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

Hello everyone!

We are currently in discussion with our prefabricated house provider regarding the flow temperature for the underfloor heating. It is a house built with wood frame panel construction and KFW40 standard. According to the construction description, the provider plans the flow temperature of the underfloor heating at 35°. We mentioned that we do not consider this up to date for a new building and would prefer a flow temperature of about 30°. Now we were told that with a flow temperature of 30°, a larger heat pump as well as a completely different design of the heating system would need to be planned and the additional costs would quickly become five-digit. Such additional costs would, of course, make no financial sense. I had thought that the flow temperature is primarily determined by the pipe spacing and would be accordingly lower if the pipe spacing is reduced. Why could a lower flow temperature lead to a larger heat pump? What am I overlooking? I have to say that the heating load calculation and the exact determination of the heat pump will still be carried out. In any case, it will be a Vaillant Arotherm Plus.

Also one more question about the floor covering. We know that tiles are optimal for underfloor heating but vinyl is almost equally good. We will also have an active underfloor cooling installed (via the air-water heat pump). Does one of the floor coverings - tiles or vinyl - have advantages when it comes to cooling? Or would laminate even be best for cooling?

I had the problem with this requirement that the low flow temperature would have required a small temperature difference. This temperature difference would not have been achieved by the Vaillant Arothoterm Plus 55/6 in my case. Only the 75 model could deliver the required pump capacity (pumping water through heating coils - not the compressor) according to the datasheet (1x00 liters/hour).

Basically, it applies that lower flow temperature = SMALLER heat pump. If it manages the necessary pump capacity for the heated water.

How does the provider calculate and what is their reasoning?

Edit: This value

In my opinion, the biggest challenge is always to get the bathroom properly warm.

This almost always requires very tight installation distances, circuits, and loops on the wall, so that it is comfortable at a flow temperature of 30 without e radiators.

…. Tiles … vinyl …. equivalent …. laminate…

Find the mistake! One word does not fit. There are two solutions ;)

Additionally, a question about the flooring.
We know that tiles are optimal for underfloor heating, but vinyl is almost equally good. In our case, an underfloor cooling system will also be installed (active via the air-water heat pump). Does one of the floorings – tiles or vinyl – have advantages when it comes to cooling? Or would laminate even be best for cooling?

Regarding the flooring, it depends on the construction you mean. Glued vinyl is very good, with cork insulation and floating installation with impact sound insulation is not.

Whether hot or cold does not matter; it is about thermal conductivity.

I am equally satisfied with glued vinyl and tiles in my case. But I naturally cannot notice a difference, since I do not have a consumption comparison to "tiles only."

I had thought so far that the flow temperature is primarily determined by the installation spacing and would accordingly be lower if the installation spacing is reduced.

That's correct.

Now we were told that with a flow temperature of 30° a larger heat pump as well as a completely different design of the heating system would have to be planned and the additional costs could quickly reach five figures.

Yes, it has to be designed differently. Different installation spacings, possibly more circuits, more heating circuit distributors. Larger heat pump? I don't see that at first. In the end, it depends on your house – 99% of the time no different heat pump is needed. Many heat pump manufacturers, for example, install the same pump in every heat pump, so that the devices are identical except for heating capacity. But five-figure costs can occur – the home builder wants to make good money from special requests. The material cost as well as additional installation effort will at most be in the low four-figure range. Apart from that, I completely agree with you that 35 degrees is not up to date. But this allows for a simple and cheaper design and the house will be cheaper...