Heat pump domestic water storage tank - What size is recommended?

A heat pump with 7kW heats water just as quickly as a gas boiler with 7kW.

No, the behavior is different. Keyword temperature difference supply/return. A heat pump delivers a lower temperature lift but a higher volume flow. Additionally, gas burners modulate and deliver significantly more power in full load operation during hot water demand than your heat pump is supposed to have. Practically, heating water with a heat pump therefore takes longer than with a gas boiler.

My personal opinion: heat pumps and rain showers are not sensible. You should really be willing to commit to a heat pump and know its disadvantages. In the hot water area, efficiency decreases, and then water-saving showerheads, e.g. Grohe Raindance with lots of air mixing, are the first choice. Especially since the constant dripping of the rain shower would really annoy me, but of course that is totally subjective.

The fact that a heating engineer recommends a large hot water tank can have various reasons.... One of them, higher price = higher margin.

We are 2* adults + 1* child + 1* baby. That means the bathtub or baby bathtub is used regularly. We adults shower daily.

We have never had to freeze. We only have a 280l hot water tank. A Grohe Raindance with "air" :-) And a Stiebel air heat pump. The heat pump has a power output of 4 - 8kW depending on the outside temperature. Hot water preparation takes between 30 - 60 minutes.

The air heat pump makes hot water once a day. And only to 45-47 degrees. 45 degrees in winter 47 degrees in summer (since a photovoltaic system is available :-) )

P.S. the heating element of the air heat pump is turned off ;-) We have no circulation.

Hello dear home builders,

I am dealing with the same issue as you. I am also currently building and there will be an air heat pump from Vaillant.

@Mastermind1: what are your electricity costs for the heat pump? I can find little about it online, and users are of course the best source to get "real" information.

I am afraid that the heat pump will eat my hair off my head due to high electricity costs!

Thanks for your feedback.

Best regards

There is a large heat pump consumption database on the internet. You can find a lot of data on it.

The important thing is the preliminary planning:

Have the heat load calculation done according to DIN -> this way you know the heating output needed at -14/16 degrees so you don't freeze (for this, a desired temperature is defined for each room that must be achieved)
This allows the correct hydraulics for the underfloor heating to be designed – i.e. the right length, correct spacing in the installation to get warm with the lowest possible flow temperature.
A heat pump is more efficient the lower the flow temperature required in winter...
Furthermore, the size of the heat pump should match the house.

The heating engineers like to calculate very generously.

If possible, one should avoid a buffer tank and only have a hot water storage...

Room thermostats should also be deactivated (the energy saving ordinance requires these parts).

This makes it possible to have as constant a circulation volume as possible...

If you pay attention to all this, you are safe