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! )

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??

Assume a lifecycle of ~15-20 years, but you can beat that pretty easily.

You do better if you're nice to them by wall mounting them above the historical snowpack depth protected by roof overhangs where they won't get directly rained/snowed on, or suffer the effects of roof avalanches, icicles, etc..

Alternatively, if you ground-mount it in an un-tended yard and let it suck rain weeds & dirt into the coils spring through fall, and suck in snow (or get completely buried by snow, which I've seen in one such installation this year) in winter, never every clean outdoor coil or the filters on the indoor head etc you may be able to kill it in a decade or less.

There may be occasional repairs- some variable refrigerant valves get sticky and fail over time, but the compressors & blowers are pretty rugged. Like anything electrical/electronic they can be done-in by lighting strikes, but they handle even pretty crappy grid-power quality without much trouble (which is how they maintain their popularity in less-rich Asian & Latin American countries with sketchy or intermittent grid power.)