Abby Normal wrote
Rod Speed wrote
Stretch wrote

5) Setback thermostats were originally designed to save money
operating oversized fossil fuel furnaces. If your heat pump is
properly sized by the cooling load, it will not be able to recover in
a reasonable amount of time whithout using the strip heat.
Therefore, you will either be cold much of the time in the winter or
your electric bill will INCREASE with a setback thermostat. NOTE:
If your heat pump is oversized the setback penalty will be reduced,
but the fact it is too big will reduce the effective efficiency all
year long. NOTE also that setback thermostats WILL save money
in the cooling season, even when the heat pump is properly sized.

One obvious approach would be to have two,
use one in the summer and both in the winter.

Be a little windy inside in the winter, but you could size
one system to the cooling load, look at its winter heat
output then size the second one to make up the difference .

Yeah, that's what I meant.

You would eliminate heat strips for maintaining comfort but
you would still need heat strips to recover from the setback.
If you want to live with the lowered temperature while the
system is trying to recover, why set it back in the first place?

The other possibility is to stop the setback well before you get up.

Wear wool socks, bunny slippers, long underwear and a sweater all winter.

Making the second heat pump large enough so that the combined
heat output was triple the design heat load, would eliminate the heat
strips on the set back but you have the problem of the second system
short cycling all the time except when revovering from the setback.

You could obviously have 3 systems but thats not likely to be
economic unless you are using close to free surplus systems.