ᐅ KfW 70? New construction – which heating system, air-to-water heat pump, underfloor heating, KfW 85 instead of KfW 70

Hello everyone,

I am currently planning a semi-detached house of about 120m² (1290 sq ft) turnkey from the general contractor. The plot is about 280m² (3014 sq ft). Like many others, I am feeling overwhelmed by all the information I have been reading. However, I am still not making much progress regarding the heating system.

First, some details about the house:

Currently offered:
Concrete slab with 60mm (2.4 inches) Styrodur (thermal conductivity 0.035 W/mK)
The house will mostly be clad with facing bricks (small plaster areas) – 140mm (5.5 inches) insulation KD 035, mineral wool with vapor barrier
Silicate plaster on Poroton bricks
Currently planned for a single-flue chimney (not yet finalized, possibly later!)
Floors: Ground floor – screed about 65mm (2.6 inches) on 90mm (3.5 inches) insulation
Floors: Upper floor – screed about 65mm (2.6 inches) on 60mm (2.4 inches) insulation
Attic floor laid with OSB boards (200mm (7.9 inches) thermal insulation WLG 035 fully bonded vapor barrier)
Insulation: upper floor between rafters 200+50mm (7.9 + 2 inches), in the collar beam area 140mm (5.5 inches) insulation WLG 035
Windows: 5/6 chamber profile, U-value = 0.6

The heating system currently planned:
Air-to-water heat pump – Viessmann Vitocal 200-s heat pump, output 7.7 kW
Vitocell CVA 300-liter (79 gallons) domestic hot water storage tank
Underfloor heating on ground and upper floors

Now to the main point:
It would be great to reach the KfW 70 standard, but I’m not a fan of an air-to-water heat pump. I would rather switch to a gas condensing boiler with solar support and domestic hot water assistance. (According to my calculations, the annual cost of a gas boiler would be only slightly higher!)

The builder told me that the KfW 70 standard cannot be reached with this setup, only KfW 85.
It would require additional heating support from the solar system – which is not really worthwhile.
Or alternatively, including a ventilation system in the calculation. Both options would add about €10,000 more.
That is, of course, a significant amount. Could anything be improved in the insulation to reach KfW 70?

Or should I simply forgo KfW 70 and choose a solid KfW 85 variant, just with gas plus solar for hot water?

I am happy to hear any suggestions...

Best regards,
snaky80

@€uro
I can’t say whether a demand assessment was conducted, but it seems that the builder often installs the air-to-water heat pump variant. It also appears to be a low-budget option based on information online.

According to the builder: simply installing gas heating plus solar thermal for domestic hot water is not sufficient. The mentioned options (heating support or a centralized mechanical ventilation system with heat recovery) must also be added; otherwise, the KfW 70 standard cannot be achieved.

Why is a solar thermal system for domestic hot water considered only useful for the "paper calculation"? Is it not effective enough?

Regards

Snaky80

snaky80 schrieb:
....It also seems like a low-budget version according to what I found online.

This is often the case with general contractors/building contractors (GU/GÜ) projects. At the very least, the air source heat pump should match the demand and be fully modulating.

snaky80 schrieb:
....Statement from the building consultant: It would not be sufficient to install only gas plus solar thermal for domestic hot water; the mentioned options (heating support or central mechanical ventilation with heat recovery) must also be added, otherwise the KfW 70 standard cannot be achieved.

This cannot be assessed remotely. One would need to review the calculation process. Sometimes GU/GÜ steer clients towards a preferred solution.

snaky80 schrieb:
....Why is a solar system for domestic hot water only suitable for the “paper calculation”?

The reason is straightforward. A reference building is used as a benchmark, where a solar thermal system for domestic hot water is predefined. Theoretical coverage rates of 50 to 60% are assumed, which are practically almost never reached. It’s a “paper optimization.” In reality, solar thermal is rarely cost-effective for single-family homes. Nevertheless, the combination of gas condensing boiler plus solar thermal system is quite common, especially in GU/GÜ projects. One might be suspicious of someone thinking ill of this.

Best regards.

... I have finally decided on the condensing boiler with solar. Now the question is which boiler is recommended?

I have been offered:

Viessmann 200-W, alternatively 300-W (both in the 19 kW (19,000 BTU/h) version)
+ 2 hot water storage tanks of 160 liters (storage placement has a maximum height of 146 cm (57 inches) due to 26° roof pitch)
+ 5 m² (54 ft²) flat plate collectors, alternatively the tube variant (not from Viessmann, as it is too expensive, according to the heating engineer)
+ 2 drip trays for the hot water storage tanks

Would you recommend the 200-W or 300-W model? Is the extra cost of 500–600€ worth it?

Are there alternative manufacturers of equal quality, or better? Or is there not much difference in the condensing boiler sector?

Regards

Snaky80

snaky80 schrieb:
... I have finally decided on the condensing boiler with solar. ..

Why?

snaky80 schrieb:
... (both in the 19 kW version)..

Is the building located on the Zugspitze?

snaky80 schrieb:
... Or is there not much happening in the condensing boiler sector there? ..

That has already changed!

Best regards

Germans pay approximately 42 billion euros in taxes each year on mineral oil taxes, plus 19% VAT, which roughly amounts to 58 billion euros in total.
When oil largely runs out in about 15-20 years, what will the government do? They certainly won’t go bankrupt; of course, they will come up with new taxes. The most obvious solution is to transfer the mineral oil tax (65 cents per liter + 19% VAT) to the kilowatt-hour of electricity in order to somehow reach the 58 billion euros again… Actually, if they’re going to do it, they might as well set it to 90 billion, so the budget balances again.

For now, I’m sticking with gas and waiting to see how things develop. After all, solar panels already reduce the load on the boiler.
The trend is moving toward passive houses or net-zero energy homes anyway. People already worry if KfW55 standard homes will still be affordable in 30 years. So why build to KfW70? The additional effort is manageable, and a ventilation system also helps prevent mold.

The Viessmann Therm 200-W seems to be very good; I want to have that in my house too.
Ranked number 1 in tests.