Air-to-water heat pump sizing in new construction

So with most general contractors/prefabricated house companies, etc., you have little chance of contractually fixing something like that in advance. The salespeople you deal with in this early phase lack the technical knowledge regarding building services engineering.

I managed to get mine to the point where he himself suggested reducing the expansion stage so that I can award the building services work externally.

Regarding the waiver of individual room control, they didn’t even know that according to the Energy Saving Ordinance, you can probably waive it in rooms smaller than 6m2. Otherwise, they would have wrapped 10 meters of underfloor heating pipe and installed a separate heating circuit in a 1.5 m2 pantry.

- Room-by-room heat load calculation according to DIN
- 30/27/21 (flow temperature/return temperature/RT) or 30/27/23 in the bathroom
- Set heating circuit length to 80m/circuit +-10%

So now we’re getting to the nitty-gritty. So far, I’ve only discussed 30°C flow temperature with the main contractor. The heating engineer wants to use a standard 35°C with 15cm pipe spacing. When I told him that 30°C flow temperature was agreed upon, he automatically refers to 30/22 flow/return.

How are flow and return temperature actually related? Does it have something to do with the flow velocity and pipe length, so that you can influence it?

Flow temperature determines the heating curve. Return temperature and thus the spread can be considered as a whole or for each heating circuit individually. Return temperature results from the amount of heat delivered and can therefore be influenced by the flow rate. Other influencing parameters are also screed/room temperature and heating circuit lengths. Modern heat pumps actually operate with a smaller spread than the 8K suggested by your heating engineer.

This is Heizungsbauer Reflex, suggest 30/26 or 30/28 to him. And keep an eye on [Heizkreislänge] as well as the pipe diameter of the connection and the circuits.

Also give him the heating circuit length, because he has this large spread and has probably tried to go over 100m.

Normally it is 60-70m

Thank you for the feedback.

First, call the GU again. Then we’ll see. I have been constantly talking to the GU about "room-by-room heating load calculations (DIN 12831)" and about a flow temperature of 30°C. However, the contract itself now states that all design is according to KFW 55. I probably didn’t pay close enough attention there.

However, I have documented various contract discussions with him in minutes and pointed out the above-mentioned points. And his response was that the calculation would of course be done and that a flow temperature of 30°C would not be a problem. He did not contradict that.

Let’s see what he tells me later.

There are various service providers on the internet who offer this as well. Does it make sense to briefly commission it myself and then make a counterproposal to the heating installer? The heating installer will mainly want to install as few pipes as possible at my place and require the shortest possible time for installation while at the same time complying with the Energy Saving Ordinance/KFW55 requirements, right?