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The thermostat at my home is set up with only the 24 power and return through the relay. I would like to add a 'smart' thermostat, however the line is fairly well buried in the wall/basement ceiling and will not be fun to rewire to add the 'c' return path. However, I have fairly easy access to all walls via the main ceiling. If I were to add another 24v transformer, could I run an independent line to power the thermostat? My concern is that the input power for the thermostat and the relay will be common, this thwarting my plan. Would the inputs be jumped that I can remove, or is my screwball idea hopeless?

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up vote 1 down vote accepted

I think that in most cases you would run into the problem you described where you can't really provide "separate" power to the thermostat. This would obviously depend on the particular thermostat, however.

One smart thermostat manufacturer provides a 2-wire accessory kit. Looks like it completely replaces the 24VAC supply in the furnace.


Also, definitely check out this Q&A on thermostat wiring.

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If there's an easy way to get from the basement to the main ceiling, then you could just run an entirely new cable for your thermostat. There's nothing that says you have to follow the existing path. Just abandon the old cable.

You could even take advantage of this to move the thermostat to a better location, if you were so inclined.

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Can You?


Should You?


How Might You?

A simplified version of a thermostat schematic, might look something like this...

Thermostat Wiring

To power the thermostat from an alternate power source, you'd have to modify the circuitry so that the additional features were isolated from the control portion of the thermostat.

Thermostat with alternate power source

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If you absolutely have to run a separate power source, all you need is another relay, like this one: http://www.jameco.com/1/1/46908-t92p7d22-24-24v-general-purpose-industrial-power-relay.html

enter image description here

  • Connect the power source to the thermostat R and C wires.
  • Connect R on the thermostat to one of the coil terminals on the relay.
  • Connect W on the thermostat to the other coil terminal.
  • Connect the original R wire to the common terminal on the relay.
  • Connect the original W wire to the NO terminal on the relay.
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I think that the 2nd line in the list of the previous response is not correct. Instead of "Connect R on the thermostat to one of the coil terminals on the relay", it should be "Connect C on the thermostat to one of the coil terminals on the relay". The relay coil needs to be powered by 24V if activated (calling for heat), which is done by the W wire (which has the 24V on if the thermostat calls for heat) and the C wire for the return of the 24V. This worked for me.

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