Heat pump for new single-family house efficiency house 55

Hello dear forum members,

I am currently working with the company Lehner Haus from Heidenheim to realize my prefabricated house in timber frame construction. During the plan discussion, the topic came up as to which air-to-water heat pump should be used. I tried to educate myself a bit beforehand. Nevertheless, I would be happy to hear your opinion on this.

First about the construction itself. A Efficiency House 55 with approximately 200 sqm living space is planned. Attached are the plans as well. Underfloor heating is planned for all rooms except the pantry and technical room. Four people will move in. Usage behavior I would describe now as normal or average. Currently, there is no one who stands in the shower for 30 minutes. The bathtub is currently used about every 2 weeks. Of course, this may change when my children are older. Temperature-wise we are rather cool in the bedroom. Living room etc. around 20-21°C. Bathroom of course somewhat warmer but at least in the upstairs bathroom there will be a towel radiator.

Lehner Haus offers 2 providers for the heat pump: Vaillant and Viessmann Currently included in the price is a Vaillant Arotherm Split 105/5 AS, a 300-liter domestic hot water buffer tank and a compact buffer tank VWZ MPS40. A complete heat load calculation is not yet available. However, a rough preliminary estimate by the company showed that the heating system is oversized and rather a 75/5 AS or 85/5 AS should be sufficient. They also recommended here to switch from the aroTherm Split to an aroTherm Plus. Lehner Haus currently recommends a Vaillant aroTherm Plus 75/6 A S2.

Here I am unsure whether this is sufficient and also ask myself why originally a significantly larger heat pump was offered. In various posts on the Internet you read that the heat pumps were planned too small and therefore now operate inefficiently because they have to be run too "hard". But you also read the same statements the other way around that the heat pump should not be planned too large. Also, as a layman it is difficult for me to compare which heat pump is better (Split or Plus). As far as I understand, the Plus is more efficient and quieter. But is it worth the extra cost? And are there possibly comparable cheaper pumps or equally priced better ones from Viessmann? In the current issue of Stiftung Warentest Viessmann also won. Of course, only Lehner Haus can tell me my final price. Removing the heat pump from the scope of Leistung from Lehner Haus is unfortunately not an option for me because then I would also have to remove both the underfloor heating and the entire sanitary installation (apparently the same team).

Can you possibly help me here and give recommendations on what the right choice is? I have to make the final decision at the beginning of November.

Many thanks Best regards ToBu1991

There is currently no complete heat load calculation available.

Then choose the heating system only when that is the case.

In various posts on the internet, one reads that the heat pumps were planned too small and therefore now work inefficiently because they have to be operated too "strongly". But you also read the same statements the other way around, that the heat pump should not be planned too large.

The heat pump has to fit. If it is too "big", especially if it cannot modulate down enough, it will short cycle. So it goes on and off constantly. This significantly reduces the lifespan. If it is too weak, the heating element has to be used additionally at very cold temperatures. This then increases electricity costs. For example, we have the 75 model from Vaillant for just under 200m² and it is clearly oversized. We only took it because it was heavily subsidized by the Bafa at that time.

In various posts on the internet, you sometimes read that heat pumps were planned too small and therefore now operate inefficiently because they have to be run "too hard." However, you also read the same statements the other way around that the heat pump should not be planned too large.

Explanation:
A heating load calculation is carried out based on the standard outdoor temperature. So maybe -15 degrees. However, this temperature is reached on very few days—if at all. So normally, the heating = heat pump only has to operate at a fraction of its capacity.
If the heat pump is too large and cannot modulate down enough, it starts to short-cycle. It turns on, pumps heat into the house, and then turns off again. This is not good for the device. If the heat pump is suitable or possibly a bit smaller than necessary, it runs at the appropriate performance range for at least 95-98% of the year. If it is so small that it cannot handle -15 degrees over several days (these are then daytime temperatures at -15 degrees), then an electric heating element is needed.
For my region, however, I cannot remember that the standard temperatures were reached any longer. At most as a "highlight" during the night. However, this does not bother the heat pump because the house would first have to cool down before it uses the electric heating element.
Experience shows that you operate very well and, above all, efficiently with a suitable or slightly smaller heat pump.

Besides, it is also difficult for me as a layman to compare which heat pump is better (Split or Plus). As far as I understand, the Plus is more efficient and quieter. But is it worth the extra cost?

The Plus is actually intended as a device for older buildings / renovations. I can’t think of a single reason to install this in new builds, unless it was cheaper.

Otherwise, wait for the room-by-room heating load calculation and then see which device fits.

The Plus is actually intended as a device for old buildings / renovations. I can’t think of a single reason to install it in new buildings. Unless it is cheaper.

The Plus is simply a completely different device. A monoblock where water is carried out instead of refrigerant lines. More efficient and a more modern refrigerant.

The plus is simply a completely different device. A monoblock where water is led out and not refrigerant lines. More efficient and a more modern refrigerant.

That is clear to me. But in new buildings, I don't need a heat pump that is tuned to 55 degrees flow temperature – especially for old buildings/renovations. A "normal" heat pump is sufficient for me. I personally also like monoblocks and have already installed two myself. Otherwise, see

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Hello everyone,

first of all, thanks for the informative answers.
I was aware that I have to wait for the final heating load calculation before making a final decision. I was more looking for an initial assessment of whether it could even work with the models mentioned.
I will also upload the heating load calculation as soon as I have it.

So the Plus costs me between 2,000 and 3,000 euros more depending on the model (75/105) compared to the Split. If it really has no added value, I am happy to do without it.
I am also happy to consider other models from the two manufacturers (Vaillant/Viessmann) if that makes sense. Are there other sensible options here or is the aroTherm Split actually the most reasonable one?