TGA planner difficulties, underfloor heating supply temperature + wastewater ventilation

I understand the approach. I didn’t pay attention if the OP has ventilation (or you ) but with ventilation, humidity shouldn’t be a problem.

We have a central controlled residential ventilation system, but unfortunately that is not sufficient for the bathroom.
Currently, humidity rarely drops below 70%. The air-water heat pump simply does not run often enough yet and when it does, currently at 25 degrees supply temperature, with 24 degrees air temperature in the bathroom, that is a delta T of one degree, so basically the heat pump is not heating at all right now and moisture is constantly introduced by showering.

And I don’t know what kind of kW monsters you have for towel radiators, but they really don’t have heating power for “normal” cases and if they do, they consume a lot.

300W.

I hope you don’t really believe your last sentence about free photovoltaic electricity!? :p
That costs you at least the feed-in tariff

On the contrary, I know that for sure. Because of KFW40Plus I have a 60% curtailment.
The 300W of the radiators can be perfectly shifted with KNX into the free 40% that would otherwise be curtailed anyway. ;)
But okay, that is probably only the case for a few.

… and in winter you use what comes down from the photovoltaic (if anything comes down at all) already with your heat pump or base load.

That is correct, but then the heat pump also brings enough into our bathrooms thanks to a fully covered wall.
But as written above, because of delta T of 1 degree, that currently does not bring anything at all.

We have a central controlled residential ventilation system, but unfortunately, that's not enough for the bathroom.
Currently, humidity rarely drops below 70%. The air-water heat pump just isn't running often enough yet, and when it does, currently with 25 degrees supply temperature and 24 degrees air temperature in the bathroom, that's a delta T of one degree, so the heat pump is basically not heating at all right now, while constant moisture is introduced through showering.

Then you have a poorly designed controlled residential ventilation system; that has nothing to do with the heating behavior of your heat pump.

And regarding the rest....

We don't have the combination of KFW40Plus and KNX here and probably it isn’t standard overall.
As far as I understand, you only use the towel radiators during the transitional period because normally (in winter) you have enough heating capacity.
Which, however, would not be the case for the original poster if they want to lower the supply temperature.

In my opinion, electric towel radiators remain a nice gimmick (with exceptions, of course ) for someone who wants warm towels or to dry them faster. I would not include them as a heating capacity. Especially because they are somewhat slow.
Especially in the case of the original poster, who might have to reckon with too little heating capacity in the bathroom if the supply is turned down, it makes the most sense to me to have something that provides heating capacity quickly in the short term. I find fan heaters a very simple solution for that.
Depending on taste, also IR panels or similar.

That sounds of course like a very good and well thought-out solution. With a bit more money and a plot of land, I would have gladly done it that way too. ;)

As far as we are concerned, there is ventilation; how well it works with towel drying, moisture in the bathroom, etc. I will just have to try out. Since towel radiators are included in the scope of work for both bathrooms, there is certainly no reason not to have them simply installed. In the worst case, we will just indulge in the luxury of pre-warmed towels.

Regarding the idea of providing a prepared wall heating, I am also skeptical. I have not met the plumber yet and will certainly do so at the earliest after the heating pipes have been laid. An expert appointment for this is already scheduled. We will look at everything in detail then.

So I would definitely have more pipe installed, reduce the spacing... in the bathroom as small stainless steel as possible. 24 degrees in the bathroom is also completely exaggerated, especially if adjoining rooms have less, it will only work to a limited extent. If floor heating is no longer possible, still try to get the flow temperature as close to 30 degrees as possible, regardless of whether for gas or heat pump, both systems will thank you in the long run, and now you still have the opportunity to persistently influence it further, it is your house and your money.

I don't think the house can provide that. I already went from 40° to 36°, the installation distances in the bathrooms are 5cm. Of course, I could lower the design temperature in the bathrooms to 20°, then maybe I can reduce the calculated flow temperature. But in the end, that is not really a gain.

I don't think the house allows that. I've already gone from 40° to 36°, the installation distances in the bathrooms are 5cm.

EVERY new build runs at 30 degrees flow temperature. A heating engineer might say that's not possible - but he only says that because he simply doesn't feel like it.