If you have an old space heater
with a thermostat, that not being used anymore, it might be useful to prevent freeze-up in unheated rooms.
We got our first sufficient snow last night!
Since we started using a Sanyo ASHP, the den isn't heated at night.
Only a small amount of heat from the main living area is allowed to
escape into the den. The room has a LOT of glass and it's been going
down into the low 50s this past week.
So, I set the space heater for 47 degrees, Just in case!