I have a Carrier gas furnace, new in 1991, and a ***** digital programmable thermostat.
On very cold nights only (when the temps dip into the teens and below), the temps in the house will fall right through the "on" setting, not triggering the furnace. We have it set to come on at 65 degrees, and we wake up and it's 59!
This only happens when it's very cold outside, including during the day. During less severe times, it works fine, day and night.
Any ideas? Had an HVAC tech over, and he did about $300 of repairs (new home warranty), but the problem remains.