KFW-55 house cools down strongly

I just wonder if a new house is allowed to cool down that much in such a short time...

You can’t say that in general.

If it is colder outside than inside, every house cools down, no matter how well insulated it is, unless heat is somehow supplied. That can be the heating, the sun, body heat, or electrical consumers (cooking, lighting, etc.) in the house.

To answer your question, it would be more interesting to observe how fast the room temperature drops, for example overnight.

Do you have no (south) windows or are the blinds down during the day? 19° in the living room after a day like today is already little. We lowered the venetian blinds of the west windows from the afternoon because it got too warm. Rooms with south-facing windows still have 22° in the morning here, other less used rooms 20-21°.

The ventilation system surely has heat recovery. With some devices, you have to set the heat exchanger manually from summer to winter. Measure the temperatures at the supply air vents.

Maybe there is still a lot of residual construction moisture in the house, which consumes energy. Is the house solid or made of wood?

What is the construction like? Nothing will reach you now if I tell you that the [OG] only cooled down by 0.7 degrees overnight.

You do write ventilation system, but there are also systems without heat recovery.

And please do not compare it with an apartment. When the neighbors heat, they indirectly also heat a non-heated apartment. Basically, you need an apartment that is in the middle, hardly needing any heating.

Possibly the bypass in the controlled residential ventilation still open at night? Then it cools down even faster.

I had suspected a bypass now as well. Last night we had 3 degrees, during the day outside 20, inside the house it warmed up to 24 degrees through the windows. In this transitional period, a fireplace is something great "we don't have one yet" but my neighbor already lit his, for example I am not that cold but my wife has already taken out her pajamas with socks.

Bypass or with us it is a summer cassette that cooled down by 2-3 degrees at night. The week before last, at night it was between 4 and 10 degrees. I only replaced it a week ago.