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

 

[Ipfreelly]

Please see edit above to correct my initial mistake

Sorry, last question. Seeing that the R8845U is rated for 120V line power, would it make more sense to wire a plug to L1 and L2 and just plug it into an outlet, removing the 24VAC adapter altogether? I had planned on putting the system on a UPS backup, which would make for a cleaner installation. With the UPS, I could be without power for weeks before it would be drained.

 

[gthomas785]

Sorry, last question. Seeing that the R8845U is rated for 120V line power, would it make more sense to wire a plug to L1 and L2 and just plug it into an outlet, removing the 24VAC adapter altogether? I had planned on putting the system on a UPS backup, which would make for a cleaner installation. With the UPS, I could be without power for weeks before it would be drained.

I think that is how it's meant to be set up and would make sense to me. Feed 120v to the Honeywell and use R and C from that to power the thermostat.

Running on a UPS, just be aware those transformers draw significant idle current although with a low power factor. Not sure how nicely that would play with your UPS inverter but should be worth a try.

 

[Ipfreelly]

I think that is how it's meant to be set up and would make sense to me. Feed 120v to the Honeywell and use R and C from that to power the thermostat.

Running on a UPS, just be aware those transformers draw significant idle current although with a low power factor. Not sure how nicely that would play with your UPS inverter but should be worth a try.

I wired up the black wire to L1, the white wire to L2, and the green to the ground screw and verified through a volt meter that I have 120V. When I tested C and R I do not have 24v, I actually do not have anything.

 

[gthomas785]

I wired up the black wire to L1, the white wire to L2, and the green to the ground screw and verified through a volt meter that I have 120V. When I tested C and R I do not have 24v, I actually do not have anything.

That's odd, did you check the fuse?

 

[Ipfreelly]

That's odd, did you check the fuse?

Well, I pulled the fuse, and it tested fine. But when I placed it back into the slot, I had power again—then I didn’t. It seems the fuse placement is a bit touchy. After playing around with it for a bit, everything seems to be working normally now.

Thank you so much for all your help! I really appreciate you taking the time out of your day to guide me through this. It truly means a lot!

 

[gthomas785]

Well, I pulled the fuse, and it tested fine. But when I placed it back into the slot, I had power again—then I didn’t. It seems the fuse placement is a bit touchy. After playing around with it for a bit, everything seems to be working normally now.

Thank you so much for all your help! I really appreciate you taking the time out of your day to guide me through this. It truly means a lot!

Glad you got it working. Not cool that the fuse holder is loose like that, if it gives you problems I'd be asking for a replacement unit... Would suck to have your heat go out because of a loose fuse...