KfW70 in 2 variants ... help wanted!

Hello everyone.

After having been in close contact with about half a dozen home building companies and our construction project (i.e. wishes vs. what is feasible) is basically "set," two home builders have now been shortlisted. Both rely on different methods to achieve KfW70.

A single-family house with approx. 135 m² living space including a basement is to be built.

Home builder 1 (A* House):

Construction will be in timber frame construction. This one relies on a gas condensing boiler + solar system + controlled ventilation. Here, the recommendation was made in favor of gas with a "buried" gas tank as a solution (which would have the advantage of being able to choose the provider). Fundamentally, the conversation gave the most competent impression in my eyes, as far as I can judge with my half-knowledge.

Home builder B (L* Solid house):

This is also a "prefabricated house manufacturer," but one that builds solidly (if it can be called that). According to the representative we spoke with today, the Klimapor walls (look like a rice cake) are built without foil, so a controlled ventilation system is not necessary. However, to achieve KfW70, he would install an air-to-water heat pump.

In all other respects including price offer, both are largely similar. And now we don’t know what to do. Home builder B explained to us today that foregoing a ventilation system has only advantages. From what I have been able to find here so far, the prevailing opinion is different. With a ventilation system, you can no longer open the windows at will, or it would affect the costs and function of the system or restrict its purpose. Besides that, you save the gas connection costs. Also, it would not be necessary because of the omission of foil in the walls anyway. Sounds somehow ... strange.

And now we are really uncertain. We basically always sleep with the window open (summer and winter). Even in the living room, we usually keep the balcony windows open permanently in summer. We do not want to give that up.

What now? Are the statements from B really correct or is that nonsense? Has anyone had experience in this regard or with the mentioned companies?

We would be very grateful for any tips!!!
Regards, Andi

Hello,

House builder 1: ... Although here the recommendation was made towards gas with a "buried" gas tank as a solution (which would have the advantage of being able to choose the supplier)...

You can already do that today with a "conventional" gas connection. In addition, you have plenty of space in the basement to accommodate the boiler along with all the required technology. What logical reason would there be for you to spend more money? With the one exception maybe that there is no gas supply in the vicinity of your property and you would have to rely on liquefied gas?

House builder B:.. According to the statements of the representative we spoke to today, the Klimapor walls (look like a rice cake) are built without foils, which is why a controlled ventilation system is not necessary ...

Excerpt from the publicly accessible BB: "The solid house exterior wall is basically 24 cm thick, made entirely of the solid and healthy natural building material xxx-clay plus a 12 cm thick full thermal insulation". What is the full thermal insulation made of?

Further in the BB: "The particularly dense exterior shell of our buildings prevents unwanted heat loss. To ensure a controlled supply of fresh air, a controlled ventilation system with heat recovery may be sensible."

What is correct now, the statement of the seller or rather the information from the BB?

Kind regards

Hello,

... Both rely on different means to achieve KfW70.

Almost all doors are open for "creative accounting" ;-)

... This one relies on a gas condensing boiler + solar system + controlled ventilation. Here, the recommendation towards gas with a "buried" gas tank was proposed as a solution (which would have the advantage of being able to choose the provider).

Liquefied gas and then also a solar thermal system? That's quite something ;-)

... According to the statements of the representative we spoke to today, the Klimapor walls (look like a rice cracker) are built without foils, which is why a controlled ventilation system is not necessary.

Aha, so you can skip a BD test? ;-) What sellers come up with is sometimes more than sad!

... But to achieve KfW70, he would install an air-to-water heat pump.

Which one?

... We would be very grateful for tips!!!

Yes, immediately get an external consultant and planner for heating, domestic hot water, and possibly ventilation!

best regards

Hello.

Thanks first of all for the feedback.

The solution with the buried tank actually came about because we would have to "bridge" about 15m from the street to the house. But that was just a suggestion. I would actually prefer a direct connection.

Regarding B: Somehow the seller’s argumentation is starting to make me suspicious.

How is it basically with such a KfW70 house, is the statement correct that with a ventilation system you can no longer open the windows or that it makes no sense, for example, to leave terrace doors open for a long time in summer? The decision between A or B currently largely depends on the system.

I will still consult an independent advisor ...

Hello,
You can already do this today with a "conventional" gas connection. In addition, you have plenty of space in the basement to accommodate the boiler and all the necessary technology. What logical reason should there be for you to spend more money? Except maybe that there is no gas supply in the vicinity of your property and you would rely on liquefied gas?

Reason see above. Whether gas is available at the property is still open (it is a partitioned sub-property of a larger one)

Excerpt from the publicly accessible BB: "The solid house exterior wall is basically 24 cm thick made entirely from the solid and healthy natural building material xxx-clay as well as 12 cm of full thermal insulation." What does the full thermal insulation consist of?

Uhm ... good question ...

Furthermore in the BB: "The particularly tight outer shell of our buildings prevents unwanted heat loss. To ensure controlled fresh air supply, a controlled supply and exhaust ventilation system with heat recovery may be sensible under certain circumstances."

What is correct now, the seller’s statement or rather the information in the BB?

Best regards

I tend to think the BB’s. What unsettles me even more.

Hello,
Almost all doors are open to "sugarcoating" ;-)
Liquefied gas and then also a solar thermal system? That’s quite intense ;-)

What do you mean by solar thermal system? What do you mean by intense?

Aha, so you can skip a BD test? ;-) What sellers sometimes put forward is more than sad!
Which?
Yes, get yourself an external advisor and planner for heating, domestic hot water, and possibly ventilation immediately!

Best regards

See above.

Regards
Andi

...Regarding B: Somehow the seller’s argumentation is making me suspicious.

That is quite justified. Once you have signed, .... Caution is always the mother of the porcelain box ;-)

..., is the statement true that with a ventilation system you can no longer open the windows or that it is pointless, for example, to leave terrace doors open forever in summer?

That is nonsense. Although a momentary, i.e. temporally limited undefined state is created, it does not play a significant role in the overall context.

..I will still consult an independent advisor ...

You can gladly send me an email if needed ;-)

..Reason see above. Whether gas is available on the property is still open (it is a partitioned sub-property of a larger one)

You don’t need a tank for natural gas! ;-) Solar thermal system, often used in conjunction with gas condensing boilers, to make the balance look good on the paper. Solar thermal systems usually don’t pay off economically in single-family homes with “normal” user behavior!
best regards

To A):
Please ask what the "controlled ventilation and exhaust" is supposed to consist of. There is currently another thread where this is supposed to be done only through window ventilation => I would advise against that.

To B):
The wall usually consists of "Styrofoam bricks" which are then reinforced with reinforcing steel and poured with concrete. This method is fast and cheap, but not really good.

Furthermore, if you want to install a heat pump, there is still a missing circuit "before it". The heat pump usually only serves to enhance an existing heat source (e.g. geothermal => deep collector). But without this circuit, the heat pump is absolutely useless. And if you are already buying a heat pump (usually costs about 5-7k€) you can usually also bear the remaining costs for a low-energy house or even a classic passive house.

I would definitely consult a specialist here, as recommended by €uro.

Best regards, Thomas