Heating during "high daytime temperatures in autumn"

Hello everyone

I have a question about heating at “high daytime temperatures in autumn”.

I am the landlord of a basement apartment (extension to the house, partially basemented, on ground level). The apartment makes up about 20% of the entire house. The relatively large heating system for the whole house is a ground probe heating system. The house is well insulated and has new windows.

My tenant complains about the high humidity in the apartment and wants me to turn on the heating or to adjust the heating settings. Since I do not have these problems in the main house (humidity always between 45-60%), I suspect that my tenant simply does not ventilate properly.

On the morning of 20.09., the humidity in the rental apartment was 74% at a temperature of 20.2 degrees (the maximum 24-hour room temperature was 22.2 degrees).

The weather during this period was as follows:

19.09. temperature max. 22 degrees, min. 13 degrees / rel. humidity outside day 50%, night 90%
20.09. temperature max. 24 degrees, min. 12 degrees / rel. humidity outside day 50%, night 90%
21.09. temperature max. 24 degrees, min. 14 degrees
22.09. temperature max. 16 degrees, min. 11 degrees -> at the latest here the heating switched on automatically

I assume that the heating did not switch on from 19.-21.09. (or only switched on briefly), since the daytime temperatures were well above 20 degrees and the return temperature also rose quite quickly.

Since the outside humidity during the day was around 50%, I assume that the tenant could reduce the indoor humidity simply by ventilating correctly during the day and that using heating is not (cost-)efficient. Is my assumption correct?

Thanks for your answers.

First of all: Yes, I have read that the primary request was about heating. IMHO the issue lies elsewhere, namely with ventilation. As a former landlord I say: Any precise and physically sound instructions for proper ventilation are just as effective as a fart in a hurricane.

What have I discussed, pleaded, shown, documented, over the years.

There was exactly ONE solution and since then peace has prevailed: ventilation with differential humidity control. Of course with frost protection and the usual best practices, obviously. But I made it clear: this is a ventilator that belongs to the house, nothing is adjusted or blocked, it just runs.

So install a sealed and unblockable ventilation system with differential humidity control if possible and the issue is solved.

And if someone still wants to keep their place at 24 degrees, they should get an electric radiator.

So install a possibly sealed and as unblockable as possible ventilation with differential humidity control and the issue is settled.

What have you installed? ... installed ... installed ... installed

That was a long time ago, back then I hadn’t built anything like that myself yet. I went to [Heizung/Sanitär] and said: Do whatever is necessary and send me an invoice. He installed a ready-made control system, a fan with an exhaust flap.