Every so often, the temperature on the 2nd floor of our house stays at 68 F (20 C) all night, even though the overnight setting on both thermostats of our houses' two-zone system is 64 (18 C) degrees. This happens quite irregularly; on most nights, the temperature falls normally to the set level. I am quite puzzled.
This can happen when the outside temperature is below freezing, as tonight. It is now 4 hours after the night setting kicked in, the setting has definitely not been altered (pressing "present setting" shows 64), and there is this 4-degree F differential. Incidentally, the temperature is holding stable at a level above even the day setting, which is 67.
Any clues as to what is going on?
This can happen when the outside temperature is below freezing, as tonight. It is now 4 hours after the night setting kicked in, the setting has definitely not been altered (pressing "present setting" shows 64), and there is this 4-degree F differential. Incidentally, the temperature is holding stable at a level above even the day setting, which is 67.
Any clues as to what is going on?