Interpretation of new heating and hot water system for a 1970 holiday home

10% triple glazing

10% window area proportion is very little. Normally, one calculates with 20% of the living area; more is nicer. The U-value for windows with triple glazing is around 0.9 W/m²K for the entire window.

22W per m³ heating power would be needed, with ytong stones about 33W per m³.

Heating loads are relatively independent of room volume. Relevant are the m², not the m³. Do you mean m²? Then it does not fit with the 250 per apartment. So calculate again and do a heating load calculation. There are online tools for that, or if you know what you are doing, it can also be done by hand: Determine U-values of all components, ignore components adjacent to heated rooms Add 0.1 W/m²K thermal bridge surcharge for each component Determine corresponding transmission heat losses at dT=30-35K (depending on region) Factor in 20% surcharge for heating up Include minimum air exchange rate of 0.5/h, better 0.7/h, multiply that by the heat capacity of air, air temperature (20°C), and room volume: determine ventilation losses Do this for every room and you have a room-wise heating load calculation. The DIN does a bit more fiddling, but this helps in practice.

Are there split units that provide about 7.3kW?

Outdoor units yes, single indoor units no, but at 100 m² GEG2020 building standard you don’t need 7.3 kW, rather about half - this should result from the above calculation.

You can imagine the apartment like a large garage without room divisions. Two large rooms, one 40 and the other 60 m², are connected by a 3-4 m large opening. Where is best to install the air conditioning? Middle?

I would recommend one multisplit unit per dwelling with 2 indoor units, one each for the large rooms, and electric surface heating in the bathroom (with temperature and time control, preferably only 40-50 W/m² power). One multisplit per dwelling for 2 outdoor units. Since you want to use the system for heating all year round, avoid cheap ones and take Daikin / Panasonic / Mitsubishi EL / LG / Hitachi or similar.

Instead of a boiler, could one also use instantaneous water heaters?

Yes and no. If possible, I would also prefer an electrically controlled instantaneous water heater and a small plug-in instantaneous water heater for the kitchen instead of the boiler, but the instantaneous water heater requires significantly higher connection power. Even better would be a DHW heat pump. For that, you need supply and exhaust air as well. My favorite would be "Daikin Multi +" or Hitachi "Yutampu". Maybe other providers have similar systems, but with these you can connect up to 3 indoor units (Mitsu even 4) to one multisplit outdoor unit plus a heat pump supplied hot water storage tank as boiler replacement. At least for the Daikin units, there are relatively small tanks that can also be wall-mounted in a niche. One system like this per dwelling and overall this should be the cheapest solution investment-wise and also efficient and flexible in operation with varying loads. Addendum: Split air conditioners should be dimensioned so that the indoor units can be run all year on the lowest level except for heating up, and the outdoor unit should be able to regulate down as far as possible, i.e. be as small as possible. The goal is that indoor units are quiet (lowest level) and the outdoor unit can run continually without short cycling and still have just enough power to heat all rooms at design temperature plus reserve time to defrost the outdoor unit.

The apartment can be imagined like a large garage without room partitions. Two large rooms, one 40 and the other 60 m², are connected by a 3-4 meter large opening.

Don't you feel like showing the plans here, that sounds interesting :) although there should be at least a WC door ;)

Hello, yes sure, from the basement we will then build with a 36.5 cm stone, everything remains open except that we will place a toilet cabin somewhere on each floor. Regarding the air conditioning on the lowest setting... how am I supposed to calculate that? So in the heating load calculation on the web I come to about 6000W. How should the air conditioning be dimensioned so that it can run on the lowest setting all year round?

Hello yes sure gladly,

from the basement on we build with a 36.5 stone, everything remains open except that we will put a toilet cabin somewhere on each floor.

Regarding the air conditioning on the lowest level....how should I calculate that? So in the heating load calculation on the web I come to about 6000W. How should the air conditioning be dimensioned so that it can run all year round at the lowest level?

The outdoor unit should then have the corresponding nominal capacity at the design outdoor temperature (-10 to -16°C depending on location / climate region). The performance values for the indoor units can be found in the technical data sheets. These are usually available from the manufacturers or also on internet providers in the product description. There you will also find performance values (min, max, nominal). The minimum value logically corresponds to the lowest operating level.
You now have to find the minimally suitable outdoor unit for your heating load - not the maximum heating capacity, but the nominal heating capacity is relevant, since the maximum heating capacity is not reached at design temperature, but at higher temperatures (lower temperature differential). This must be a unit suitable for 2 indoor units (unless you want 3 indoor units to be able to run longer on minimal level).
Then you choose the indoor units so that you can connect as much capacity as possible in order to operate them at a low level. You can also oversize the indoor units, i.e. connect more heating capacity in total than the outdoor unit has. Manufacturers also provide information on this. However, at the highest level they cannot deliver the respective nominal capacity anymore – but you don’t need that. It should only be approved by the manufacturer so that there are no problems with the control.

The whole thing requires a bit of brainpower or experience or you ask a qualified heating engineer / refrigeration technician who does not just do the job by the book.

Thank you very much for the explanations, that already helps. I have double-checked per apartment and had it calculated online, I need 3.9KW. What does the power supply to the [Durchlauferhitzer] look like? I need to include it in the wiring diagrams. Is it three-phase current or is 230V 16A sufficient?

Better remove the link, forum rules say: no external links. And that is 1. a single split system and 2. probably the cheapest you have found. I don't know if it is sufficient because I don't want to convert btu/h to kW. However, it contradicts everything I have posted so far regarding efficiency, noise level, and durability. DLH have around 20kW. 3*230V*16A < 20kW. if you don't know that, either stay away or look it up. The easiest is always in the device datasheet/installation manual. That is included for all non-China junk.