"Bill" wrote in message
ups.com...
I have a Trane XL80 two-stage with a Honeywell PC8900 programable
thermostat. Both were installed about 30 months ago, and everything
was functioning normally.

Twice in the last three weeks the temperature dropped a few degrees
below the set-point during the night, even though the furnace continued
to fire and blow at the lower stage and fan speed.

The first time it happened, I turned the system off at the thermostat
and turned it back on and the furnace turned and and the second stage
kicked in and the blower came on high. HVAC repairman came out and
could find nothing wrong with the furnace, but he didn't check the
thermostat.

This morning the same thing happened, but I didn't reset the
thermostat. After a time, though, it did "kick-up" itself and is now
operating normally, again.
Any ideas would be greatly appreciated!

Thanks.

Bill

This is Turtle.

The Honeywell PC-8900 you have may be set up like some that are done and when
you see a temperature range away from the set point . You getting into the two
averaging of the temperature in the room. one temp is the wall temp and the
other is a remote temperature sencer reading and then it will average that temp
to give the reading which will give you a fall back temp on the display and it
will run the low stage heat to run longer to even out the over all temp in the
house. Now when this condition is in effect it should not read lower than 4.2ºF
lower than set point for the second stage heat should come on to bring the temp
back up to about 2ºF range of the thermostat setting. So you could see a
temperature on the thermostat display of room temp and it be below the set point
by 2ºF or so and still not kick in the second stage heat because it is a average
temp of the house. So this thermostat can let you see this condition sometimes
if the right conditions happening at the right time.

Now you can read about this condition at this Honeywell website stated below :

http://content.honeywell.com/yourhom...enttemperature

Go to the site above here and scroll down to Question # 32 which will speak
about this condition

TURTLE