"TOM" wrote in message
...
Air-exchange heat pumps stall out at about 35F outside air temperature, so
electric resistance
heating kicks-in.

That statement might have been correct 15-20 years ago. Today, it just
isn't the case.


If you have a ground-exchange heat pump, it works against
the ground
temperature (59F average across the US) and does not have this problem. It
thus uses a lot
less electricity (heating or cooling) but that has to be balanced against
the much greater installation
cost (buried heat exchange tubing). Last time I investigated, the
incremental cost of installing the
ground-exchange in a residential situation achieved payback in about 20
years. This is usually
considered a poor investment. With rising energy prices, the payback
period
may have changed
since then.

-- Tom

Geothermal's are very efficient but as mentioned, installation expense is
much higher than air-air.

- Robert