Air-water heat pump in KFW55 house / heating load, standards?

Hello dear heating professionals,

we are currently in the process of planning our heating system with the heating engineer. In the last few days, there has been a lot of talking and discussing between the architect, energy consultant, and heating engineer, as the calculations from HB and the energy consultant were far apart. But first things first.

The energy consultant originally calculated the following heating load for our house using the simplified method (DIN EN 12831-1:2017-09 sec. 8):
NAT -16°
Room temperature +20°
Ht= 0.261 W/(m2K)
Heat transfer surface area A 584.8 m2
Building air exchange at NAT nGeb 0.50 h-1
Air volume V 745.6 m3
= Building heating load 10.1 kW

The heating engineer offered us a Stiebel Eltron LWZ 5 S Plus. I then noted as a layman that 10 kW vs. 5 kW somehow sounded illogical and that 10.1 kW seemed quite high to me. The calculation was then adjusted, as the NAT for my postal code is -10°. With otherwise unchanged parameters, the energy consultant now calculates a building heating load of 8.4 kW. But even here, from my layman’s perspective, 8.4 kW stands opposite the 5 kW of the heat pump.

However, the product information states that the air-to-water heat pump is recommended up to 8.0 kW (see attachment).

The reason for all the differences is supposed to be that the building heating load was calculated according to DIN 12831, and the heat output of the LWZ is based on EN 14511. Thus, everything is supposed to match.

The statement from the heating engineer was also that in all living rooms we lay 15 cm and in the bathrooms we put in as much as possible.

A few more info about our single-family house: KFW 55 monolithic with 36.5 Poroton. Ground floor + upper floor 171 sqm (completely heated) as well as a partially heated basement. The heated floor area in the basement is an additional 29.3 sqm.

And to add to the confusion, I used the often quoted online heating load calculator (1st Google search result for “Energieeinsparverordnung-Heizlastabschätzung”) and come to a heating load of 5.25 kW with the listed parameters.

I’m completely lost now. Three heating loads, two standards, what now?

Thanks very much for your input. I would be happy if some light is shed on the matter.

Choose a specialized office for the calculation of the underfloor heating and the heating load.
Costs a few hundred euros but helps.

Specify temperatures for individual rooms, and the supply temperatures should not exceed 30 degrees.
Clarify with the contractor whether he works according to these installation plans (there has been a case here where the contractor dropped out because he didn’t like it).

You need a proper calculation of the underfloor heating.
The length of the individual circuits should not exceed 100 meters.
The heating circuit manifolds of the individual floors must be connected with adequately dimensioned pipes.
Install them in the bathrooms also under the shower and bathtub if possible.
Keep wall heating in the bathrooms as an option in mind.

Do you have a controlled residential ventilation system with heat recovery?

This must be taken into account in the calculation.

https://www.hausbau-forum.de/threads/lwwp-dimensionierung-im-neubau.36049/page-16

Who used the wrong NAT for the calculation? The HB or the energy consultant?

Who took the wrong NAT for the calculation? The HB or the energy consultant?

The energy consultant. The -10° now fits our postal code. I just called the HB again and asked what supply temperature he calculates with, for example, or whether he can guarantee me 30°, and what spacing he intends to use. The answer was no, he cannot guarantee that. After all, he doesn’t know the parameters of the house ;-) and the pipes are usually laid at 15cm spacing in living and bedrooms and 10cm in the bathroom. I then asked whether the bathroom would also get warm at 24° with a supply temperature of 30°... Then the conversation suddenly headed in the right direction. It switched from general statements to the fact that they can calculate after all. Another coordination is now taking place between HB, EB, and the architect. What I have not understood, however, is what the different outputs in the product datasheet mean: 8 kW according to DIN 12831 and 5.5 kW according to DIN 14511. Would the heat pump basically be suitable (without considering efficiency) to heat the house without the heating element constantly having to supply additional heat? Can someone enlighten me? Many thanks.

And as if the confusion wasn't enough, I used the often-cited online heating load calculator (1st search result on Google "Energy Saving Ordinance - heating load estimation") and came to a heating load of 5.25 kW with the parameters given.

So calculated with about 2 people and centralized controlled residential ventilation (-10 degrees NAT). The value fits perfectly. The values from my engineering offices (room-by-room heating load calculation and the design of the underfloor heating) and those of the calculator you used matched almost exactly for me.

The statement from the contractor was also that in all living rooms we install 15cm and in the bathrooms we put in as much as possible.

That alone would be a reason to change the contractor immediately. Idiot!

Layman's opinion: The 5 S Plus fits well.

But: Do you really want to have the heating designed for only 20 degrees? That would be too cold for me. A professional should calculate that. Flow/return 30/26 at 22 degrees in living rooms and 24 degrees in the bathroom.

Do you have a controlled residential ventilation system with heat recovery?
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Yes, the controlled residential ventilation system has heat recovery.