C wire from transformer when I have both Rh and Rc

I have a CT100 thermostat running on batteries since I do not have a C wire. I have seen guides that you can get a transformer and connect to C and Rh or Rc. My house has separate heating and cooling so I have 5 wires in total. That means I currently already use Rh and Rc. Can I still get a transformer and then use Rh or Rc? If I can, which of the two should I use?

You have a dual transformer system, adding a third transformer is not a good idea, running a C wire from your air handler is the proper way.

I have a same set up (boiler and a separate ac unit in the attic and just went threw all this getting nest a C wire). I’m
not a 100% sure, but I think adding another 24v to Rc or Rh will cause your HVAC trouble.

IF you had a typical furnace with air handler it would be one transformer connected to Rh… You could then easily connect another to Rc and C.

I’m in no way a HVAC professional, this is just something I learned trying to get a C wire to my Nest.

I figured that would be a problem. Thanks. I guess I will just have to continue using batteries. It will be cheaper than hiring or me trying to get a cable to the thermostat.

@dom89 no need for batteries. i have similar thermostat by radio thermostat and this solution works perfect and only $22!

item to buy
Venstar ACC0410 Add-A-Wire Accessory for All 24 VAC Thermostats (4 to 5 Wires), White https://www.amazon.com/dp/B01IF3QXMC/ref=cm_sw_r_cp_api_WaqdybHK2P9G6

how to

ive also added a 24v power adapter for another feiend with similar issue because their hvac was too old and looked nothing like a conventional board.


Do you have a jumper between Rh and Rc?

Are you sure there is not a spare wire in your wire bundle?

Also Google about how to use the G wire as a C wire.

The add a wire mentioned above looks interesting, I haven’t tried it, I just ran a new wire. (in my case it was easy to do)

I just did this in my camper. I added a 24v transformer and I used C and Rc but I don’t think it matters.

I would do this but I don’t have access my units. I was only able to change the thermostat.

No jumper. As mentioned my heat and cool are different units. There are two different cable bundles. I use replaced an old honey well round thermostat.

I thought of using the G wire but I’m one of the few people who actually turns on just the fan so I can’t.

how do you not have access? if not buy an power adapter, cut the wire and insert in slots to power wifi


He has a two transformer situation his heat supplies 24v to Rh and his cool supplies 24v to Rc. If he added a third 24v transformer to Rc or Rh wouldt that increase it to 48v and be way too much for is HVAC? this is the point I got to and just decided to run a C wire… I really don’t know what would happen but didn’t want to chance blowing a circuit board.

However without access to the units he’s out of luck running a C wire as well as (add-a-wire), or even the possibility of using a G wire.

I assume you live in a condo or apartment where the units are outside of your living space?

@dom89 have you seen this? i know not same thermostat but same kinda setup right?
see page 4,5

Correct, the units are outside of my living space. I could do the G wire but didn’t want to lose the fan functionality.

The CT100 instructions are the same. My setup is the same as seen on page 14 section 5 wire heat/cool. That means no jumper between Rh and Rc. Both are in use.

Wouldn’t you have to have access to the units to move the G wire to the common?

you would need access to the HVAC board and he does not.
@dom89 see bottom of page 6. i do not think it will overload your system at all. because nothing about it saying it would in manual.

You’re right. I was not thinking that I would have to move the wire at the board.

That’s promising. I guess I’ll have to research it further to make sure I don’t end up blowing anything up lol

this is what i did for my sister and she covered with a canvas painting

What you could do is get a 24v normally open SPST relay (about 10/15 dollars) from your local HVAC supply or Amazon.

Remove the Rh and W1 wire from the thermostat and set aside for now… Take your 24v transformer put one wire in Rh the other in C. Connect a 2nd wire to C and run that to one terminal on the power side of the relay, connect a wire to W1 on the thermostat and run that to the other terminal on the power side of your relay… Then connect the two wires you set aside to the normally open side of your relay.

When your thermostat calls for heat it will just send 24v to the relay and close it, when the relay closes it will turn your heat on. Meanwhile the Transformer you plugged in being connected to Rh and C will let your thermostat slurp all the power it wants and not need batteries.