Does it make sense to retrofit an air conditioner?

A device in the hallway at the highest point, which circulates the air for cooling by a few degrees is sufficient. Of course, it will result in an ice-cold hallway. But if all the doors are open upstairs, a pleasant temperature will settle as the air mixes and yes, the ground floor will also get some of it.

Surely you will consume more energy this way and need a larger system because you are cooling the entire house with it and not just zonally as is possible with several indoor units in each room.

But the investment is worth it. I myself faced the same problems a year after moving in and then also retrofitted an air conditioning system in the house and have since also modified it because more power was needed due to more people in the house.

Mobile air conditioners are inconvenient. They are more for a shed or a garden house.

Above all, with mobile units it must be considered that - even if the warm exhaust air is ducted outside through a hose - air must flow back in somewhere to equalize the negative pressure created in the house. That means warm air from outside is brought in, which also contains quite a bit of moisture. If this air is then cooled down again, the relative humidity increases further.

If an air conditioner is used, then a split air conditioner. Unless it really cannot be helped.

You build a KFW 55 house and think it is nicely cool inside because it is so well insulated, only to realize unfortunately then that it wonderfully retains the heat once it is inside....

Well, simplest physics...

I really have no idea about the topic, but couldn't the same effect be achieved with mobile devices in the bedrooms?

I tried that once in the summer of 2006 on a whim with an offer from Metro, since we had our bedroom in the terraced house under the roof and my dear wife is not so happy with heat. It was exactly as Rick says. I then gave the thing to my tire dealer - he had his office in a scorching hot container at the time. If you do it, then fixed installation and coordinated with disciplined closed room volumes.

Just by the way:
Our Mitsubishi air conditioner is broken. A bit older than 2 years, and now it was noticed last week during the first operation of the new year that it no longer works.
Error analysis showed that the outdoor unit is broken, the compressor and inverter probably need to be replaced. Warranty is initially being denied because we are 4 weeks (!) past the 2-year deadline.
I thought that by choosing a renowned manufacturer I would avoid such problems.
I will now try directly with Mitsubishi, I am really fed up...