KFW70? New construction, which heating, air-to-water heat pump, underfloor heating, KfW85 instead of KfW70

Hello everyone,

I am currently planning a semi-detached house of about 120m² turnkey from the BU. Plot about 280m².
As is the case for many, my head is now smoking from all the reading.
But I am also not really making progress when it comes to heating.
First some information about the house:

Currently offered:
Base slab with 60mm Styrodur (WLG 035)
The house will mostly be clad with facing bricks (small plaster surfaces) - 140mm thermal insulation KD 035, mineral wool with moisture barrier
Silicate plaster on Poroton bricks
Currently a single-flue chimney (chimney not yet planned, possibly later!)
Floors: Ground floor – screed approx. 65mm on 90mm insulation
Floors: Upper floor – screed approx. 65mm on 60mm insulation
Attic floor laid with OSB boards (200mm thermal insulation WLG 035 with vapor barrier fully glued
Insulation: Upper floor between rafters 200+50mm, in the collar beam layer 140mm insulation WLG 035
Windows: 5/6 chamber profile Ug value = 0.6

For the heating system, it is currently planned:
Air-water heat pump - Viessmann Vitocal 200-s heat pump power 7.7 KW
Vitocell CVA 300 liter domestic hot water tank
Underfloor heating ground floor + upper floor

So now to the essentials:
It would be nice to achieve the Kfw 70 standard, but I am not a fan of an air-water heat pump. I would rather switch here to a gas condensing boiler with solar and domestic hot water support. (According to my calculation, the annual consumption of a gas boiler would be slightly more expensive!)

The BU said that KfW 70 standard cannot be achieved with this, only the 85 standard.
You would have to choose heating support by the solar system – which is not really worthwhile.
Or add a ventilation system in the calculation. Both options would cost about €10,000 more.
That is of course quite a lot of money. Could something perhaps still be changed about the insulation to achieve KFW 70?

Or should one simply forgo KFW 70 and choose a solid KfW 85 variant, only with gas + solar for hot water?

I am happy to listen to suggestions...

Regards

Snaky80

@€uro I can’t tell you whether a needs assessment was carried out, but the BU seems to install the LW heat pump variant quite often. It also seems to me, according to the internet, to be a low-budget version. Statement BU: It would not be sufficient to install only gas + solar for hot water, but the mentioned options (heating support or central residential ventilation) would have to be added, otherwise the KFW 70 standard cannot be achieved. Why is a solar system for hot water only suitable for the "paper calculation"? Is it not effective enough? Regards Snaky80

....It also seems to me to be a low-budget version according to the internet.

This is often the case with general contractor/developer projects. At least, the air heat pump should match the demand and be fully modulating.

....Statement BU: It would not be sufficient to install only gas + solar for hot water, but the mentioned options (heating support or central residential ventilation) would also have to be added, otherwise the KFW 70 standard cannot be achieved.

This cannot be assessed remotely. One would have to trace the calculation process. Sometimes general contractors/developers "steer" the interested party in a direction favorable to them.

....Why is a solar system for hot water only suitable for the "paper calculation"?

The background is simple. A reference building, in which a solar thermal system for hot water is predefined, serves as a comparison basis. Theoretically, coverage rates of 50..60% are assumed here, which are practically almost never achieved. It is a "creative accounting" on paper. In fact, solar thermal is hardly economical in most cases in single-family houses. Nevertheless, the combination of gas condensing boiler + solar thermal system is quite frequently found especially in general contractor/developer projects. A knave who thinks badly of this.

regards

... I have now finally decided on the condensing boiler with solar. Now the question is which boiler is recommended? I am being offered:

Viessmann 200-W, alternatively 300-W (both in the 19KW version) + 2 hot water tanks of 160 liters each (placement on the storage tank - max height of 146 cm there, due to 26 degree roof pitch) + 5m² flat plate collectors, alternatively the tube variant (not from Viessmann, as it is too expensive, according to the heating engineer) + 2 drip trays for hot water tanks

Would the 200-W or 300-W be more advisable, is the extra cost of 500-600€ worth it?

Are there equivalent alternative manufacturers, better ones? Or is there not much happening in the condensing boiler sector?

Regards

Snaky80

... I have now finally decided on the condensing boiler with solar. ..

Why?

... (both in the 19KW version)..

Is the building located on the Zugspitze?

... Or is there not much happening in the condensing sector there? ..

That has already "happened"!

best regards

The Germans pay about 42 billion euros in taxes per year on mineral oil taxes !! + 19% VAT = roughly estimated 58 billion euros If oil largely runs out in about 15-20 years... What will the state do? It certainly won’t go bankrupt, of course it will come up with new taxes, nothing is more obvious than to transfer the mineral oil tax (65 cents / liter + 19% VAT) to the kilowatt hour of electricity, so that somehow the 58 billion come together again.. oh well, if we’re at it, let’s make it 90 billion, then the budget will balance again I’ll stick with gas for now and wait for development, after all, the solar panels already relieve the boiler. The trend is anyway towards passive/plus energy houses. People even worry about whether Kfw55 will still be affordable in 30 years. So why build Kfw70 then? The extra effort is manageable, such a ventilation system also prevents mold. The Viessmann Therme 200-W seems to be pretty good, I want to have it in my house too. Number 1 in the test.