Is there a 24VAC transformer for a thermostat, that fits in 1-gang wall-cavity?


Any transformer you use needs to be listed specifically for use in 120VAC wiring subject to the National Electrical Code. Do not use some random transformer out of an electronics supply catalog, even if it is UL listed as a component.

One of the Code requirements is that high voltage and 24VAC must be separated by a divider etc.

Generally these transformers are readily available in forms which solve that problem. For instance, a common form is a transformer built into an electrical box lid. The low viltage terminals are on the outside of the lid, solving the separtion problem.

I see these often as 2-gang box covers (4”) and I swear I've seen them in a 1-gang (2”x4”). We're not a product recommendation site but your question is do they exist; the answer is yes.

Phase issues

The problem you have adding a second transformer is phase. The two transformers will both be attached to R. If your transformer is out of phase with the furnace transformer,...

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Assuming the heater is in the basement, you can fish it up through the wall that the thermostat is on. Frame walls are hollow - and you can usually find the wall in question as other wires or plumbing is likely already going up into it.

There are special "fish" wires made to help you pull such wires, but I would do it this way....I'd cut a hole for the thermostat about 1" round or so and then, after doing careful measurement, drill a hole up from the basement into the hollow of the wall right below that. Then I'd push a stiff wire up from the bottom until I could grab it with a tool through the hole. Then tape your thermostat wire to that and pull it down.

If you are uncomfortable with this you can mount it on the outside of the wall surface using "wire mold" which is available in plastic or...

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