Max Metral wrote:

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

If your thermostat takes batteries then replace them. A battery powered
t-stat with weak batteries will drop out when it attempts to bring in
heating or cooling, having enough power for the display, but not enough
for the internal relay circuits.

If your thermostat doesn't take batteries then it is a power stealing
stat, using Y to tap into common back through the compressor contactor.
Depending upon model Y may or may not go directly to the compressor
contactor--it may go instead through an electronic circuit on a main
control board. This type of system may require that a resistor be wired
across Y and C in the unit in order to provide a low resistance path
from Y to C. This type of stat usually comes with a large ceramic
resistor packaged with it.

There are also power stealing stats that also use batteries for backup.
The batteries in these are only provided to hold program and temp
settings in the event of a power failure.

Barring all of this it may be that your unit is shorted somewhere
internally which could cause power to drop at the transformer, and thus
from the t-stat when it calls for heat.

hvacrmedic