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I have a thermostat that controls the AC/Heat upstairs which is located on my bedroom wall. Right below that I have another Millivolt thermostat for the gas fireplace in my bedroom. If I want to upgrade the fireplace thermostat to a smart wifi that needed a c wire can I pull the 24 volts from the other thermostat? Thanks

isherwood
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Chris
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    Don't mess with millivolt thermostats unless you know what you're doing. And never grab "just one wire", always use wires in matched sets. – Harper - Reinstate Monica Dec 19 '19 at 19:35
  • Does this answer your question? [Can I share "C" wire on two thermostats?](https://diy.stackexchange.com/questions/77307/can-i-share-c-wire-on-two-thermostats) – The Ghost of Jon May 13 '21 at 12:31

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No, same as any electrical circuit, you need a two way street on the same circuit and the c wire is only one of those lanes. The R wire is the second lane (most cases). Additionally, you risk frying your furnace's circuit board, and your fireplace blower still needs switching that converts back to millivolt.

Your solution is to pull from a transformer (install a low voltage power supply of some kind) and provide an appropriate switch relay.

See here:

The Ghost of Jon
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This is possible, but it will take some additional components, or a smart thermostat made for the purpose. The main issue is that a regular smart thermostat is going to use power provided to it on the R wire and the C wire. You can think of those as positive and negative (but it's not really because its alternating current). When the thermostat takes an action to turn on heat, what it really does is connect the R wire to the W wire. That is connecting 24v to the W wire, and that is just incompatible with the millivolt system.

So, yes, you can run an R and C wire from the existing thermostat to power the new one, but then you also need to wire up some additional relays to provide the signaling the millivolt system expects. The basic idea is that the 24v that the thermostat switches would be used to drive a relay to provide the contact closure the millivolt system expects. The relays would need to be properly spec'ed for the job.

If you happen to have a smart thermostat made for millivolt that requires 24v only to power itself, that would be very straight forward. I just don't know if such a thermostat exists.

JPhi1618
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  • Here is a [link to a forum post](https://www.doityourself.com/forum/thermostatic-controls/579942-help-installing-nest-millivolt-system.html) talking more about using a relay. You can ask a question about it here if you need more help, but I thought it was a little beyond the scope of this question. – JPhi1618 Dec 19 '19 at 18:05