Harman P38+ with thermostat

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New Member
Dec 2, 2023
Hello everyone,

When I connect a thermostat to my Harman P38+ pellet stove, the stove acts like the tstat is calling for heat all the time, without regard to the thermostat's on/off switch and temperature set point. I would appreciate any advice, please. Here are the details:

The stove's control board has two spade connectors for a thermostat. I'll jump ahead and note here that I measure 55 VAC across those contacts with and without a thermostat connected. I don't know what that voltage is supposed to be, but the two thermostats I tried are rated at 24 VAC, and so maybe I shouldn't be surprised they act like they are being shorted. Both thermostats work fine with my oil boiler, whose tstat contacts are rated at 24 VAC @ 0.2 A, and where I measure 29 VAC. And that's the extent of my HVAC experience, I have no idea what I'm doing.

The stove manual and a technical bulletin Harman published both say to connect the tstat directly to the stove with no mention of a relay as discussed in other threads, including this one from last month: https://www.hearth.com/talk/threads/wire-to-thermostat.201198/. Someone already uploaded that bulletin elsewhere in this forum, so I attached it here. The bulletin says to put the tstat in series with the room temperature probe, but it acknowledges the P38 doesn't have that. The expected behavior is when the tstat calls for heat, the stove runs at "turbo" feed rate, and otherwise it runs wherever you set the feed rate knob. I confirmed that behavior in my stove by bridging the stove's tstat contacts with a 15k resistor. I didn't leave the resistor in place, I just added and removed it a few times to confirm the feed rate change.

I tried the Honeywell RTH230B thermostat that Harman's bulletin recommended, as well as an RTH2300. Both run on AA/AAA batteries, both are rated at 24 VAC, in both cases I removed the Rh-Rc jumper as directed by Harman's bulletin, and I ran 18/2 thermostat wire from the stove's tstat contacts to the thermostat's Rh and W terminals. As soon as I connect the thermostat, the stove goes turbo, no matter what the thermostat's set temperature is. With the RTH2300, if the heat/off/cool switch is "off" then the stove doesn't react, but switching to "heat" makes the stove go turbo without regard to the set point. With the RTH230B, the stove goes turbo no matter what the tstat switch and set point are doing.

I would appreciate any help. Thank you all. Stay warm, but not as warm as you would be if you had the problem I have where my stove wants to run at turbo when it smells a thermostat.

Harman P38+ with thermostat Harman P38+ with thermostat