In our part of the country heat pumps make a lot of sense since we get few cold snaps below the teens. In the last two instances, when temps dropped to single digits and slightly below the heat pump efficiency was amazingly close to nameplate specifications. In '21 we shut off the old 90s vintage central system heat pump because it had gone negative. Not producing air as warm as the conditioned space. Amazingly, an 18 kbtu and 9 kbtu mini-split kept the whole house at 68 or above. They are located at the ends of opposite wings of the house. To be sure, distal places were cool but not cold and nothing froze.
The LG Red mini-splits are pretty efficient all the way down to about 10 degrees. The Mr. Cool unit in the shop sips juice keeping 900 sf at 62 in the winter and about 80 in the summer. I am not happy that 62 is the lowest setting. We have added the last 1000 sf to the house and it is a three zone LG Red mini-split system. Anxious to see how it works. I'm finishing up the install this week.
As I write it is around 40 and the only unit that is on is the little 9 kbtu unit blowing air down the hall and into the rest of the house. We just foamed the roof decking in the old portion of the house and one addition, a total of about 1800 sf with the total being about 2400 sf. I'm not declaring success yet and we still need to freshen the blown fiberglass but something has changed. I am anxious to have conclusive data.
I wanted earth source but the mini-splits come close most of the time. The earth source units last forever since the compressors just never work very hard. A neighbor had one of the originals installed in the early 70s and three owners later, believe it or not, it still works.
Ducting is what puts the earth source heat pump efficiency down in the mini-split range for normal conditions. If you put the ducting in the conditioned space I don't think you could beat it.
The wood stove back-up heat does not have an outside air source. That needs to be changed. It is so inefficient since it pulls air into the house to vent I only run it for ambience or just when necessary.
With the fools in DC getting their way we are all going to have to learn a lot about efficiency to manage to keep warm or cool for as long as we can.
Re: foaming
I’ve owned a well-renovated house of about 2kSF for the last 2-1/2 years, done 7 years ago. They spray foamed the heck out of this thing. I have been *very* impressed with the energy performance. I am now a spray foam believer / advocate. They didn’t replace the HVAC when they did the studs up renovation, one of a few things I still find to be head shakers. The renovation was architecture-driven, infrastructure was not on the radar, other than the spray foam and cementitious siding. I’m planning on an HVAC upgrade to get zoning and efficiency, probably involving ducted mini-splits. Perhaps one unducted.
Anyway, I think the foam stopping air leaks plus getting all the duct into the insulated envelope are huge. Old house had ductwork in the attic above the fiberglass insulation and did poorly there. I predict you’re about to be very pleasantly surprised.
—”Heat pump efficiency eventually plumments.”
Yes, it does!
The good news is that each year they gain efficiency at lower temperatures.
By design heat pumps and high-efficiency furnaces/boilers are the laziest equipment I have ever used.
Left to its own settings the compressor and fans will produce the minimum output to meet demand in NO hurry.
To keep an eye on it I have S/R and room temperature sensors also a watt meter on the power supply to the system.
Simple split coils on the mains, a ten-minute installation.
2 Phase 3 wires 110V+110V 100A Din Rail 18mm Tuya WIFI Smart Energy Meter Power Consumption Monitor kWh Meter Wattmeter
Even at minus 15F it is not at max power draw unless you select Turbo mode and it does not lock in that mode.
Being a cheap SOB, I approve.
I did want to watch the Coefficient of Performance in real-time to optimize changing over to the gas boiler floor heat system. It is usually done on a ground source system, air to air /not so much.
Have you seen a COP graph for your unit?
Some publish all of it Pioneer only shows a few key points.
There are third-party reports that usually show more...
https://ashp.neep.org/#!/
My unit has a vacation mode heating of 46F and you can hang the stat near the ceiling,stratifaction will drop it ~4F.