Towel wall heating with heat pump with low return temperature (30 °C)

In the last house, we left out the towel radiator in the (innenliegenden) bathroom. It was never too cold for us in the bathroom. In the new house, they will not be installed either, but we have planned an outlet to retrofit one if it does get too cold.

It could have been due to the heating effect of such a towel holder tending to zero

I can confirm it. We have installed a towel radiator with a heating cartridge in the bathroom because we thought we needed it. For years, we have only hung the towels there but have not used the heating cartridge. I would not make the investment again.

In the last house, we left out the towel radiator in the (internal) bathroom. It was never too cold for us in the bathroom.

It could be because the heating effect of such a towel holder tends toward zero

It could just as well be due to a misadjusted heating system. If you have to slow down the heating with the ERR, it of course won't get too cold, but efficiency is another matter. Moreover, an internal bathroom is likely to have different energy losses than bathrooms with 2 exterior walls and windows. Besides, I don't believe the heating effect tends to zero. Purely based on the heated surface area, such a radiator could easily contribute 20% or more in a smaller bathroom. Clearly, a wall heating is certainly more attractive. But it would probably have to be considerably larger again.