We have a Lennox gas fireplace that’s about 10 years old and we live in New England. We have the thermostat set to turn off the unit overnight and back in at 6am. Often it doesn’t turn on even though the thermostat triggers on.

During the day after the room is warm, the thermostat and fireplace seems to work ok, turning on and off to hold the temp. In the morning to turn it on properly, I open the control panel and flick the switch to “on” and then “thermostat” and then it lights up. If I then turn off the thermostat, it goes off. It I turn the thermostat on after that, the fireplace works.

It seems that after the unit has been off for a while, or if it gets cold (the room gets to around 64 degrees), the unit doesn’t work in thermostat mode. Occasionally it does work, though.

Please let me know if you have troubleshooting ideas.



1 Answer 1


With a millivolt control valve your problem may be in the timer that shuts it down at night. You said it was a thermostat but not all thermostats are millivolt compatible. If it worked correctly and just recently started having problems I would suggest replacing the thermostat as it sounds like the contacts are becoming pitted or carbon fouled and that is probably why it is intermittent.

  • Changed my brother's thermostat for doing the same thing.... amazing how dirty the contacts and mercury were.+
    – JACK
    Dec 22, 2019 at 20:28
  • Ok thank you. This thermostat is about 15 years old — an inexpensive Honeywell programmable. Is there anything I need to look for in the new one? Would a Nest be compatible?
    – Kulluk007
    Dec 24, 2019 at 13:00

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