Underfloor heating air-water heat pump. House too warm when the sun shines

Just a question related to the topic: Is there any reason against keeping the towel radiator (hot water) in the bathroom (connected to the same heating circuit as the underfloor heating) running permanently (for support)?

Firstly, you hardly have any heating output with the normally low supply temperatures of underfloor heating. How long do you want to hang a towel to dry at a supply temperature of maybe 28°C or less? Secondly, you also have to greatly slow down the flow to avoid a thermal short circuit. This further reduces the heating output. Ergo, it is better to install an electric towel radiator without disturbing the underfloor heating. It is then better for occasional supplementary heating.

Just a question related to the topic:
Is there any reason not to keep the towel radiator (hot water) in the bathroom (connected to the same heating circuit as the underfloor heating) running permanently (for support)?

I did it that way myself because the bathroom was/is the "problem area" for me. Larger heating surface means lower flow temperature required.
I also did it in the guest bathroom, where you can then really well dry the kids' wet clothes on it.

Does it make sense with 28 degrees flow temperature? Better to decouple the towel radiator and heat it with a heating cartridge.

It already feels like it makes a difference. A towel radiator just has a fairly large surface area. Ultimately, the floor also does not have a higher temperature and heats the entire room. A heating cartridge would probably make more sense, but unfortunately, I don’t have any power nearby. So I’m making the best of it.

That's right, the increase in heating surfaces is a pro.

I miss my old 40-50 degree hot towel radiator.