On Sun, 19 Dec 2010 02:43:09 -0500, Edward Reid
wrote:

On Sat, 18 Dec 2010 11:43:15 -0500, wrote:
According to what I have been reading, heat pumps do not work well below 40
degrees

What you read is incorrect. Yes, there is a point at which the
efficiency of the heat pump is less than that of the resistance heat,
but that point is roughly 15F, not 40F. Newer, more efficient heat
pumps might lower this point a little, but the problem is basic
physics, so it won't get much lower.

And as already pointed out, the heat pump might switch to the
resistance heat automatically. This likely depends on age and
manufacturer. Also note that most heat pumps will add the resistance
heat when the inside temperature drops about 5F below the set
temperature, but this is separate from the efficiency cutover. This
auto-add at 5F below is the reason you are always advised not to set
the thermostat down (very much anyway) at night when you have a heat
pump. If you disable the resistance heat (which is a viable course of
action in the southern US), then you can ignore that advice and save
some money, albeit at the tradeoff that it may take quite a while to
bring the temperature back up.

Ours throws in the resistance (aux) heat with only a 2F hill to climb; dumb.
With our house in VT (hydronic system), we'd set-back the thermostat about 6F
(65F down to 59F) at night and weekdays. Here we have to keep it set at
66-67F (no set-back capability and the forced-cold-air system) to be anywhere
close to comfortable.