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