KfW 70 without ventilation system

Hello everyone,

maybe someone here can give us a good tip, because we are torn back and forth regarding a ventilation system. We are planning to build a house (solid house) according to Kfw 70 requirements, with underfloor heating, a fireplace, and a solar system. That is already certain.

Now we have received an offer from Company A for a ventilation system with heat recovery, and the seller also recommended it to us.

At Company B, the ventilation system is significantly more expensive, and the seller also told us that the house is so well insulated that there would be no problems with moisture... where should it come from, and the rest could be regulated by normal ventilation anyway. (I also think he realized that his ventilation system would have blown our budget and now I am worried that he only advised us against it for that reason!)

Now my husband and I are laymen... but my instinct would tell me, the better the house is insulated, the more problems I have getting the moisture/old air out of the house, right? After all, such a blower door test also shows that there is not much air circulation. Am I seeing that completely wrong? We are now somewhat torn, after all, such a system is not exactly cheap! But is it necessary, or just a nice extra?

Thank you very much for your help!

Hello, we faced the same problem a few months ago and then decided on a controlled residential ventilation system. You hear different opinions about it, but most builders nowadays choose a controlled residential ventilation system, as do most energy consultants.

For us, the point was that with the controlled residential ventilation system we fall well below the KfW70 standard. Just above the KfW55. This doesn’t bring advantages in terms of KfW (subsidy), but the less energy demand, the better for the electricity bill later on. In addition, our heat pump, an AlphaInnotec, is fully integrated with the ventilation system (KHZ 60), so that the interaction between both systems significantly increases the COP value.

Furthermore, with the controlled residential ventilation system, there is the option to insulate the exterior masonry again with an external thermal insulation composite system later on, but then a controlled residential ventilation system is almost essential.

The gain in comfort is also not to be underestimated.

Hello,

Now we have received an offer from Company A for a ventilation system with heat recovery, and the seller also recommended it to us.

You should continue negotiating with this seller and remove the others from your list of potential candidates.

The houses built today according to the latest technology and the requirements of the Renewable Energy Heat Act as well as the Energy Saving Ordinance are so tight that normal ventilation as a rule is not enough to get the moisture out of the house. Keep in mind – especially if you and your husband both work – when you can ventilate?

There are many good ventilation systems available and of course with different price ranges; if a central system will exceed your budget, get advice on decentralized systems. The latter have, for example, the advantage that you do not have flat ducts running under the ceiling and can relatively selectively ventilate the rooms you find sensible; not every room has to be ventilated individually. Central systems take up a lot of space in the utility room, so it should be planned to be over 8 sqm if such a system is used.

According to the latest results of blower door tests in our new buildings, I see a ventilation system not as a nice extra, but rather as an investment to avoid trouble (mold).

Kind regards

Thank you already for your quick responses!

I am also rather "pro" ventilation system. That’s why I was so confused about what the salesperson was trying to convince us of.

Of course, he immediately started to badmouth the competitor’s system (Vallox Controlled Residential Ventilation 090 SC) and to sell his own system (Helios Controlled Residential Ventilation) as better.

BUT I would still prefer a system (Vallox) rather than not being able to afford the Helios, right?

@Bulla I also think that the gain in comfort should not be underestimated, not to mention that mold is the last thing I want in the new house!

The only disadvantage with provider 1 is that they use lightweight construction for the interior walls upstairs instead of solid like provider 2. But I think I could live with that... they don’t fall over just from leaning against them either.

Such decisions are really difficult!!

Our DEURA builds everything completely solid. Nothing had to be changed, we just had to accommodate the ventilation shafts here and there, for example on the ground floor right behind the door. The ventilation system is indeed from Aplhpainnotec, but is supplied by Pluggit. You can roughly calculate the costs with them as well.

Hello,

...We are planning to build a house (solid construction) according to KfW 70 standards, with underfloor heating, a fireplace, and a solar system. That much is already clear

Solar thermal systems are rarely economical in single-family homes, but they do help meet primary energy requirements

...
We have now received an offer from Company A for a ventilation system with heat recovery, and the salesperson also recommended it to us.

Of course, he’s selling it after all
Ventilation systems are convenient and comfortable, but neither mandatory nor absolutely necessary in every case.
Here, one can have the necessities checked before investing 4..7 thousand euros!

...
Now my husband and I are laypeople... but my instinct tells me the better insulated the house is, the more problems I have to get rid of moisture/old air from the house, right?

Modern buildings are relatively airtight, which is sensible and necessary. However, simple window ventilation is still often fully sufficient for standard buildings (Energy Saving Ordinance, KfW 70), even though the fear of mold business is booming. In doubtful cases, this should be checked.
For buildings with very low heating loads as well as passive houses, however, one can hardly get by without a CMV system with heat recovery. Here, controlled ventilation plays a major role.

Best regards