Natural Cooling / NC-Box Experiences

A 0.4-fold air change is usually sufficient for high air quality in a single-family house. Another rule of thumb is ~30m³/h per occupant, which is generally even lower than a 0.4-fold air change. This amount is enough, for example, to keep the CO2 level low during normal operation, much more consistently than is possible without a ventilation system. I run ~35m³/h per person; increasing the airflow further hardly improves air quality for me (in normal conditions, not during parties).

Of course, during parties with ten times the number of people in the house, this is not sufficient, but then you just open all the windows... however, for these cases, an excessively oversized ventilation system... I see no advantages, just a huge waste of energy (heat recovery is nevertheless <100%..., plus the electricity for the system for this amount of air, high installation costs, ...). There is also nothing worse suited for cooling than a ventilation system, since air is known to be an incredibly poor heat conductor... that works with an air conditioner because it cools actively, but cooling continuously with outside air (even if slightly precooled by a geothermal heat exchanger) ... this is only really possible in the transitional season, when outside temperatures are still fairly low, but not during long heat periods, when it doesn't drop below 20 degrees at night.

The Daikin system I linked is an active cooling system that is only integrated into existing or ventilation ducts. However, you can forget about that if a ventilation system only provides 0.4 times air exchange. As you have already correctly written, air is a poor conductor of heat, so more air needs to come in.

Additionally:

However, of course, the system should not be designed too small so that it does not operate at full capacity (quieter, reserves when there are fewer visitors, etc.). In the example above: 130 sqm x 2.5 m = 325 m³ room volume. When designed with 5 times the air volume, that would be 1500 m³/h... which is a system for a single-family house... Here, a typical system with 350 m³/h is more than sufficient, which then runs at 130 m³/h under normal operation.

Ok, so 150m² / 2.5m = 425/450m³/h that one should have.
With the first property developer, although it did not really seem to be known what I meant by "decentralized to centralized", it was said that the surcharge for "Pluggit Avent P 310" is 4000€. (Air volume flow range 70 - 300 m3/h) and would actually be too small for the calculated living area of 150m².
In general, as an addition it states:
Cross-counterflow heat exchanger made of aluminum with 85.4% HRV
4 ventilation levels, nominal ventilation capacity 230m³/h
Front control panel + alarm function
Automatic summer bypass
Air volume control with setback periods via a weekly program
Frost protection function
Condensate drain

What I missed, however, were all the pipes that would have to be installed more when switching from decentralized to centralized?

For active cooling, either air conditioning units or, for example, the Daikin system should be provided. The disadvantage of the central Daikin system is, of course, that this device has to cool down any room instead of being able to cool down individual rooms (e.g. 1x bedroom, 1x living room, 1x study).

Ok, so 150m² / 2.5m = 425/450m³/h that one should have. At the first property developer, where it also did not really seem to be known what I meant by "decentral to central," it was said that the surcharge for "Pluggit Avent P 310" is 4000€. (Air volume flow range 70 - 300 m3/h) and would actually be too small for the calculated living area of 150m².

It depends! A somewhat normal design is 0.4*volume, which corresponds to about 170 m³/h air exchange for a 425 m³ room volume (and in reality, for good air in the house under normal operation, you actually need less). The P310 can manage that. Of course, you cannot cool with the air exchange, but to cool effectively, you need MUCH larger volume flows and are far from standard ventilation systems in single-family house areas. From the 170 m³/h, it would rather have to become 1xxx m³/h.

We decided on a Viessmann heat pump with Natural Cooling. It cost us about €1000 more, which I find manageable. Since the system has only been running for 3 days and the temperatures have only been above 30°C for 3 days as well, I can’t say much yet.

We have a Ytong house with 36.5 cm thick walls without additional insulation; so far, we always only had 21°C in the house in the evening, and without cooling, we were already freezing on the sofa in the evening.... But okay, the summer hasn’t been great so far either.

Of course, such cooling can’t match an air conditioner, but considering that you basically get cooling for free, it’s something. Only a 50W circulation pump is running at the moment, and whether you have 27°C or 24°C in the house is already a noticeable difference.

Currently, it’s 23.5°C inside the house and 36°C outside. I’m curious how it will develop in the coming days; the temperature is supposed to stay above 30°C for a while longer. Of course, I can’t make a comparison to having no cooling....