KfW70 with gas condensing boiler and solar is definitely not possible

Hello everyone!

Yes, that is the statement from my building planner that has been occupying my mind for quite a while now and with which I am not really willing to settle, especially considering the existence of exactly such houses along with the certificate I desire.

But briefly to the key data of our planned single-family house, hopefully including the relevant values here:
- 2 full floors with expanded and habitable attic (35-degree gable roof) with a total of 220 sqm heated area without basement
- heated building volume: 828 cubic meters
- usable area according to the Energy Saving Ordinance: 265 sqm
- primary energy demand QP: 61.02
- transmission heat loss HT: 0.337
- ventilation heat loss HV: 128.35 W/K

The whole thing will be realized with the following technology and insulation:
- gas condensing boiler with solar for hot water (and possibility of heating support)
- controlled residential ventilation with heat recovery
- orientation of the house: SSW/NNO
- exterior masonry 365 aerated concrete, WLG 035, U= 0.229
- pitched roof U= 0.19 (currently 20cm insulation between rafters)
- ground floor slab U= 0.298
- aluminum front door U= 1.3
- triple-glazed windows U= 0.84

As a result, the KfW70 standard is met in terms of transmission heat loss. However, the annual primary energy demand still leaves something to be desired.

When I asked how the last approximately 11 required kWh could still be saved in a (also economically) reasonable way, I received the rather general answer: "with a heat pump or a pellet boiler." But I have deliberately chosen conventional gas and do not want to deviate from that. Moreover, I suspect that this is hardly the only solution, but simply the easiest one.

So before I take the (already planned) step to an energy consultant, I would like to ask the experts here in the forum whether my prospects would really be so bleak if I stick with gas as the primary energy source.

Oh, and… the house is not being built by a developer/general contractor/general planner. So I am relatively flexible in purchasing. Only the cost/benefit factor should be maintained somewhere.

Finally, in advance, the answer to the question: "Why KfW70?"
I am aware that a KfW85 house can have a lower energy demand than a KfW70 house. Much of this can be cleverly calculated (keyword heat pumps). But ultimately, it is the "good feeling" of being able to prove at any time with a certificate that one owns a particularly energy-efficient house. How paradoxical that sounds... Ultimately, I just want to get the best possible energy standard for my budget.

Best regards
Der TeuPhil

Hi,

I am certainly not an expert in these matters. There are others here in the forum who know much more about it.

When I used to deal with the topic of heating technology, a consultant mentioned the zeolite gas heat pump as a particularly energy-saving gas heating technology. But it also depends on your budget, because this costs quite a bit more than a normal gas condensing boiler.

Further information on this can surely be provided by search engines and also by the experts here in the forum.

Hello f-pNo,

thank you for your reply. I had actually already dealt with the zeolite gas heat pump. Definitely a nice piece of technology. However, the disadvantages here are so significant that this technology is only suitable for builders who don’t have to watch their money and can absolutely follow a purely green conscience.

Such a system would be more than twice as expensive as a conventional gas condensing boiler. My heating installer quoted me additional costs of around €14,000. Amortization can probably only be dreamed of.

It must be possible to achieve the KfW70 standard with a conventional modern gas condensing boiler, a controlled residential ventilation system with heat recovery, and (the unfortunately almost essential anyway) solar thermal for hot water and (!) heating support.

By the way, my already approved building application is still based on the Energy Saving Ordinance 2009, if that is even of any relevance here. According to my building planner, I can almost be glad to have been granted a building permit at all despite gas as the primary energy source. …Oh really?!

What is the result of the calculation if you add a bit of insulation, e.g. 42er walls?

...allegedly no significant improvement in primary energy consumption either. Here, the transmission heat loss would rather be avoided even more. However, with the current values, I am already within the desired range for the THL. It fails due to the still too high primary energy consumption.

Maybe someone in this forum has already built a solid house with a single-layer construction and gas boiler and achieved the KfW70 standard... How did it go for you?

I know this is not the answer you want, but still: we left out all this KfW nonsense. Financially, the loan conditions offer little advantage, but you are already restricted in the building technology or forced into some nonsense (my opinion). We simply build according to the Energy Saving Ordinance 2009 with a gas condensing boiler and insulate in such a way that we can use the 15% rule.

Just question everything