Terry, that's not true of modern heat pumps. In places where the temperature falls below freezing recently developed heat pumps are fully capable of heating interiors.
If the climate is very cold but not frequently in the winter then resistive heating may be added to the system for those few really cold nights. Only in the coldest of climates does it pay to use geothermal heat sourcing.
It looks like for current air only cold climate air-source heat pumps there's no need for supplemental heating down to 17F/-8C.
Bob
I've been completely out of the business since 2004, and I'm sure improvements have been made.
It's not that a heat pump unit isn't capable of heating an interior at below zero temperatures, rather that the built in resistive heat strips, which run along with the compressor whenever the unit is in defrost, increase the wattage until any savings over straight resistive heating has been lost.
The "sweet spot" for AC or heat pumps is 20 F, so whenever the ambient temperature is <52 degrees the outside coil will remain below 32 F and, even in the desert water vapor begins to form as ice on the coils. In more humid climes ice forms more rapidly.
The ice slows air flow past the coil and this causes the temperature to drop further, which causes ice to form even more rapidly.
To deal with the frozen coil, the unit goes into defrost mode and stays there until the ice has all been melted. During this cycle the gas flow has been reversed and as the outside coil is heating, to remove the ice, the inside coil is chilling the air inside the house - or at least it would be if not for the resistive heaters which kick in to keep the occupants cozy inside.
The old rule of thumb was that at less than 40 F (~4.5 C), ambient the wattage drawn through a complete cycle of heating and defrosting was more that it would be if the compressor was left off and only resistive heating was used. This could have been improved upon by more efficient compressor design, but the changes would be incremental. Resistive heating is not going to change.
A few caveats. I was engaged in designing large systems as opposed to wall units, window units, or even residential units. Think Caesars Palace, but not a Strip Motel. As such almost all of our work was with Water Source Heat Pumps, primarily because air/air cooling systems are not very efficient during desert summers, and all of the majors have chiller systems which allows water to be used.
Some isolated buildings nonetheless were separated from the main buildings and required air/air systems for one reason or the other.
These systems were generally 5 Tons and up, and management was very aware of operational costs. Regular maintenance and filter changes were expected so we could run a little closer to the line than most.
I can't think of any way to change the physics. Ice forms at 32 F, ice impedes airflow over the coil and acts as insulation, it requires energy and time to melt the ice, the enclosed space needs to be heated during this time. A 24 hour watt meter is required to observe the unit through time to determine the electricity used.
Have fun
Terry
