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Comment by NullPrefix | Hacker News Reader

Heat pumps have COP of 4 when reaching temperatures needed for room heating. Hot water is way hotter than that. The more the difference between outside air and heated water, the less the COP becomes. I don't expect COP of 4 at 200 fahrenheit

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True, but you never need 200F for DHW. This is even dangerous (scalding) and harmful to the piping. Typical temperatures for heat pump derived DHW are around 120F (45-50C), with some (bi)weekly cycles to avoid legionella build up at 150F(65C). Your statement is generally true, because a heating installation that is optimized for heat pump works at 85-100F, however in practice not many installations in old buildings are like that.

If your concern is storage you want as hot as possible, less boiling. Thermostatic mixing valves can bring the temp down to a safe temperature for use.

Losses are higher but you store more energy per L, which is often the limiting factor.

But you only need to store enough for a day or two. So you can alternatively just go bigger

Is energy per liter often the limiting factor? Suppose a 70m² apartment with 3-meter ceilings costs US$3000 a month because it's in an expensive city center like San Francisco or Manhattan. That's still only 17¢/liter/year. Expanding your hot water storage space from 45 liters to 200 liters consumes space worth US$27 per year of your rent. Even that cost seems far too low to be a limiting factor, and the vast majority of people live somewhere much cheaper than that.

I think that what actually costs money is not the space but the tank. Higher temperatures mean not only more expensive materials and shorter lifetimes for tanks and piping but also higher conductive losses.

As hot as possible is not the way to go. Heat pump COP will degrade dramatically if you try to boil the water. It is not even possible with the popular refrigerants (R32, R410a and even R143a), because they will exceed their critical temperature. If you cannot afford the space for a bigger DHW tank, then there are two options:

1. Consider PCM heat storage (still relatively new technology, but works well with heat pumps)

2. Maybe the problem shall be solved at the building level, not individual apartments.

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