Heat pump underdimensioned for a 6-family house?

We have recently purchased an existing condominium in a six-family house completed in 12/2014. The house has a built-up space of 3,114 cubic meters, and the total living area is just under 640 sqm. Installed is an air-to-water heat pump from Stiebel Eltron WPL 33 HT with the hot water storage tank SBB 1000 heat pump SOL. The heat output is specified in the description as 12.38 kW at A -7/W35, and 7.45 kW at A2/W35 partial load. I am not an expert, can make little sense of these figures, and have also found little enlightening information on the internet so far.

The electricity consumption of the entire house was extremely high in 2016 at 30,360 kWh (= 7,650 euros). Could it be that the system was chosen significantly too small and therefore constantly has to be over-heated? According to the seller, the construction company subsequently had to install a slope reinforcement at six-figure cost. I wouldn’t be surprised if savings were made elsewhere...

If you enter the search options for new buildings > 240 sqm on the Stiebel Eltron homepage in the product selection, the above-mentioned pump is not suggested. Possibly a first indication of a non-ideal configuration?

In my opinion, the rents for various heat meters are also disproportionately expensive. Can they be replaced by our own meters, and what does something like that cost? An air-to-water heat pump is supposed to be very low maintenance. Are the maintenance costs of 672 euros reasonable?

Thank you very much in advance for competent answers.

I can only compare with an air-to-water heat pump offer that we received from Alpha Innotec for our single-family house with about 200sqm of heated area. Although this does not replace a heating load calculation, ait roughly estimated with 45 W/sqm, which means 9000W or 9kW. In view of this rough calculation, I would already find 12kW quite strangely low for more than triple the area. Others roughly calculate sometimes with 35W/sqm, but also 50W/sqm. But even then, 12kW seems rather low. Couldn’t you ask for the heating load calculation? It should be there in black and white...

Sufficient for a 1,000-liter tank - you just have to calculate. With old heaters, it is rather disadvantageous for consumption if they are oversized.

Man man man.. It could also simply be that the heat pump is running inefficiently due to excessively high supply temperatures. This is often caused by incorrect heating behavior. And this rough estimate can be completely discarded and simply forgotten. For a heat pump, it is simply mandatory to have a heating load calculation. And that room by room. If this is not done, there will be too little underfloor heating installed in one room, and at efficiently low supply temperatures the room will not be warm enough.

Man man man..

Why so annoyed?

No one claimed that a heat load calculation is pointless or unnecessary. On the contrary, the only truth can be derived from it if anything.

Until it is available, one can only make estimates. What is so wrong with that?

And since a manufacturer of air-to-water heat pumps also makes rough estimates for orientation until the heat load calculation is available, their approach was explained here to give the OP a first, albeit very rough, indication.

For an exact verification of the dimensioning, the heat load calculation is mandatory. Nothing contrary was claimed.

Joedreck, are you a professional and therefore perhaps understandably annoyed by rough estimates because they cause various problems in your professional practice? If so, that would also be important information for me, since I am not, but I could then confront the air-to-water heat pump manufacturer with it...

Hi, no, I am not a professional. But I have been interested in it as a hobby for over 7 years and am relatively good at it for an amateur. And I am not annoyed by the people here, but by the heating engineers who do not want to calculate and therefore mess up a lot, and by landlords/construction companies etc., who never care what happens because they do not bear the costs. Often in multi-family houses, everything is simply set to "the main thing is that it gets warm." With gas and oil, that's still somewhat manageable, but heat pumps are absolute low-temperature systems. Every degree more in the supply temperature costs efficiency, which means real money.