On Tue, 7 Feb 2017 21:29:16 -0000 (UTC), Jerry Peters
wrote:

(I still think the best Thermostats made were those old round Honeywell
ones with the mercury switch. They lasted decades and never needed any
repairs. So much for so called "improved technology").


My programmable thermostat has an auxiliary thermoswitch that will
turn on the furnace if the temperature goes below around 50F or so,
even if the battery fails or the thermostat itself fails. It's a
Robertshaw, about 20 or so years old. The thermoswitch is clearly
visible on the baseplate assembly. The thermostat itself is removable
from the baseplate so you don't have to stand at it when setting
the program.

I'd expect most decent battery powered thermostats to have a similiar
feature. Otherwise I could see lawsuits against the manufacterers or
the heating contractors when the battery fails and the pipes freeze
and cause extensive water damage.

Maybe mine will turn on at 50deg. (or so). It does not say anything
about that in the PDF manual I downloaded. It's a Honeywell made around
2009. I can easily pop it off the wall and program it.