Lowering the supply temperature in underfloor heating more complex than expected?

Thus, you have much more water in the underfloor heating, and need a higher volume flow, ergo a larger heating system, which you actually do not want,

That is not correct. At least not the larger heating system. First, there are many manufacturers who install the same pump model in their heating systems – regardless of whether it is a 5kW or 9kW heat pump. And the volume flows do not increase that drastically.
In addition, I would add that not only the supply line but also the return line must be taken into account. There is a big difference whether I design for 30/26 or 30/24.
Here, the original poster should provide their data on the heating design and the heat pump.

So with the Vaillant Arotherm plus series, the technical data is already correct. The 55/6 supposedly does not achieve the required volume flows. However, in the neighboring thread, there was already suspicion that the pumps might actually deliver more.

On page 1 of the thread, you see the possible volume flows. They do change with a larger heating system (580 --> 790 --> 1418 l/h). And of course you can run the water slowly through the underfloor heating, resulting in a larger temperature difference (30/24). But whether the bathroom will still get warm so that the house's lady likes to take a shower is questionable. Then we are talking about a bathroom radiator (of course electric). Costs money and additional energy. Coming back to my initial question: what does the OP want to achieve by reducing the theoretical 35°C to 30°C? Saving energy -- unlikely. Saving investment -- also not. Increasing lifespan -- also not. So?

on page 1 of the thread you can see the possible volume flows.

Sorry, I didn’t see the edit on page 1. But I’ll just increase the data that’s listed there now. Namely from the 55 model from 580 l/h to 860 l/h. Or from the 65 model from 790 l/h to 1,205 l/h. These are the official maximum values you get from Vaillant (see Vaillant). Adjustable in the device. Therefore, I would be surprised if anyone has real problems with the volume flows for an average KfW40 house. With 860 l/h, i.e. 14 l/min, you can easily get any normal 40-class house up to temperature (I assumed about 140-160 sqm) with 30 degrees flow temperature. Aside from that, I find Vaillant rather simple and suboptimal in terms of installed components and options anyway. I much prefer the Asian manufacturers like Pana, Samsung, LG, Hitachi who generally install decent pumps right away. Even the entry-level sizes run with well over 1,000 l/h, often even up to 2,000 l/h. My small Pana manages max. 2,100 l/h – which I of course never need.

Yes, so I have the 75/6 and I would not recommend it, see our arotherm thread...
https://www.hausbau-forum.de/threads/vaillant-arotherm-plus-vwl-35-55-75.42822/

It’s actually logical, the Asians have been producing heat pumps (air/air as split air conditioning units) for much longer than the established Central European companies.
The plumber does the hydraulics anyway, so the devices themselves are much more mature.
If only I had been that far two years ago...

We have vinyl in wood look with a very rough surface in every room except the bathrooms and the utility room.

Which vinyl have you installed? We definitely want wood look too - no matter if tiles or vinyl.

edit: Much more important for efficiency is that your heat pump can modulate down FAR ENOUGH! Don’t let yourself be sold a big heat pump and then it switches on and off all the time!

I understood that - but it should be irrelevant whether it’s the 35/6 or 55/6, right? Or am I reading the data wrong?

Bonding is always direct. Although that also has consequences for impact sound. In your buildup you then have BP/ceiling, insulation and then underfloor heating in the screed. And if people walk barefoot on warm floors, you hear it. The culprit is the heel strike you’re used to from wearing shoes. With ball-of-foot walking you don’t hear anything, and neither with slippers.

That’s interesting - I would have bet that it’s louder with slippers than barefoot. Does it make a difference for impact sound if you choose tiles or vinyl?

First of all: what is your goal? Because that’s not quite clear to me. You have to distinguish the theoretical calculations from the real conditions here.

My goal is to size the heating so that all rooms get warm enough and at the same time do not consume unnecessary electricity. Since I have read everywhere on the net that 35°C leads to higher electricity consumption and is no longer up to date, that made me suspicious.

The 35°C flow temperature is normal because it refers to the NAT. The calculations say that at NAT your house must also be warm. So your heating must manage a flow temperature of 35°C. In real life this only occurs for 2 days every 5 years. Most of the time your flow temperature oscillates between 26 and 30°C, rarely above.

Thank you very much for the explanation. But when you read here and also in other threads about supply temperature of 30 degrees, they always refer to the NAT, right? And if the supply temperature at NAT is lower, isn’t the supply temperature at, for example, 2 degrees outside temperature also automatically lower?

Aside from that, I find Vaillant rather simple and suboptimal in terms of installed components and options anyway. I praise Asian manufacturers like Pana, Samsung, LG, Hitachi who usually install reasonable pumps directly, for example. Their entry-level sizes run at well over 1,000 l/h, gladly also up to 2,000 l/h. My small Pana manages max. 2,100 l/h - which I of course never need.

Well, we’re tied to Vaillant. That is specified by the provider. Surely they are not the best but I also don’t think they are the worst. At least we have some acquaintances who are quite satisfied with their Vaillant. If the heat pump ever breaks, can we choose any other manufacturer as a replacement? Or is it a case of once Vaillant, always Vaillant?

Yes, well I have the 75/6 and I would not recommend it, see our arotherm thread...

The thread has very, very many pages ;). Could you please briefly summarize why you would not recommend it?