KFW 55 - ground source heat pump - which hot water storage, etc.?

Hello dear experts,

we have a new building coming up in a few months, finally.

Following data:
Living area: 205 sqm
People: 3
Heating device: Ground source heat pump with deep drilling (2X75m). Most likely MTEC WPS 412 (max. 12KW)
Type of heating: Underfloor heating with partial wall heating - which manufacturer is recommended here?
Controlled residential ventilation with heat recovery: Zehnder Comf. q350tr with enthalpy
Masonry: 42.5 cm
Solar/photovoltaic support is not planned.
Hot water storage ????

The hot water storage is still the big question here.
We are only being offered buffer tanks with 1000l at prices starting from 3000 euros.

However, I have read that buffer tanks are rather counterproductive, especially with geothermal heat pumps.

The only condition here, since it is being built under the 10000 houses program in Bavaria, is that a hot water storage tank of 360l (at least 30l/kW) is necessary with corresponding U-value/insulation.

What would be the best hot water preparation with what size, etc.
Not to just throw thousands of euros into the heating engineer’s throat and then even have increased electricity costs, etc., because the whole system is not coordinated at all, which unfortunately happens very often, even though they are "professionals."

I would also be very happy about experiences with Mtec WPS 412, etc., or any possible suggestions (underfloor heating, etc.). Now everything can still be taken into account.

Thank you very much in advance for your support. Happy Easter.

12kW sounds completely oversized for the heat pump. What is the heating load of the house?

There are good storage tanks starting at €600 ([shwt, twl]), branded products for about €1000 (Juratherm hdw). It is important to have a large heat exchanger of 2, preferably 3m2 in the storage tank. Regarding size, the usual question is whether 200l or 300l. A smaller tank has the advantage that the water stands for a shorter time (legionella), 300l makes sense if there is a lot of bathing/frequent consecutive showers. You can best estimate that yourself. Larger is not necessary.

Do not let anyone sell you a buffer storage tank.

Underfloor heating: Have a room-by-room heating load calculation done with your own desired temperatures. Based on that, have the underfloor heating planned for a maximum supply temperature of about 30 degrees. Heating circuits all around 80 meters. Plan the bathroom especially well; if necessary, heat electrically occasionally there and/or lay the underfloor heating in the wall as well. Goal: low supply temperature for the whole house.

Thank you. 12KW of the heat pump is only the max. output. It is a heat pump modulating from 4-12KW. I only took this from the offer. For brine heat pumps, the offer is more limited than for air-to-water heat pumps. Usually, you then have no choice here. Whatever is in the heating engineer’s program must be installed. Regarding the hot water storage tank, as mentioned, according to the requirements of the 10000 Houses Program, it should be at least 360 liters accordingly. The bathroom will definitely be optimized with wall heating, my heating engineer said you don’t need that, but he would also try to sell me a towel warmer. And not just one heating engineer. They all just want to take money out of your pocket. Invest tens of thousands of euros and have electricity costs like an old oil heating system.

[What is in the heating engineer's program must be installed.
Regarding the hot water storage tank, according to the requirements of the 10000 Houses Program, it should be at least 360 liters as mentioned..

The heating engineer should also install what he is familiar with; you should only be worried if he installs something he doesn't know. If it has to be 360 liters, then take that. We have 230 liters for 2 people, which has been sufficient so far; when the little one grows and we are really three, we'll see.

The bathroom will definitely be optimized with wall heating, although my heating engineer said it is not necessary, but he would also try to sell me a towel warmer. And not just one heating engineer. They all just want to take money from your pocket. Tens of thousands of euros invested and electricity costs like an old oil heating system.

We have towel radiators in the bathroom, works great, nothing against it directly. Towels dry until the next shower, bathroom comfortably warm at 24 degrees without high consumption.
If your heating engineer doesn’t make a big mistake, despite the really large over 200 sqm house, thanks to KFW 55 and ground source heat pump, you won’t have high heating costs anyway. At least that is our experience with a 160 sqm KFW 55 house with air-water heat pump. Just stay away from buffers for heating and keep the flow temperature as low as possible. After moving in, adjust the heating curve for the house and it will be fine.

If it has to be 360l then just take those.

The 360l are not necessary for the heating but are just one of the knock-on effects of the poorly planned 12kW heat pump.
The heating engineer may well be very familiar with this heat pump, but above all he will earn well from it, to the short- and long-term disadvantage of the builder.

Unfortunately, it is a mistake to just let the heating engineer do as he pleases. He is not measured by what makes sense for the customer in the long term; technically, it's enough if it gets warm enough. Good planning is not necessary for that. Instead, more power and buffer storage are simply sold because it is easier and more profitable.