ᐅ What size hot water storage tank is needed with a heat pump?

I am currently wondering what size is more suitable for a single-family house with 4 people (without a bathtub): 200 liters or, to be on the safe side, 300 liters?

Leaving guests aside — the typical scenario would be all 4 people showering in quick succession in the morning. Another case would be everyone taking a quick shower in the evening after swimming outdoors in summer.

With a water storage temperature of 50-55 degrees Celsius (122-131°F) and showering at 35-38 degrees Celsius (95-100°F), cold water is still mixed in. Calculated, at 7 minutes of showering per person and 10 liters per minute, the demand would be 280 liters (74 gallons) of mixed water. If you shower at 40 degrees Celsius (104°F) and only have 50 degrees Celsius (122°F) in the tank, it gets tight — then 210 liters (55 gallons) would be needed.

Still, the difference to 300 liters (79 gallons) seems quite large to me. How does this look in practice?

No one can give a general answer; it depends on you. I would go with 200 liters (53 gallons). However, I also think 7 minutes per person is quite a lot. Even with my long hair, half of that is more than enough.

WilderSueden schrieb:

No one can give a general answer; it depends on your needs. I would go for 200 liters (53 gallons). However, I think 7 minutes per person is quite generous. Even with my long hair, I easily need only half that time.

The 7 minutes is more of an estimate, possibly including a teenage factor. So, 200 liters (53 gallons) would probably be sufficient—increasing to 300 liters (79 gallons) would mean heating about 100 liters (26 gallons) of water unnecessarily most days of the year, which could also increase the recovery times in the tank.

The issue with the teenager can also be resolved by having the parents shower first.

That really seems to be a very personal preference.
For me, not having a bathtub is a “does not compute,” but to each their own.
Showering at 35°C (95°F) and washing hair in 3.5 minutes? Did you all go to military training?
We have 190 liters (50 gallons) and there are three of us. We manage to have two people shower one after the other if the first one is quick.
If I were you, I would go for 300 liters (80 gallons). If needed, you can always lower the storage temperature so it empties more regularly.

BackSteinGotik schrieb:

How does this work in practice?

Speaking from experience as a father of three teenage girls – with 200 liters (53 gallons), we couldn’t all shower one after the other. At least not in a way that the last person would still have really warm water – for example, when all three girls come back from the barn (so on Mondays, Fridays, Saturdays, Sundays).
I prefer to be on the safe side and size the hot water storage tank a bit larger, choosing 300 liters (79 gallons). Even then, with three long-haired girls showering thoroughly and using care products (which teenagers tend to use), it would be tight for me as the last person showering.