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.