KFW 70 is not achieved, target values for gas condensing boilers

Hi guys,
I need some advice from you. Our single-family house new build is currently being calculated and the person doing the calculations called me and said that the house does not reach Kfw 70 and I was shocked because I actually thought we would almost reach KFW 55.
Now to the house data:
186m² living space
Basement insulation 8cm XPS 0.04 WLG
Exterior wall 1.5cm gypsum plaster, 24cm Poroton T16, 18cm 0.035 WLG mineral wool, 2cm air layer, 11.5cm clinker
Windows triple glazed UG 0.7 Roof 6cm insulation over rafters, 24cm 0.035 WLG insulation between rafters, 6cm insulation under rafters WLG 0.035
Heating system Buderus GBH172 T75S 14KW with 750L buffer tank, 5SKS 4.0 collectors at roof pitch 45° south orientation no shading
Underfloor heating on ground floor and upper floor
I hope I didn’t forget anything. So according to the transmission heat losses it should already be close to KfW 55, but the heating system is also taken into account. Although it is a gas condensing boiler, it should still reach KFW70.
Since I of course obtained offers from some house manufacturers and construction companies, I know what they do for KfW70 to achieve this and there was not even a ventilation system included and almost always only a gas condensing boiler with hot water preparation. Regarding insulation, no house planned had what we had planned. My question to the experts is: is my calculating person right, which would actually mean that almost all house providers are cheating,........ or did she make a calculation error. I understand that opinions here do not replace a proper calculation but a rough assessment should be possible.
Thank you in advance!

Good evening,

€uro will surely go into the topic in more detail.

186m² Wohnfläche

A lot of concrete might have to be built into the exterior walls – after the structural calculation.

Dach 6cm Aufsparrendämmung, 24cm 0,035 WLG Zwischensparrendämmung, 6cm Untersparrendämmung WLG 0,035

Why did you put so much effort into the roof? What is the purpose of that?

Meine frage an die Fachleute hat meine berechnende Person recht, was ja eigentlich bedeuten würde das nahezu alle Hausanbieter bescheißen.........

That depends on which providers you asked

Gas condensing boiler & solar is always a hot topic; anyone who says that KfW 70 can always be achieved with this combo should change profession. Whether your advisor is right or miscalculated no one here can answer reliably. Many factors – for example the concrete mentioned at the beginning – play a role; also the climate location.

Rhenish greetings

Hello,

.....and said that the house does not reach KFW 70 and I was shocked because I actually thought we would almost reach KFW 55....

On what is the assumption of KfW 55 based?

...Now to the house data:....

Purely based on the initial data, actually formally suitable for KfW 70 (no guarantee for this statement without verification!!!)
Wherever people work, mistakes can happen. I am not error-free either! Therefore, I use plausibility checks in my calculations to prevent such, possible issues.
One should give the creator the opportunity to review. Sometimes it is formal small matters or simply incorrect entries!
On the other hand, a software error can be present, absolutely no rarity!!!
I am always surprised how carelessly software is used. For liability reasons alone, every user should check results in sample calculations manually, which is rarely successful for pure "key operators" or "form fillers" due to a lack or absence of professional competence. The main thing is kg of paper produced!

.... So based on the transmission heat losses, it should be just about KFW 55, but heating is also considered....

Correct, this is legitimately intended by the legislator, because the efficiency of a system has a significant impact on the overall result.

.... My question to the experts is whether my calculating person is right, which would actually mean that almost all house providers cheat.........

Here one should definitely distinguish between verification procedures and the reality actually to be expected.
Verification procedures only have to formally fulfill or comply with the "paper form", real consumption costs are something completely different!

Best regards

Hello

thanks for the answers.
Now to the questions asked.

Why did you put so much effort into the roof? What is the point of that?

In my opinion, the roof is the most important component and I simply believe it should be well insulated. Besides, in my old house I only had 18cm of insulation; in winter OK, at least I didn’t notice it getting cold, but in summer it got warm far too quickly for me, so significantly more insulation and more mass, instead of vapor barrier OSB boards, stone wool instead of mineral wool, and insulation above the rafters.

On what basis is the assumption KfW 55 made?

I only thought of KfW 55 because I know the U-values from the Energy Saving Ordinance 2009, e.g. exterior wall, ceiling 0.28 W/m²K, of which 30% is for transmission heat loss, for KfW 55 that is 0.196 W/m²K my value according to U-value calculator 0.144 W/m²K and then did that for all components and thought it should come out close to KfW 55 but definitely KfW 70. I’m aware that the U-value calculator only provides a rough estimate and thermal bridges and so on cannot be represented, but even then there is actually still enough room to improve.

Here one should strictly separate the verification procedure and the reality actually to be expected. Verification procedures only need to formally meet the "paper form", actual consumption costs are something completely different!

That is clear to me as well, but in my case it is about getting the KfW funding program 153, 2.1% interest, and therefore the paper is important at first.

It seems to me as if the house were calculated according to the Energy Saving Ordinance 2014 and then I could understand that KfW 70 is not achieved because that approximately corresponds to KfW 55 now. But according to my information, 2014 is only valid from 2016 for new buildings.

Greetings from Lower Saxony

Hello,

In my opinion, the roof is the most important component and I simply think it should be well insulated. Besides, in my old house I only had 18cm of insulation; in winter, OK, at least I didn’t notice it getting cold, but in summer it got warm way too quickly for me, so definitely more insulation and more mass, instead of a vapor barrier OSB boards, rock wool instead of mineral wool, and Aufsparrendämmung.

Basically, the insulation effect does not increase linearly with the insulation thickness; in my opinion, you could have saved yourself that money.

**Regarding summer heat protection: the purpose of insulation materials is to limit the heat flow and not to store heat. A good solution for the roof area is a so-called Aufsparrendämmung, which, applied on top of the roof frame on the outside, fully envelops the entire construction. The roof is – cost-effectively, as less thickness is required – excellently protected against sweltering heat in summer.

It seems to me that the house was calculated according to the Energy Saving Ordinance 2014 and then I could understand that KFW 70 is not achieved because that now roughly corresponds to KFW 55. But according to my information, 2014 is only valid from 2016 for new buildings.

The date of the building application counts for the calculations. In the Energy Saving Ordinance 2014, practically nothing changes initially for new buildings; only from 2016 on must an additional 16% be saved. Therefore, your consultant “probably” could not have miscalculated by human standards.

** I borrowed the formulation from the RGZ; I can’t explain it any better myself


Rhenish greetings

Hello again,

I think I found the mistake.
I looked through the building permit documents again and noticed that my application preparer (construction company) stated the solar yield at only 15%, which seems a bit too low for a system with heating support. I then took the trouble to calculate the solar yield for my house and the other specifications such as location, etc., using various portals. The result was always a total coverage of 30%-35%, significantly more, even though I know these are only approximate values, but compared to 15% it's about twice as much. Also, at least in the application, it was not indicated that a wood stove with a water jacket for heating support would be installed, which, if I am correct, can also be rated at 15%. And that would then be a total of 50% (I know, a bit nicely calculated) from renewable energies.
The question to you is, can it be like this or have I mixed something up?
If not, then KFW 70 shouldn't be a problem at all, right?

Regards