On Feb 11, 11:20 am, Tony Hwang wrote:
Mr. R wrote:
I had a Honeywell digital thermostat in a good interior wall location. It
seemed to behave well except when it was really cold outside. When that
happened, my setting of 68 would invariably go to 72, and sometime more.
Thinking that the thermostat was defective, I bought a new one and it still
continues. Can anyone advise why the thermostat won't cut out the furnace
when the setting temperature has been reached ... only when it's extremely
cold outside?

Hi,
On some like my Vision Pro 8000, I can set the temperature band tight or
little loose from service option.(Like +/- 2 degs. or +/- 1 degs. from
target temp.) You can look into this. Here right now it is -20C outside
but 'stat works to 20.5C bang on all the time. No over/under shoots.

If this is a new, off the shelf thermostat and it's doing this with
the default settings, as did the old one, something is very strange.
ARe you saying the thermostat is set to 68, but the furnace continues
to run and the thermostat display continues to go on up to 72, which
probably takes another 30 mins+ of furnace run time? I could see how
a defective thermostat might do this, or one where you screwed with
the settings, but not one right out of the box.