Add-on 24v A/C common to the C terminal on the thermostat

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Glen G

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3 wires come from the ac unit outside, 2 from the main unit.
They don't seem to come together except at the thermostat except the common wire from the 24v A/C transformer.
Main unit no screw terminals, just wire nutted.

Here is my question.

My thermostat is mounted in a main hall on a wall and behind that wall is my master bedroom closet with power 110v available.
Can I just add a 2nd 24v A/C transformer for my C wire?
I read where some have done this.
Some say only use the Common side a the C-wire and don't use the hot side of the transformer.
Others say piggy-back the hot side of the transformer to your R wire on the Thermostat.
I have a RC and a RH wire on my thermostat now.
So I see 3 ways to do this.

Add-on 24v A/C common to the C terminal on the thermostat (one wire used only)
Add-on 24v A/C common to the C terminal on the thermostat and the hot on the transformer to RH on the thermostat
Add-on 24v A/C common to the C terminal on the thermostat and the hot on the transformer to RC on the thermostat
I can do any of the above 3 options easily.
 

JerryR

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Yes, you can simply add a small transformer to substitute the c wire.

You connect the transformer outputs to the C and Rh terminals on the thermostat.

I've used 18vac alarm system transformers I had extra and worked fine.

Here is a typical YouTube video that should help you.
 

Jadnashua

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Just throwing in a new transformer could be a big problem unless it was done right. You have 240vac coming into the house, and the current transformer in the furnace may have 240vac going into it (unlikely), but may also only use L1 or L2 and neutral to supply the transformer (much more likely). If yours used L1-N, and the supply you want to tap into is L2-N, you'll have problems. Plus, it will unbalance things...the transformer is designed to have any power be able to be turned on/off and to complete its path from just that transformer in the furnace, not from some other supply.

If you think about it...all the thermostat is is a smart on/off switch. The power in gets switched to one of various things: the fan, the furnace, or the a/c unit. That power goes into that unit, and to complete its circuit, must get back to the transformer that supplied it...that won't happen if you tap power into the thermostat from somewhere else...you'd always have some power running in things that wasn't designed to be there.

The safest way to do this is to run a new cable from the furnace that would bring its common wire to the thermostat. You could do things with an isolation relay(s) if you wanted, and keep things safer. You might need as many as three of them if you needed to control heat/air-conditioning/fan.

I was lucky on mine...the existing cable had an unused wire in it, and I just connected both ends to the appropriate terminals.
 

JerryR

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External transformer providing voltage between the C & Rh terminals of digital thermostat is safe and a common solution. The only thing it provides is to keep the digital thermostat working when internal batteries can't keep up due to age etc. also if the thermostat has a wifi module it supplies power for the wifi.

In fact Nest, Honeywell, Filtrete all recommend adding an external transformer if pulling an additional wire is not feasible.

Here's a proven, safe 24 vac transformer that's commonly used. Being it's an a/c voltage , polarity is not an issue.


If you have any doubts just read the reviews on the Amazon link above.
 
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Jadnashua

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This is a good tutorial of how thermostats are wired and work

https://www.epatest.com/store/resources/images/misc/how-a-thermostat-operates.pdf

Power needs to go back to the transformer that supplies it to complete the circuit properly. Putting a different power source on the Rh terminal, without properly providing a path back to it's common in the things it is powering or being designed for it, can be problematic. Proceed at your own risk. If you haphazardly insert a second source of power into the thermostat, in this case the Rh terminal, when the thermostat calls for heating, it would apply that voltage to the W wire, and the commons are different. It might work, it might not. Worst case, they'll be on opposite phases, and burn something up.
 

Stuff

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With these two prong plug-in transformers the secondary 24 volt wiring is floating. As long as the C terminal on the thermostat is only connected to this transformer what you are doing is providing a voltage (differential) between R and C that the thermostat's power supply needs. The Rs will all still need to be connected so that the circuit can be complete to the furnace/air handler. It doesn't matter what the furnace common is - voltage or phase - as it is not present at the thermostat. Same reason a bird can sit on a 22kv electric line.

The only time not to connect all of the Rs would be if you have separate Rh and Rc going to separate transformers (such as boiler with a/c).
 

JerryR

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First, the C wire is only used to power the thermostat clock, backup and if included, charge thermostat rechargeable batteries (nest).

Unless the system is OLD line voltage controls with big wire nuts or millivolt control there is no risk providing an external 24 vac source between the C-Rh. Ecobee, Honeywell, Nest phone support routinely advises using an external transformer as an alternate solution.

I understand your theory of out of phase but it's unfounded with only the Rh commonly connected to system and external transformer. Step down transformers isolate from line source. What you would never want to do is connect an external transformer AND a system wire to the C terminal of a thermostat. If you do that then you're asking for smoke.


Read the reviews at the above link for a typical 24vac transformer. Reports from Nest users! Honeywell users! Ecobee users and Filtrete users. Exceptional 92% positive 4-5 reviews. Not one of the negative reviews stated damage to thermostat. There are a few early transformer failures and some obvious reviews by incompetent DIYers.
 
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DonL

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If you wire it wrong you can end up with 48 volts, if the transformers are in series. :eek:
 
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