We have a KozyHeat wood fireplace connected to our older gas furnace. The idea is that the furnace serves as a fan when the fireplace is hot, or will use gas to heat the house if the fireplace is cold. This works great, as we can save lots on gas by heating with wood. We would like to upgrade our older central thermostat to a smart one, and need some guidance on which one would support this system. KozyHeat told us we need something with a “low volt/no volt” setup...whatever that means. And a local KozyHeat dealer told us it can’t be done. Help?

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    Millivolt control is normally used on older gas systems, the issue is controlling the fan separately from the gas , not a big issue but we would need to know more about your furnace most older units do not have a fan only but it can be set up with relays and controlled as you want. Is there a schematic on the furnace? If you can post it we can help you wire it up. – Ed Beal Feb 25 at 6:11
  • Yes, can you post wiring diagrams and/or manuals (or at least make and model) for all pieces of equipment involved? – ThreePhaseEel Feb 25 at 12:41

Do you get much heat flow out of the fireplace without the fan running? Would it be possible to locate a Tstat to sense the heat from the fireplace? I'm thinking of using a Tstat that supports air conditioning, but repurpose the A/C terminal and connect to the "fan on" terminal on the furnace. That way, when the Tstat detects warmth from the FP it would turn on "A/C" which is now actually just the blower on the furnace. When the fire goes out and the house cools down, the Tstat would turn on the furnace normally.

I'm not an HVAC guy, so take this with a grain or two of salt. You might need a pro to get this right. But it seems to me like an approach that might work. Ed, Jack and Harper? Ideas? Comments?

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