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.