ᐅ Design of a New Heating and Hot Water System for a 1970s Vacation Home

Audiobampa schrieb:

10% triple glazing

A 10% window-to-floor ratio is very low. Normally, you would expect about 20% of the living area, and more is usually better.
The overall U-factor for windows with triple glazing is around 0.9 W/m²K (0.16 Btu/h·ft²·°F).

Audiobampa schrieb:

22W per m³ heating power needed, with ytong blocks about 33W per m³.

Heating loads are relatively independent of room volume. The relevant measure is the area in m² (square meters), not m³ (cubic meters).
Do you mean m²? But then it doesn’t match the 250 per apartment. I suggest recalculating and performing a heating load calculation.
There are online tools for this, or if you know what you’re doing, you can do it manually:
Determine the U-values of all building components, ignoring those adjacent to heated rooms.
Add a thermal bridge increment of 0.1 W/m²K (0.018 Btu/h·ft²·°F) for each component.
Calculate the related transmission heat losses for a temperature difference of 30-35 K (depending on your climate zone).
Include a 20% margin for warming-up periods.
Incorporate a minimum ventilation rate of 0.5 air changes per hour, ideally 0.7/h; multiply this by the heat capacity of air, air temperature (20°C (68°F)), and room volume to calculate ventilation losses.

Do this for each room, and you will have the heating load per room. The relevant standard (DIN) involves more details, but this is sufficient for practical purposes.

Audiobampa schrieb:

Are there split units that deliver around 7.3 kW?

Outdoor units of that capacity exist, but individual indoor units usually do not. However, you don’t need 7.3 kW for a 100 m² (1076 ft²) apartment built to GEG2020 standard; you’ll likely need about half that power, as the calculation above should show.

Audiobampa schrieb:

The apartment can be imagined like a large garage without room partitions. Two large rooms, one 40, the other 60 m², connected by an opening 3-4 m (10-13 feet) wide. Where is the best place to install the air conditioning? In the middle?

I would recommend one multi-split system per apartment with two indoor units, one for each large room, and electric surface heating in the bathroom (with temperature and timer control, ideally only 40-50 W/m² (1.3-1.5 W/ft²)).
Per apartment, use one outdoor multi-split unit to support the two indoor units. Since you want heating throughout the year, avoid cheap brands and opt for Daikin, Panasonic, Mitsubishi Electric, LG, Hitachi, or similar reputable manufacturers.

Audiobampa schrieb:

Instead of a boiler, could you use an instantaneous water heater?

Yes and no. If possible, I would prefer an electrically controlled instantaneous water heater (tankless water heater) and a small plug-in model for the kitchen over a traditional boiler, but tankless units require significantly higher electrical connection capacity.
Even better would be a domestic hot water heat pump (DHW HP), which requires both supply and exhaust air.
My favorite systems are the Daikin Multi+ or Hitachi Yutampu. Other manufacturers may offer similar products, but these systems allow connecting up to three indoor units (Mitsubishi up to four) plus a heat pump–powered hot water storage tank as a boiler replacement to one outdoor multi-split unit. At least for the Daikin units, relatively small hot water tanks can be wall-mounted in a niche.

One system per apartment usually offers the most cost-effective investment and flexible, efficient operation under variable loads.

Addendum: Split air conditioners should be sized so that the indoor units can run year-round on their lowest setting, except during heating-up phases. The outdoor unit should be able to modulate down to a very low level—that means it should be as small as possible—so the indoor units operate quietly (lowest fan speed), and the outdoor unit can run continuously without short cycling, providing just enough power to heat all rooms at the design temperature plus reserve capacity for defrost cycles.

Audiobampa schrieb:

You can imagine the apartment like a large garage without any room divisions. Two large spaces, one 40m² (430 sq ft) and the other 60m² (650 sq ft), are connected by an opening about 3-4m (10-13 feet) wide.

Don’t you feel like sharing the plans here? It sounds interesting 🙂 Although there should definitely be a door for the toilet 😉

Hello, sure, no problem,

Starting from the basement, we will build with 36.5cm (14.4 inches) blocks. Everything else remains open except that we will install a toilet cubicle on each floor.

Regarding the air conditioning on the lowest setting... how should I calculate that? In the heating load calculation online, I get about 6000W.
How should the air conditioning system be sized so that it can run all year round on the lowest setting?

Audiobampa schrieb:

Hello, sure, no problem,

from the basement upwards, we’ll build with a 36.5cm (14 inch) block. Everything will remain open except that we’ll place a toilet cubicle on each floor somewhere.

Regarding the air conditioning on the lowest setting… how should I calculate that? In the heating load calculation I found online, I get about 6000W.
How should the air conditioner be sized so that it can run all year round on the lowest setting?

The outdoor unit should have the corresponding rated capacity at the design outdoor temperature (-10 to -16°C (14 to 3°F), depending on location/climate zone). You can find the performance data for the indoor units in their technical datasheets. These are usually provided by the manufacturers or can be found in product descriptions from online suppliers. They list capacity values (minimum, maximum, nominal). The minimum value corresponds logically to the lowest operating mode.

So, for your heating load, you need to find the minimally suitable outdoor unit—not the maximum heating capacity, but the rated heating capacity that matters, since the maximum heating output is not reached at design temperature but rather at higher temperatures (smaller temperature difference). The unit should be suitable for 2 indoor units (unless you want 3 indoor units to keep them running longer at minimum output).

You then choose indoor units so that you connect as much capacity as possible in order to operate them at lower output. You can even oversize the indoor units, meaning the total heating capacity connected exceeds the outdoor unit's capacity. Manufacturers provide guidance on this as well. In this case, at the highest setting, the units cannot deliver their rated capacity individually—but you don’t need that anyway. Just make sure the manufacturer allows this setup to avoid control issues.

This requires some thought or experience, or you can consult a qualified heating/cooling technician who goes beyond just following regulations.

Many thanks for the explanations; that already helps a lot. I have double-checked for each apartment and, based on online calculations, I need 3.9 kW. What does the power supply for the instantaneous water heater look like? I need to include it in the electrical wiring diagrams. Is it three-phase power, or is 230 V 16 A sufficient?

Better remove the link, forum rules say: no external links.
And this is 1. a single split system and 2. probably the cheapest one you found. I’m not sure if it’s 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 units are around 20 kW.
3*230 V*16 A < 20 kW.
If you don’t know this, either stay away or do some research. The easiest way is always to check the product datasheet or installation manual. This information is included for all reliable brands except cheap imports from China.