Have been on ground source for over 20 years now. It was an initial investment and leap of faith for pay back. Installation expertise is really important.

I have inverter heat pumps in my shop they work great from zero to 98 F
I bought larger units then called for but I'm quite happy and the energy cost savings have been impressive

Yep I have a Mitsubishi 2 ton mini split in my 24x26 man cave garage . I have 10' ceilings but insulated quite well . It heats and cools very easily . It never sounds like it is working hard . If I run it 24/7 regardless of heating or cooling it only adds about 25 bucks to the electric bill per month . Hard to beat that .

Man, that is the truth.
11 years on my system. Total nightmare when first installed. They kept coming out to flush and bleed the ground loop, it kept getting air in it. They finally had to bring a mini-excavator in and dig a hole next to the house and fix a joint that wasn't glued properly.

Of course, they hit the house with the excavator. Lucky the siding was new and easy to match.

Of course, they refilled the hole by putting the topsoil in the bottom, and finished with sand. I still have issues getting the grass to grow on that side of the house, even after getting rid of some of the sand and bringing in more topsoil.

Could be glycol contamination that is causing the grass not to grow? One of the biggest concerns with ground source is groundwater contamination.

My installation is also 11 years. Not ground loop but 170m borehole. I remember that plastic "elbow" joint between borehole pipes and pipe going to house was special item. It is one-time welded together by passing current through built in wire which then melts and fuses the elbow to pipes pipe. I suppose this is done to prevent leakage. A powerful pump in a drum was used to flush brine before connecting it to heat pump.

Glycol is not allowed here (poisonous) so our brine is 50/50 bioethanol/water.

I have two of them on my RV, they are good to about 40-45 degrees, but in an RV the idea is at an RV park you are using their electricity instead of your propane. On a $150K RV they were a few hundred bucks, well worth it when ordering the RV.

My leak was a faulty King Valve.

We moved to North Carolina last May. The HVAC system (heat pump) of our home was upgraded in 2021. Although we didn't really expect single digit temps here in NC, the heat pump worked perfectly throughout the cold snap, so maybe they've improved over the years.

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This is a great thread.
If I have it correct - the AC (cooling) function of the heat pump is not necessarily more efficient than a normal AC unit. The potential "gain" is to be able to provide heating, to a point, without using NG or propane. Does it cost more in electricity to provide heating compared to using e.g. NG? Why not have an AC unit and a NG heater?

The cooling part of the heat pump is AC. The heating part is the same, but flipped around backwards. I think it's cheaper/more efficient to heat with a heat pump if the pump and your cold temps support it. On top of that, having NG/LPG, heating requires another service and more complexity and presumably, greater cost. And some environments don't have/use a lot of NG/LPG.

I think ^that's^ correct in general.

I believe it's the opposite, that heat pumps use less energy to cool than to heat.

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I wasn't intending to compare cooling vs heating. I was intending to compare heating with a heat pump vs heating with other means. Or maybe I misunderstand what you're saying.

But I'm not 100% certain on all of the details, and certainly don't have a dollar/BTU break down. And it probably depends on your location and market prices.

^^^ Nah, I think it was I who misunderstood you.

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For geothermal, this is not necessary the case. My borehole delives brine at 5-ish deg C during the summer. There is an add-on where I can pipe that brine into fan convector in order to cool the house.
Energy consumed is brine pump (around 80W) and convector fan (also around 80W). Theoretical "cooling" power of my borehole is around 10kW. You might be able to get half of it. But it is basically "free".
Disclamer: Borehole temp will vary depending on yearly mean temperature at your location. So "free cooling" will not be as efficient in Arizona as in Minnesota.