Hi. I have a Lenox furnace, about 15 years old I guess. It was working
fine, but then all of the sudden the thermostat seemed to just die. So I
swapped thermostats. It worked again for a couple of days, and then failed
again. So I debugged it a bit more carefully, and it turns out the
thermostat "resets" whenever it tries to call for heat. The fan will come
on for about a tenth of a second, and then the display acts like it does
when you first plug it in. I'm not sure exactly what's happening, but it
SEEMS that the furnace is shutting off 24v when heat is called. It's not
the blower though, because if I just turn the fan on (from the thermostat)
it works fine. It's also possible that the therm is somehow resetting
itself, the only real test I did here was connecting 24v to the heat line
directly, and the heat didn't come on, but I'm not sure if that's the proper
"protocol" for requesting heat.

Any thoughts before I call the techs?

Thanks,
--Max