A family member took a few of my comments further than I expected and installed (2) Mitsubishi (..12FE Model) units as the sole source of heat for their new house located in West MI. As a long time geothermal fan and owner, I was surprised and then leary that if (when) these ASHP units wouldn't keep up, I would be the blame. To my surprise, the units have performed FLAWLESSLY even though there were all time record cold months this winter and last in West Michigan. On the coldest mornings (-10 F) they've lost a degree or two from setpoint on the mainfloor (due to open stairs - upstairs unit was running at part load only), but have never had to use backup heat.
They couldn't be happier with their decision as they aren't stuck with the fuel oil or propane price roller coaster, and saved a bunch on the upfront install compared to any other heating/cooling option (except electric baseboard maybe). While year around efficiency isn't quite what a typical geothermal system would achieve, it isn't too far off and the decision to go with a bit more insulation and a much cheaper heating /cooling source)was a big factor in making the whole upfront cost of building/moving feasible for their budget. He ended up doing 1.5" exterior XPS, which is still rare in this region especially for a house in this size/price range.
The lower the heating/cooling load (due to smaller size house, or better insulated/airsealed), the less the difference will be in operating cost between an ASHP system and WSHP (geothermal). As the operating cost difference decreases, the payback horizon on the premium to go to WSHP from ASHP gets longer, and the ASHP will start to look better in some more cases. While the average stay in a house is around 7 years, the best evaluation should factor in the actual life of the equipment, which would favor a WSHP.
The only thing I see holding the ductless minsplits heat pumps back from more widespread use is the planning involved in distributing the heating or cooling in non-open areas of the floor plan (for example bedrooms!
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I'll be curious to see how fast the low temp, ductless ASHP systems become a bigger share of the market, and more important how long they last when regularly operating at such high compressor lifts (more than 100 degrees on the cold days). Should one expect 10, 15, or more? yrs of life??