the issue now that they're combined additional wires need to be hooked into the back to work? I have manually touched together the heat and AC wires separately to make sure they work and they do. It's an older thermostat as well, so maybe that's part of what it doesn't like, I don't know. I've attached a picture of the wiring to show you.

https://photos.app.goo.gl/vJ9aLS2KqrU6Q7h66 (and yes I know images are allowed here but it never works)
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Old 10-09-2019, 02:15 PM   #2
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Re: Combining Thermostat Wires


Strangely the AC just started working, but still nothing on the heat. I have my doubts about how long the AC will work. I intentionally did all of this when it was the time of year that I didn't really need either that much.
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Old 10-09-2019, 02:23 PM   #3
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Re: Combining Thermostat Wires


did you remove jumper between r and rc? you must.
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Old 10-09-2019, 02:48 PM   #4
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Re: Combining Thermostat Wires


There never was a jumper wire. These were on separate thermostats prior and there's no jumper wire installed in the new digital thermostat unit.
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Old 10-09-2019, 02:50 PM   #5
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Re: Combining Thermostat Wires


check the install manual carefully - you may need to do something to it to use a two transformer system.

you should have a g wire.
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Old 10-09-2019, 04:05 PM   #6
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Re: Combining Thermostat Wires


I figured it out. I think typically your C wire comes from your heat. Or if your heat and air are combined then comes from central air unit in general. But since there was no C wire for the heat I had to use the C wire from the AC. Which previously, was unused so it wasn't connected to C in the thermostat. That, and there's no standard color for C really. So I took a guess that it was the blue wire and now heat and AC work respectively!
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Old 10-10-2019, 09:27 AM   #7
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Re: Combining Thermostat Wires


No typically the C is from the AC air handler.

Also some thermostats need a jumper between W1 and W2 when you're not using the separate heat stages.

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