Heat pump type water heaters fail miserably in real life use; it might be a function of the number of duty cycles but I think they’re just too complex for something that needs to be 100% reliable ,It shouldn’t matter if it’s a heat pump pulling heat from air like your A/C or it’s a water to water system as this guy wants to build.
I’ve had 2 tankless electric heaters fail over a 5 year period BIG 3*60 AMP ones ,, with 3 chambers theres a lot of possible failure points and don’t try to reason with the wife when she has to take a cold shower...
What works in any climate is/are GAS tankless water heaters ... my Aunts home in Brooklyn still has their original tankless water heater from 1926 ,, just updated to eliminate the pilot light... amazingly simple , just a copper coil about 6” in diameter and 2 feet in length(THICK COPPER) inside a tube that narrows to become the chimney/exhaust.
The 1970’s/1980’s solution was to capture waste heat from the A/C pressure/liquid side with a sealed heat exchanger box... WORKED GREAT and improved A/C efficiency also... don’t know why it has fallen out of favor.
Best solution now in warm climates is passive solar heat with electric backup.
>> “Best solution now in warm climates is passive solar heat with electric backup.” <<
Warm climates, with soft water only!
Put it in a cold climate with hard water and the freeze valves will fail within a year. Warm climate with hard water, about two years.