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I have searched for this topic and the previous answers are not conclusive to me so I will try again with my specific situation. Thank you in advance for your help!

Current situation: I have a 16V 10VA doorbell transformer with a mechanical door chime and regular doorbell button. My current thermostat is not a smart thermostat and it is battery-powered. I don’t have a c-wire but my furnace has a C terminal on its board. My current thermostat wire is 18/4. It is challenging for me to run an extra thermostat wire from furnace to my current t-stat location in order to add c-wire.

My goal is to add ac-wire so I could update my current thermostat to a Google Nest thermostat (link). (Yes, it could be battery powered but I want it to have power from c-wire for solid performance). The picture is its specs from Google website (Vin = 29-42V, Voc=29-42V, Iin = 100-200mA). My plan is as following: I’ll upgrade my current doorbell transformer to a 24V 40VA transformer (like this one from Home depot). Since the door chime is right above my thermostat, I think I could run 2-wire from power supply lines of the door chime (which is 24V) to power my Nest. One is red for 24V supply, 1 is neutral for C wire. And my future Nest connects to the furnace using the current 18/4 wire.

Will this work?

I think it will just because you could alternatively buy a plug-in 24V transformer to power the smart thermostat that requires a c-wire.

Picture 1: thermostat specsthermostat specs

Picture 2: transformer specs transformer specs

Picture 3: wire at the t-stat wire at thermostat end

Picture 4: Wire at the furnace board Wire at furnace board

Picture 5: door chime wiring. If I upgrade the transformer to 24V 40VA, I plan to run wires from white and red "Trans" wire to my smart t-stat Current Door chime wiring

Picture 6: current 16V 10VA transformer current 16V 10VA transformer

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  • I'd go with a separate transformer. I'm not sure how much the doorbell would pull down a shared transformer's output.
    – keshlam
    Commented Feb 15, 2023 at 15:48
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    Does the door chime get continuous power? Sometimes it's a big loop with the button and the chime.
    – KMJ
    Commented Feb 15, 2023 at 16:25
  • Your Nest wants DC, your transformer provides AC. So at the very least, you will need to add an AC/DC conversion (diodes+capacitor+stabiliser).
    – Paul
    Commented Feb 15, 2023 at 16:43
  • I have confirmed that The Google nest t-stat uses 20-30V AC (with C-wire)
    – HongVani
    Commented Feb 15, 2023 at 17:46
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    @Paul HVAC thermostats typically use AC, I'm sure the Nest doesn't care if you give it AC or DC.
    – KMJ
    Commented Feb 15, 2023 at 21:06

2 Answers 2

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This is a total hack, but... the C terminal is present at your outdoor condenser unit. It's the red wire in that two-conductor cable on the furnace control board. Would it be reasonable to run new cable between the thermostat location and the condenser rather than going back to the furnace?

Nobody says the thermostat has to remain in the place it has always been. Is there a reasonable, central location for a thermostat that would be easier to reach with new cable than the current location is? Feel free to abandon the old cable and pull new cable to a new location. Don't forget that in many existing homes there is an exhaust flue that goes straight up from the furnace and water heater area into the attic. You can get a cable from the furnace to the attic through that space, then drop down from the attic into an interior wall.

Finally.. yep, add-a-wire gadgets exist for the times when installing new cable is just not in the cards.

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  • Thanks Greg. I will try to see if the flute in my house has some room for a thermostat wire. I'm aware of "Add-a-wire" adapters but the problem with those is that I would loose my fan's manual control.
    – HongVani
    Commented Feb 19, 2023 at 0:04
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You're better off with a "common-maker" or "add-a-wire" gadget

The problem with your idea is that while many thermostats can accept power from two different transformers, they don't provide you with any control over which transformer energizes which thermostat output -- it's almost always the case that the heating (W/W1 and W2) outputs are energized from Rh, while all other outputs are energized from Rc. Given that you need W, G, and Y to be energized from the same transformer to make this work, I'd scrap the second-transformer idea. Instead, you'll want to look into the various "add-a-wire" or "common-maker" gadgets designed to solve the problem you have -- they multiplex multiple outputs over the same wire in the cable so that you can dedicate one of the newly-freed-up wires to a C wire for your thermostat.

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  • thanks for your explanation about two different transformers. But I'm still wondering how a plug-in transformer would work to add a c-wire to a t-stat?
    – HongVani
    Commented Feb 19, 2023 at 0:02

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