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Old 05-31-2020, 10:44 AM   #1
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Wifi Thermostat wiring two transformers


Hello all back with a new problem. I'm currently running a wifi thermostat for heat only with an external 24v transformer because the gas heat only has two wires running to the Thermostat so no chance to grab a third wire for "C" this works fine. Now I want to add my AC unit's thermostat to the wfi. the problem is when they installed the AC handler they only ran a 3 wire to the AC thermostat so again not chance to get a "c" wire back to the Handler.

Now what? I now I have two separate systems and I'm sure I can't just take the Y & C wires to the wifi and have it work, can I?

Thanks
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Old 05-31-2020, 11:04 AM   #2
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Re: Wifi Thermostat wiring two transformers


If you are already powering the wifi stat with its own external transformer, you don't need a C (common) from any other source.

To control the a/c system, you just need to connect the R, G and Y terminals to the stat. That gives the stat the ability to start the compressor while making sure the fan will also be started. It also allows the stat to run the fan manually without starting the compressor.
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Old 05-31-2020, 11:19 AM   #3
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Re: Wifi Thermostat wiring two transformers


Thanks I should have said one key point I have the red connected already to the "R" for heat and the external transformer is connected to the "C" and the "RC" of the wifi. Can I also take the red from the AC and connect it to the "RC" also ? I thought that may cause a problem?

thanks
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Old 05-31-2020, 11:31 AM   #4
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Re: Wifi Thermostat wiring two transformers


Yes, that could be a problem. You should remove the external transformer connection from the rc terminal before connecting the R from the furnace.

FYI, it could work with both transformers connected if you first verified that they were phased properly, such as is done when twinning thermostats. You don't need to do twinning in your situation so not to worry... just remove the cross connection jumper you have in effect between the rh and rc.
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Old 05-31-2020, 11:47 AM   #5
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Re: Wifi Thermostat wiring two transformers


Quote:
Originally Posted by surferdude2 View Post
Yes, that could be a problem. You should remove the external transformer connection from the rc terminal before connecting the R from the furnace.

FYI, it could work with both transformers connected if you first verified that they were phased properly, such as is done when twinning thermostats. You don't need to do twinning in your situation so not to worry... just remove the cross connection jumper you have in effect between the rh and rc.
Got you. I thought about making a switch for summer and winter that would allow me to swap the external transformer lead between R and RC and cut off the other transformer
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