Help With Adding on a New Thermostat

[Ipfreelly]

I have a gas fireplace that is currently wired to a Millivolt thermostat, and it’s working fine. After reading multiple pages, guides, and suggestions, I decided to buy a 24VAC Model MGT2420 Transformer and a Honeywell R8845U Universal Switch. I have the transformer wired to L1 and L2. The gas fireplace is wired to X1 and X2, and I’ve connected the fireplace wire to the thermostat (a new Honeywell 9000) with the corresponding connections: C=C, W=W, and R=R. However, the thermostat never turns on.

I also tried using an old Nest thermostat I had lying around, but it didn’t power on either. When I directly connect the 24VAC to the Nest’s Rc and Rh terminals, it works.

Any help would be much appreciated! I’d prefer not to connect my thermostat directly to the gas fireplace.

 

[Lennox65]

I have a gas fireplace that is currently wired to a Millivolt thermostat, and it’s working fine. After reading multiple pages, guides, and suggestions, I decided to buy a 24VAC Model MGT2420 Transformer and a Honeywell R8845U Universal Switch. I have the transformer wired to L1 and L2. The gas fireplace is wired to X1 and X2, and I’ve connected the fireplace wire to the thermostat (a new Honeywell 9000) with the corresponding connections: C=C, W=W, and R=R. However, the thermostat never turns on.

I also tried using an old Nest thermostat I had lying around, but it didn’t power on either. When I directly connect the 24VAC to the Nest’s Rc and Rh terminals, it works.

Any help would be much appreciated! I’d prefer not to connect my thermostat directly to the gas fireplace.

I have a gas fireplace that is currently wired to a Millivolt thermostat, and it’s working fine.

It seems like you are trying to add a whole level of complexity to what is a simple, functioning system. I would do nothing.

 

[Ipfreelly]

I see your point, but my goal is to integrate the fireplace with a smart thermostat for better control. Since this is my main source of heating, I’d like the ability to start and stop it remotely, not just on a set schedule. I believe this would improve both convenience and efficiency. Given that the current setup isn’t powering the thermostat as expected, I’d really appreciate any insights on what might be wrong or how to resolve it.

 

[Millbilly]

Yeh why are you trying to use anything other than a millivolt tstat for a millivolt appliance? Is this specifically so that you can use the nest?

 

[Ipfreelly]

Yes, the main goal is to integrate a smart thermostat, like the Nest or Honeywell 9000, so I can control the fireplace remotely rather than just on a set schedule. The Honeywell R8845U is intended to bridge that gap, but that’s where I’m running into an issue. I suspect that the 24VAC power supply might not be the correct fit for the way everything is wired. Any thoughts on what might be causing the problem?

 

[gthomas785]

Coupe things, the R8845U has a transformer built in so you could just use that. But the main issue is You put the millivolt wires in the wrong place. X1 and x2 are the transformer outputs. You want them switched by the relay which would be the COM and NO terminals.

Edit: nevermind, I was mistaken. However, L1 and L2 should be the line voltage, not 24V. If you are supplying 24V externally you would leave L1 and L2 unhooked.

• 24vac goes to R and C on the thermostat.
• Same 24vac also goes to R and C on the switching relay.
• W from the thermostat goes to W on the switching relay.
• Millivolt wires go to X1 and X2

 

[gthomas785]

Please see edit above to correct my initial mistake