I inherited a freestanding direct vent fireplace and I'm trying to get it to work with a thermostat. It functions fine using the ON/OFF/PILOT switch but will not work with a thermostat. It is a SIT millivolt system and the previous owner had a thermostat hooked up to it, so it was working at one point.
With that said - the fireplace will turn on with the thermostat but it will not shutoff with the thermostat. Could it be a bad thermopile?

When the fire is lit I get a .330mv reading for the thermostat and .220mv for the thermopile.

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    remove the thermostat and just short the two wires together .... does the fireplace main burner light? .... separate the two wires .... does the burner extinguish? – jsotola Feb 4 '19 at 19:13
  • @jsotola Shorting the terminals ignites the burner but separating them does not shut off the burner. Also the ignition behavior is now intermittent. Sometimes the burner will not ignite with the thermostat any more. – Landoperk Feb 5 '19 at 20:05

The issue was the thermopile. I ordered a replacement and after installing it the thermostat control started working.

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  • That is unusual, usually a bad thermopile will cause a no light problem, I have not seen a bad thermopile allow the flame to continue. – Ed Beal Feb 8 '19 at 21:43
  • @ed Beal Do you think there still might be an underlying root issue? – Landoperk Feb 9 '19 at 22:11
  • I have never seen this case where a bad thermopile would prevent shutdown, so I would be looking for something else just in case. – Ed Beal Feb 10 '19 at 1:01

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