Trying to control millivolt fireplace with Honeywell Z wave thermostat

Hey, I’m new to the forum, and new to Z-wave. I have a VeraLite and have so far successfully set up locks and lights, but I am having big problems with my thermostat setup. We replaced the fireplace in our condo as part of a larger remodel. The fireplace is a natural gas insert, Regency model P33CE. My instructions to the installer were to provide me wiring for a thermostat hookup about 15 feet away, as this fireplace/blower is to be the primary heat source for the upstairs of the condo. What I got out of the wall was a red/white 2-wire setup, running at 750 mv. If I touch the wires together, the fireplace lights as desired.

My research for Z-wave millivolt-compatible thermostats left few options, and I ended up with the Honeywell TH8320ZW, which also shipped with the wiring module THP9045A. Although the cover page for the thermostat installation guide clearly states that it is compatible with 750 mV heating systems, the wiring examples in the guide ALL state that 24VAC must be connected at the common pin. The wiring module is designed “to be used with applicable thermostats in which a 24V common is required and there are not enough wires.” That sounded promising, but there are no wiring examples showing how to connect to the thermostat with only a 2-wire cable at the thermostat location. My calls to Honeywell (India) were met with confused tech reps who could not resolve my issue.

Because the 2-wire cable is already behind drywall and not easily replaced, I am not sure if I can successfully use this thermostat (or any other Z-wave thermostat for that matter). This is my main question–am I defeated at this point? I have considered adding a 24V transformer at the fireplace location, but I don’t know how to wire the relay or if it will ultimately work given my 2 wire limitation. I could relocate the thermostat location to minimize drywall disruption (if more wires are required).

The other possible solution I am considering would be to find another Z-wave temp sensor, and then wire my fireplace as a Z-wave switch, and then use the temp sensor as a trigger to light the fireplace. This presents new problems, however, as the wall switches I am aware of are designed to work with line power, and actually require that power to work.

I am hoping to avoid an expensive HVAC service call, so if you have any ideas for ultimate success, please provide. Thank you!


You need to get a 24V source for the Z-Wave thermostats.
That will hook up to Red and Common.
You will continue to use the Red and White wires from the Fireplace as
Red - R and White -W on the thermostat.

Note: Two wires attached to “R”
I would recommend that you replace the thermostat wire with at least 3 wires
and since your at it … do 4. Then put the 24V transformer somewhere where you can get power and tap this thermostat wire.

Thanks, Richard.

I didn’t mention it before, but I have a triple light switch very close to the thermostat location. Could I tap off that for line voltage, buy a 24V transformer, and put in an electrical box under the thermostat with a blank cover? I would think this was OK with code as long as the connections are in the electrical box and the box is accessible. This is the only way I can think of not having to replace the 2 wire 15 foot run (and associated drywall carnage).

You might be able to put something like this below the box … then patch the wall.

This keeps the 110V in the electrical box and the 24V outside.

You really only need a thermostat that has a on/off relay.

Not sure if it’s available to you but a DHS/ZHS 110 Thermo should do the job.

It’s either 24v or internal battery operated.

See this link (other forum) for a brief discussion.

I’m using it in a HVDuctedAC with a twin wires successfully, it should be OK with your fireplace.

I assume that you want to use the thermostat that you’ve already purchased. However, there are Z-wave thermostats that can run only on batteries and would support your millivolt application (e.g. 2GIG CT100, Remotec ZTS-100). Otherwise, like Richard said, you’d need to hook up a 24vac transformer; something like this. Assuming you have a 110V source to one of your light switches, and not just travelers, you certainly could tap that to power an outlet for the transformer. If everything is over the same empty wall cavity, you’re golden for fishing wires.

Honeywell has received one report of gas accumulating and combusting when the consumer lit the fireplace, resulting in a minor injury when the hair in the back of the consumer?s hand was singed.

You would not want to hook up an automatic thermostat if the fireplace did not already have safety lockouts for the burner if an ignitor or pilot flame was not already present!