Correct calculation by the energy consultant? Why solar for heating?

Hello everyone,

we have received our documents for the KFW loan. We want to build a Kfw-70 house.
Plan bricks T-18 (24 Poroton stones) 16 cm insulation. Floor slab insulated with 10 cm Styrodur.

10% of the value is now supposed to be covered by heating support through solar!! We didn't want that at all. Solar was supposed to be for hot water but not for heating. We come to a value of just over 40. The maximum value is 69,xx for the Kfw-70 house. But I don't understand why we have to use solar for heating support when it only accounts for 10% of the heat?

What can we do?

Details are missing. It looks to me as if this is needed for the "Anlagentechnik", or the share of renewable energies.

Hi Kerstin,

there are really some details missing, I guess,

gas heating?
Was the building application not submitted last year? So new version of the energy saving ordinance?

Then you probably can’t avoid the large ST.

There are two values that both have to be below 70%. One is the transmission heat loss. You are doing very well there with just under 40% (which surprises me a lot. We have the same wall construction 24 cm T18 Poroton with 16 cm insulation with good windows and end up at 68%, that’s quite different?!?)
But there is also the primary energy factor, which must also be below 70%.
And for that, in the case of gas heating, you need the solar thermal system.

Best regards,

Andreas

Hello Andreas,

calculated value primary energy demand 46.15 kwh/m²
transmission heat loss calculated value 0.24 W/(m²K), maximum permissible value 0.4.

These values are in the documents. Yes, it will be a gas heating system.

The application was submitted at the end of last year.

Hi,

with gas heating, a part of "renewable" energy is mandatory in Kfw70.

Honestly: It doesn’t bring anything at all. But it’s regulation. Even if you reach and fall below the values.

Domestic hot water solar thermal: Never pays off. 30-40 cents yield/day ./. electricity costs for the circulation pump (5-10 cents/day) on at best 170 - 200 days/year. A joke in a bag.
With heating support: The whole thing only makes sense when using vacuum flat-plate collectors (about twice as expensive as normal collectors). Because in summer/(large parts of) autumn/spring you don’t need heating in a KFW70 house. I know this because I also have one (with Q_p and H_t rather in the Kfw55 range). In winter (let’s say from October to mid-March) a conventional solar thermal collector does nothing anymore, except look good on the roof (and cause capital costs). The vacuum collector delivers significantly more there. And as is well known: Especially in winter you need heating and exactly then the nonsense with solar thermal and normal collectors doesn’t work at all. But in high summer you get 74-degree hot water for showering and nice hot baths.

So take the cheapest solution. Because you never get the investment for this nonsense back in this latitude here. The whole thing is nothing but a nice and unnecessary gimmick for the environmental conscience that was imposed on the KFW70 house with gas heating.

Best regards
Thorsten

PS: Last year, large parts of Germany had very hot, good weather. Here in NRW/Velbert, demonstrably not. Constant rain + storms. Sun? None. Only a very few days.

The system barely produced nearly 200 kWh in 2015 with great difficulty. Corresponds to: gas for Euro 12,- (I neglect the proportional meter fee for gas) and mercifully leave out the electricity for the ST circulation pump.

Every little water wheel with a dynamo in the rain gutters would have produced more here, every small wind turbine too.

Best regards