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I'm replacing an old battery-powered thermostat with one requiring a C-wire. Unfortunately, the wires I currently have sticking from the wall are only two: White (W) and Red (R and Rc jumpered together).

If, as seems apparent, I must add a separate 24VAC transformer, what are my options? There is a standard 1-gang dual power outlet nearby, which is rarely used. Is there a transformer on the market small enough to fit into the 1-gang box instead of the outlet?

I'd then pull the wire behind sheetrock and be happy...

Update:

  • The new thermostat I'm trying to install is Honeywell TH8110R1008. Its basic functionality works without C-wire (on 4 AA-batteries), but to talk to remote sensors (the functionality, for which I bought it), 24VAC power is required.
  • The thermostats throughout the house are all connected to a TACO ZVC 406 box in the boiler room. The box has a transformer inside it, but the cables running from it to each thermostat have only two wires each: white and red. The installer must've saved a fraction of a penny per foot of the cable, but it is too late now.
  • To folks worrying about low/high-voltage separation -- I'm looking for a transformer, that would fit in a 1-gang box instead of the dual power socket currently there. There are external transformers out there, which plug-into the power socket and can be semi-permanently wired (like this one). But I'd like it to hide inside the wall, if at all possible, so that no wires are visible.
  • Regarding phase -- if I do add a separate transformer, I'll remove the jumper currently connecting the R and Rc terminals on the thermostat. The red-wire will remain on the R-terminal and the additional transformer will connect to the Rc and C-terminals.
  • Finally, the furnace and everything else throughout the house all use the same grounding. Can I "cheat" and simply connect the C-terminal of the thermostat to the nearby socket's ground?
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    Thermostats which require a C wire, are made for furnaces which have 24V controls. Which means they already have a 24V transformer somewhere on the furnace. Almost universally, people who need 24V pull it from there. Using a second transformer is tricky. Some systems use millivolt thermostats (which have a notable ability to function even when the power is out). There are ways to make Nests etc. work with those, but it's a little complicated. Which do you have? – Harper Aug 25 '16 at 4:33
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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, that means there would be 24v between your C and R, and another 24v between R and W when the furnace is off. So the thermostat would see 48V between C and W. You'd need to check with the thermostat maker to see if it can handle that.

May not need C

Some thermostats (perhaps not yours) can also scavenge the power they need simply by being in series with the two wires R and W. They do not need a C wire. However this trick only works if the furnace's control relay draws a fair amount of current. If it is too efficient, the thermostat can't scavenge enough power to function, and then they give you the advice to get a C wire.

So, you can fix this by making your furnace relay less efficient. I would say put a resistor in parallel with W and C on the furnace, except for that "must be listed" rule. So, use a furnace or air conditioning relay, which are readily available. Make sure your furnace's transformer has enough power to drive it. The purpose of the relay coil is to waste energy; the contacts are connected to nothing.

  • My "furnace" transformer is really a TACO ZVC 406 box -- it drives 5 thermostats throughout the house -- I don't think, power is an issue. So, how would I wire things -- is there a schema somewhere you can link to or post (with or without relay)? Thanks! – Mikhail T. Aug 25 '16 at 14:36
  • As for phase -- people seem to recommend removing the jumper between R and Rc. Would not this make two completely separate circuits with two different transformers solving the phase question? – Mikhail T. Aug 25 '16 at 14:38
  • One thing -- Code does allow for resistors that are not part of equipment (just don't stick them in a plastic box, and wire them using 90degC wire, more or less, is the gist of Art. 470) – ThreePhaseEel Aug 26 '16 at 0:39
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Ok, I ended up thus:

  • Use the existing two wires for power -- the red remained where it was, the white I switched to the C-terminal on the thermostat and to the "common" contact in the boiler room.
  • Use Honeywell's "Equipment Interface Module" (EIM) to allow the thermostat to call for heat wirelessly -- over the same RedLink radio it uses to talk to remote sensors.

Hooking up the EIM was not easy in itself -- because I already had the thermostat initialized, I had to reset it to defaults first. But now it all works and I did not need to add a separate transformer after all.

  • That's an interesting solution. But aren't you tied to those thermostats now? – Jack Dec 12 '18 at 9:25
  • Tied? You mean vendor lock-in? Well, if ever I decide to go for a different brand, I can undo this, I still control the wires... – Mikhail T. Dec 13 '18 at 0:57

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