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

In the apartment with mechanical ventilation with heat recovery, we had two air conditioning units – even running continuously for 3-4 hours during our absence led to a cool living room and bedroom – the bathroom and children's room, however, remained warm.

Therefore, I believe that a single unit is not effective – but it might also have been due to the poorly planned mechanical ventilation system... I will find out for myself in the future.

I never claimed that it is cozy... I only wrote that it is possible...

Sebastian79 schrieb:
In our apartment with a mechanical ventilation system with heat recovery, we had two air conditioning units – even running continuously for 3-4 hours during absence led to a cool living room and bedroom – the bathroom and children's room were still warm.

Therefore, I believe that a single unit is not effective – but it might also be due to the poorly designed mechanical ventilation system with heat recovery... I will find out myself in the future.

I don’t think the mechanical ventilation system with heat recovery contributes much, but it doesn’t conflict with having an air conditioner either, since the heat exchanger always works toward the room temperature. The air exchange rate and heat capacity are too low.
@Mycraft:
It’s about comfort. If it’s uncomfortable, I’m not willing to spend a lot of money on it and just live with a few degrees more (or choose a different approach).

Yes, I’m familiar with the principle of heat recovery ventilation, but if the climate control is only in one room, it will still receive heat from other rooms—so I believe the cooling distribution effect is rather limited. The same applies in reverse, as heating just one room would transfer warmth to the other rooms very slowly.

My main goal with the air conditioner was to create comfortable nighttime temperatures in the bedroom.

We have a bedroom facing south, and despite shading, it heats up quite a bit when the outside temperature reaches 30°C (86°F)... to the point that during the first year, I could hardly sleep at all. So, the air conditioner was purchased and installed in the bedroom. The problem now is that if I only run the air conditioner for a few hours a day, the bedroom still remains relatively warm at night... as Saruss correctly pointed out, the law of conservation of energy applies. Therefore, the air conditioner needs to run long enough to actually cool down the room, including the furniture and everything else, which in our case means about 16 hours per day... so that I can turn off the air conditioner for the remaining 8 hours and sleep comfortably. It is switched on again in the morning after waking up.

I also noticed that the system, when running for 16 hours, can provide cool air to the entire upper floor. However, the other rooms warm up much faster again, although there is still a temperature drop of 3-4°C (5-7°F) during the day.

That brings me back to my initial point... yes, it is possible to cool an entire floor with a single air conditioner. Whether this is inefficient remains to be seen. That is why I am considering installing a ceiling cassette unit in the stairwell on the upper floor and then upgrading to a 7 kW (possibly 9 kW) system. This should be able to cool the whole house.

Please keep in mind that a 9 kW system also produces 9 kW of cold air... It doesn’t stay comfortable near the cassette for long! Even on the lowest setting, it cools very effectively. I had a 7.4 kW system, but that was in a Mediterranean country... Oversizing can also be annoying. Put a system like that on power mode, and it’s almost like a walk-in cooler. Still, you can’t cool a 150 m² (1,615 ft²) apartment evenly with it everywhere.