Leaking coolant is a major problem. The standard for most small heat pumps is r412a and it is problematic.

To make the installs cheap, these systems use flare fittings that almost all seem to leak. Couple that with the fact the 412a systems run at high pressure and you get a lot of leaking refrigerant.

No HVAC techs I’ve met like the 412a systems and most seem to be hoping for more sanity from r32, though that will have its own set of problems (flammability)

A system like Sanden CO2 or Chiltrix will heat your water with outside heat, so it's always 2x+ the efficiency of resistance heat. If your climate is predominately hot then thermal collectors are far more efficient, but interior hot water heat pumps would be an okay choice where that's not possible.

I live in a cool place. This winter we had extended periods of -20C. Heat pump efficiency drops, but still stays above 2. SCOP (for year round average for heating coefficient of performance) units sold now are better than 5.

Also, if you are worried about the refrigerant leaking, you can sacrifice a little bit of efficiency and buy a unit using CO2 as a refrigerant with a GWP of you know: 1.

I think the biggest problem here for leakage are random construction dudes cutting the pipes when they demolish or remodel homes. If only they could gather the gas in the outdoor unit before, all of it would be saved.

If the hot water heat pump is a single unit (which is the vast majority of residential installations) and indoors, it is just stealing heat from your regular heating system. You're paying for that heat, so you aren't getting COP > 1.

You're only paying for that heat in the winter though.

Hence the preface of "cold area", but my heating season is over 6 months. The ROI is already long in HWHP even assuming the best numbers, and often negative once you account for reality in areas which get cold.

Who has that, a "indoor heat pump"? That's just stupid. Sure, exhaust air heatpumps "use" the (excess) heat from indoors, but that would just get expelled anyways.

https://www.hotwater.com/water-heaters/residential/hybrid-el... this is what people are talking about when they reference hot water heat pumps without qualification, because they are the vast majority of the market.

There is no excess indoor heat in winter in 99.9%+ non-passivhaus homes.

Right, I stand corrected. I had never seen such a heatpump (based in the nordics, that concept would never fly), and I'll agree with you that in homes heated by "regular means" (resistive/electrical, fossil fuels) they add little to no value, simply adding complexity. In heating season they would just siphon heat energy from the rest of the house...

(They would however de-humudify the space they are in, but that's another thing...)

I'd argue that every house that has any sort of ventilation, even naturally aspirated ones, have excess heat in that any amount of air leaving the house "unreclaimed" while there is any heating need contains wasted energy.

Heatpumps using that energy are very prevalent in the Nordics and Scandinavia, even with a majority of the time being heating season here.

Nah. It does not necessarily have to be an air-water heatpump either, colder climates more often use geo-thermal or ground-water loops as the energy-well. Then the surrounding climate have much less of an impact.

Refrigerant leaks are still an issue though, but moving to hydrocarbons and CO2 will mitigate that. Even R32 is a step up in that regard.