System check new build KfW55 - Remko heat pump / 500L buffer / fresh water module

Hello everyone,

my girlfriend and I want to fulfill the dream of owning a home in the form of a semi-detached house with KFW 55 energy standard and have a few questions regarding our heating system for you.

First of all, our data:
Currently a 2-person household; planned in the future is a 4-person household
Basement + 2 full floors + inhabited attic
Total house area 267 m²
Heated floor area with underfloor heating (including heated basement rooms) 230 m²

We can only install an air-to-water heat pump because drilling is not allowed, no gas line is installed, and there is no space for a tank (pellet, oil, gas).

Our chosen heating contractor has now proposed the following system to us:
- Remko Monoblock LWM 150 10 – 13 kW
- Buffer tank PSFW 500
o (500 L should be enough? And the system can heat up hot water faster, he argues)
- Fresh water module FWS Easy 30
o the flow rate of 30 l/min is only specified at 70°C flow temperature. How it will behave with a buffer tank of approx. 50°C is still unknown to us.

Furthermore, we are installing about 7 kW photovoltaic modules on our south-facing roof. Here we want to either use the generated electricity for the heat pump or supply an electric heating rod in the buffer tank that heats the buffer tank to a set maximum temperature without the heat pump. Otherwise, cover our own electricity consumption as best as possible.

Now I have the following questions for the experts among you:

1. Is the heat pump oversized? We initially planned for a heat pump of about 8 kW.
2. Would you use the electrical energy generated by the photovoltaic system for the heat pump or the electric heating rod?
3. How large should the buffer tank be? Is 500 L too small, or does faster heating of 500 L outweigh the larger storage capacity of an 800 L buffer tank?
4. What type of buffer tank is the best choice here?
5. What do you think of fresh water modules? Would parallel showering be possible with this rather small fresh water module?
a. Rainforest showers and hand showers each consume 16 l/min without Eco, only 9 l/min with Eco. What do you think of Eco variants? Besides the sustainability aspect, we naturally also see lower water consumption.
b. Our chosen faucet consumes max. 5 l/min

Thank you very much in advance!
We are very curious about your assessment.

Is there a heating load calculation? The proposed heat pump seems much too large. 500l buffer? How is it integrated, what does it buffer, domestic hot water, heating water, combined buffer, decoupling buffer? Is there a proper calculation of the underfloor heating or is the underfloor heating, as so often, built according to the motto "we always do it that way"? How many heat pumps has the heating engineer already installed, references? Why use the heating rod when the heat pump can also use the self-produced electricity with a better efficiency than any heating rod in the world?