Heat pump, water storage tank, instantaneous water heater, wfK, underfloor heating, heating and cooling

Hello everyone,
I am concerned with the design of our renovation project.
Key data: year of construction 1930, ground floor area: 90 sqm, upper floor area: 40 sqm, basement (but only storage space), semi-detached house, extensive renovation is being carried out (basement ceiling, facade, windows, roof - so that BEG EM funding eligibility is possible).

We are converting everything to underfloor heating on the ground floor. On the upper floor, it will come down to a wall heating system in the space between the rafters.
According to calculations, after the renovation we will have a heating demand of 5.3 kW.
A air-to-water heat pump (Vitocal 252 - the 08 model) is being used. The heat pump therefore has no integrated water storage tank.
The system is to be combined with a water-bearing fireplace, which only delivers 2 kW of heat output to the room and 8 kW to the water.
In addition, one or more buffer tanks are to be integrated in order to
a) absorb the heat from the fireplace
b) be able to feed the solar yields of a photovoltaic system via an electric heating rod during peak times
and thus provide hot water for heating and domestic water.
Due to our space in the basement, I will probably have to fall back on two buffers side by side. I have targeted a volume of 800 to 1000 liters. However, in order not to necessarily store hot domestic water in winter all the time, I am integrating an instant water heater (solar-compatible).
The idea behind this: to use the temperature level in the hygiene storage tank and then, if necessary, only increase it by e.g. 15 degrees, ideally less.

So much for the theory, which has also been discussed with the heating engineer. However, I would like to fully understand the system and I am stuck on some thoughts and hope for your support.

1.) How best to connect the buffers? From my point of view, a series connection should be recommended, with the hygiene storage tank with the domestic water loop being the rear, warmer tank, right?
2.) Where should the heat pump feed in ideally? Actually in the middle, right? But where would the middle be with two tanks?
3.) Where does the wfK feed in?
4.) Up to what temperature in the buffer does it make sense to let the heat pump operate?

Additional consideration, which is not a priority: there would be the possibility of a cooling function in summer. Does it make sense to integrate the buffer here or would that just make everything more complicated? Otherwise, in summer there would be the option of a cold and a hot buffer? One for domestic water and one for heating water.

Thank you very much for your interest!

Thank you very much for your interest!

Wow - rarely seen such a complex, cumbersome and expensive solution. I cannot answer your individual question, but I’ll just throw a 5kW monoblock for heating and hot water into the room. Fireplace for the look. For the saved smaller five-figure amount, treat yourself to a new ID3 and charge it with the photovoltaic system – which also supports the heat pump for hot water (probably 100% in summer) and partly for heating. P.S. Cooling via underfloor heating is greatly overrated!!!

According to calculations, after the renovation we come to a heat demand of 5.3 kW.

Per minute, per hour, per day, per 24 hours, in summer or in winter? Such a complicated system for a single-family house, did a representative for building services engineering advise you?

kW is power, so independent of time ;) But surely the heating load at standard outdoor temperature is meant

My advice would be to design the hydraulics properly, omit the buffer, omit the chimney, omit the heating rods.

Wow - rarely have I seen such a complex, cumbersome, and expensive solution.

Well, the system was developed considering many factors.
The instantaneous water heater, to have immediate hot water and to always keep the heat pump operating within the comfort range.
The wfK to ensure safety on cold winter days and to further reduce heating costs.
The buffer tank(s), because wfK makes little sense without a buffer and to optimally use the solar yields of the photovoltaic system.

Certainly, all this costs money, but the goal is to reduce costs in the long term.

I may not be able to answer your specific question, but I just throw a 5kW monoblock for heating and hot water into the conversation. Fireplace for aesthetics.

My advice would be to design the hydraulics properly, leave out the buffer, leave out the fireplace, leave out the heating rods.

I believe we have moved beyond the point of generally modifying the system. Ultimately, it is also a matter of belief. Either invest a lot of money into the system now and reduce consumption costs in the long term or install a cheaper system now and then live with somewhat higher ongoing costs in the long run...

As I said, for me currently the optimal connection of the buffer tank is really the most important question and here I lack the background knowledge.
I would be grateful for valuable advice here!