I have a Lennox G23Q2/3-75-1 unit with the Johnson Controls G776RGD11 IPI. Also, I have a mercury switch thermostat.
It's an odd one here guys and gals. The furnace will run fine for hours and then stop. When it stops I have no 24VAC at the thermostat (measured with meter when the problem first started). When I suspect it has stopped I can verify by moving the fan switch to ON and the fan doesn't come on.
To reset and get things back to normal, I set the thermostat to below room temp, move the fan to ON and wait for a few minutes until I hear the fan start working again.
When the fan starts working again I can move the thermostat above room temp and everything is back to normal. The furnace works like normal and I can return the fan switch to AUTO.
I have noticed that if I don't return the fan to AUTO it will continue to heat and not break. But obviously I don't want to run the fan 24-7. The problem only happens when I return the fan to AUTO.