"George" wrote in message
...

Isn't that a little different than it can't work and it must have
resistive elements?

Heat pumps can work at low temperatures, but depending on the unit, the
efficency is such that below a certain temperature, the resistive elements
cost less to run than the heat pump compressor system. I don't know what it
is now or with their system, but it used to be around 25 deg F. was the
break even point.

Where I live, it seldom stays below 20 deg and most of that time it is night
and warms up during the day. It has gotten to just below zero, maybe to
minus 10 about every 15 years or so during the night. The heat pumps work
fine in this area, with the resistive heat strips comming on only if the
heat pump can not maintain the house at the setpoint.