Is the hot water storage tank too small for the air-to-water heat pump?

Hello everyone,

we are planning a solid house (semi-detached house, two full floors, about 140 sqm), no KfW, air-to-water heat pump with underfloor heating, central controlled residential ventilation with heat recovery.
For the air-to-water heat pump, we have the following system on offer:
Split heat pump system of the brand Viessmann "Type Vitocal 200-S 201.D10"
The system achieves a coefficient of performance (COP) of 4.0 during heating operation at A2/W35 according to EN 14511. The energy efficiency class according to EU regulation No. 811/2013 is A++ for low-temperature application (W35) as well as medium-temperature application (W%%).
Additionally, a hybrid storage unit WPU, white color, with 300 liters of drinking water and 100 liters of heating volume will be installed.

My questions are:
1. The heating volume of only 100 L seems far too small to me (two adults and a small child, with another child to come). Or is the system dimensioned in such a way that 100 L will be sufficient?
2. Can a larger hot water storage tank simply be installed, or will that become impossible for the air-to-water heat pump, requiring a different system with higher capacities to be selected?

Thanks for your recommendations!
Best regards
Nils

My questions are:
1. The heating volume of only 100 L seems much too little to me (two adults and a small child, and another child in the future). Or is the system dimensioned in such a way that the 100 L will be sufficient?
2. Can you simply install a larger hot water storage tank, or does it then become impossible for an air-water heat pump, and another system with higher capacities must be chosen?

You have to separate two things:
Buffer storage for heating – that is actually almost always completely unnecessary and in doubt costs efficiency. So get rid of it.
Hot water storage. That is responsible for the shower water. You have 300 liters there and I can tell you that you will manage with that. Sure, for two bathtubs back to back and both kids showering extensively at the same time it won’t be enough (I can also report that from experience with three kids), but under normal circumstances that works well.

What you are being offered is a combined storage tank that unites both of the above points. If the heating engineer agrees to it, have only a hot water storage tank installed.
Apart from that, the heat pump has no problem heating 300 liters. Even 500 liters is no problem – it just takes longer, costs electricity, and you also have 500 liters that lose a few degrees overnight. That would, in my opinion, be oversized.

Oh yes, before I forget. If I look at the performance data of the heat pump, I would call it so ridiculously oversized. With that heat pump, you can easily heat a 300 sqm house. Who did the planning? How is the room-by-room heat load calculation?
That would be the question I would ask. Not the one about the buffer. Although with such an oversized heating system, you can definitely use one well...

Many thanks for such a detailed answer! That means "drinking water" possibly also means "hot water." That is my concern - that if everyone showers one after the other/in parallel in the morning, it will take forever for hot water to be available again. I don't see a buffer for now, but I am informed for further discussions. The offer will only come from the general contractor’s construction advisor and is included in the fixed-price offer for the house; so far, we have only expressed our wishes regarding the type of heating.

I have 300l solar and 200l electric myself and both separately easily suffice for 4 people even with a large bathtub. Solar also lasts several days, but it also gets hotter. You can still shower under 40°, for bathing the water should already be 50°.