ᐅ Control the climate using underfloor heating or through the ventilation system?

I have to disappoint you—I’m not wrong. I have real experience and have already described this in another thread where we discussed this issue.

The heat recovery ventilation (HRV) system is out of operation during the summer anyway... it has absolutely nothing to do with air conditioning cooling. Yes, air cooling is definitely inefficient in terms of performance... nonetheless, for two summers now, I have been able to maintain comfortable temperatures throughout the upper floor with a single 3500-watt (3500 W) unit.

I believe I also mentioned that I will probably get a similar unit for the ground floor to cool the entire house.

But it would probably be enough to simply install a 7-kilowatt (7 kW) unit with a ceiling cassette in the stairwell area.

You can’t really compare commercial and residential buildings because there are completely different expectations and requirements for the systems in each case.

Well, with our 3kW system, we just managed to keep the living room cool – the adjacent rooms didn’t feel any effect at all.

And with the controlled ventilation system, the heat recovery was always active... and that does affect the air conditioning because they work against each other.

That’s been my experience, which is why there is a unit in every bedroom and living room...

I don’t want to argue about this, but I have noticed the effect myself and see it every summer... you can clearly feel the temperature threshold when going up the stairs.

Any air conditioner just briefly “coughs” at the heat recovery from the controlled mechanical ventilation system and simply continues to cool... this effect is negligible given the volumes of moving air and the system is usually off in summer anyway, since there is nothing to recover.

First of all, the mechanical ventilation with heat recovery (MVHR) really has a much lower airflow compared to an air conditioning system, and the heat recovery is actually beneficial for cooling because it works both ways. This means the incoming warm air is cooled by the heat exchanger, which is also the case with my system (and for that, my MVHR has a condensate drain/connection). In contrast, the bypass in my (by the way, also a Helios MVHR with a really good easycontrols user interface and well-documented Modbus protocol) is set so that it only opens in summer when the outside air is cooler than inside.

Regarding Mycraft:
You can definitely feel the temperature limit, especially when the air conditioner is in the hallway, but do all rooms really cool down effectively? I notice the limit clearly in summer when I go down to the basement, which by the way is pleasantly cool completely free of charge.
It strikes me as a bit odd, especially since cold air naturally tends to sink to the ground floor rather than stay upstairs.

The floors are not completely sealed off from each other, so of course a significant portion of the cooled air also ends up on the ground floor...

So, I just can’t imagine that the upper floor is significantly cooler everywhere. In our case, the open (!!) basement stairs managed to maintain about a 7°C (13°F) temperature difference on the hottest days.

Despite my skepticism about how the "coolness" spreads through the air, you have to agree with the law of conservation of energy: if you use an air conditioner to remove energy from the house, it gets cooler. Over time, thanks to good insulation, the temperature should generally even out throughout the entire room, as the remaining heat energy is evenly distributed.

I can believe that it works, but I don’t think it happens quickly or comfortably; because where the unit is running, it would have to be significantly colder than in the other rooms, which doesn’t sound very pleasant.