Sanitary shell inspection: What should we pay attention to?

2. In my chain of logic: "15-20 cm pipe spacing and 35°C flow temperature are generally too much, because closer spacing and lower flow temperature are better," am I making a mistake in reasoning or can the argument "15-20 cm fits best for our house" already be correct?

No, you are not. How is the pipe spacing planned in the bathroom? At 15cm it will be more than tight to have a comfortable temperature. There is no factual reason not to specify the flow temperature at 30 degrees (and your desired indoor temperature) and then plan the heating accordingly.

He said he could basically understand that, but whether it makes sense he still has to recalculate (have calculated).

Just ask him if it makes sense to drive permanently at 200 km/h on the highway, or if fuel consumption might be lower at 120 km/h. It is exactly like that – at 35 degrees you have higher heating costs. At 30 degrees lower heating costs, but also a slightly higher initial investment (more pipes, larger manifold, etc.).

Just ask him if it makes sense to drive at 200 on the highway permanently, or if the fuel consumption might actually be lower at 120. That's exactly how it is - at 35 degrees you have higher heating costs. At 30 degrees lower heating costs, but also a somewhat higher initial investment (more pipes, larger distributor, etc.)

True, the comparison between 120 and 200 is somewhat exaggerated though. As far as I am informed, with a heat pump you assume about 14% higher heating costs if you increase the supply temperature from 35 to 55°C. That's not nice, no question. In a well-insulated new build (150m² 50kWh/m²/a) with 7,500 kWh, that results in "only" 1,000 kWh heat per year, so with a heat pump around 300 kWh electricity consumption.

Yes, it's not nice, should be avoided if possible and if you remember. But it is far from your comparison. At 120 I consume less than 4l/100km diesel, at 200 it's easily 7.

As far as I am informed, with a heat pump you assume 14% more heating costs if you increase the flow temperature from 35 to 55°C. That is not nice, no question.

Unfortunately not correctly informed. 5 degrees correspond to about 12.5% increased electricity consumption (about 2.5% per degree). Therefore, from 35 to 55 degrees about 50% more consumption (matches the data of my own heat pump when it makes hot water at 50 degrees. ==> The comparison to a car fits quite well ;)

I found a comparative study from Fraunhofer on this... annual performance factor shown over flow temperature. Your 50% does not match that.

However, the variation is large, it also depends on the heat pump. But what do they know, right? ;)

I found a comparative study from Fraunhofer on this... Annual performance factor represented over the flow temperature. Your 50% do not match with that.

However, the variation is large; it also depends on the heat pump. But what do they know, right?

Just google "increased consumption heat pump increase flow temperature" (or similar). The results are consistently around 2.5% per degree. And you will find articles from the last 15 years that have always stated this figure. And if you also look at the electricity consumption of your heat pump, you can confirm these values.

Got it... I found a nice study from Fraunhofer and a graphic from ifeu on this.
For an air-to-water heat pump, the seasonal performance factor goes from 3.5 at a 35°C supply temperature down to 2.8 at 55°C supply temperature.
Well, that's not 14%, but 20%... but also not 50%...

For geothermal, it looks significantly worse, that's true. It drops from 4.8 to 3.2. That's 50%. But very few people have geothermal, and 3.2 is still more than 2.8 for air-to-water.

It was a field study, so measured on real devices. Presumably, the value for geothermal has been generally applied for 15 years and also used for air-to-water? I wouldn't be surprised, since craftsmen/companies like to make everything as dramatic as possible to their advantage ;)

PS:
Solar installers do the same nonsense, by assuming constant photovoltaic system yields throughout the year and then calculating miracles about how much you save... nobody tells you that you feed most of the energy into the grid in summer and hardly have any yield in winter... just when you need it... ;)