ᐅ 2 water-proofing layers and three ventilation units are required in a duplex with a granny flat.

Hello dear community,

I have already gathered so much information from the forum, so first of all, thank you for the many interesting and informative contributions. Since I wasn’t able to find an answer through the search, I’m trying my luck with a post here. Our house planning is in full swing, and currently, I am reviewing the first offer from a general contractor (GC).

My children are building a semi-detached house in Munich on an existing plot — each unit is 130 sqm (1,400 sq ft) with a 45 sqm (485 sq ft) fully basemented granny flat. I have now received the first offer from a GC and have the following questions:

• Are two air-to-water heat pumps always mandatory for KFW 55 standards and a total living area of about 300 sqm (3,230 sq ft)? The building specification lists heat pumps from Viessmann, Vaillant, or Daikin (Rotex) as options. A heat demand calculation has not been provided yet.
• The central ventilation system must be installed three times. The contractor’s reasons are the size of the total living area, the three housing units, the high cost, and the required fire protection. It was not included in the offer. When asked, a price of about 30,000 € was mentioned.

I always thought that, for a multi-family/semi-detached house, there is potential for savings by sharing the heat pump and ventilation system.

Best regards Ludwig

LudwigMuc schrieb:

Are two air-to-water heat pumps always necessary for KfW 55 and a total living area of about 300sqm (3,230 sq ft)?

No.

LudwigMuc schrieb:

The central ventilation system must be installed in three copies.

Must? No.

However, since these are two houses and one apartment, I would install separate systems for each house. The apartment would then share the system in one house. Decentralized ventilation for that.
Examples of why I would do this: House 1 hosts parties with raclette and lots of grilled fish. You want the ventilation system to run at full capacity. House 2 is occupied by someone ill and bothered by drafts. They want the ventilation system turned down very low.
House 2 is meant to be sold but has no separate heating system. Think about what that might do to the selling price.
There are many more reasons.

P.S. Significant savings are probably unlikely either, since the system becomes more complex. Normally, for example, two semi-detached houses are completely separate and decoupled. You are reconnecting them with one ventilation system. Maybe then in House 1, you can hear what's going on in House 2’s bedroom through the ventilation ducts in the children’s room…

@nordanney
Thanks for the quick response. How does the noise level behave? Does having 2 heat pump outdoor units also mean a doubling of the noise level, or does the noise level not increase?

LudwigMuc schrieb:

How does the noise level behave? Does having 2 heat pump outdoor units also mean doubling the noise, or does the noise level not increase?

That is a) subjective, b) dependent on the installation, and c) dependent on the device.
Two outdoor units, each properly sized for the heating load and not always running at full capacity, are definitely quieter than one large unit that has to work hard.
It’s like two small compact cars being quieter than a single sports car engine roaring – metaphorically speaking.

There are also large and quiet units.
But separating the technical components, as described by @nordanney, makes sense.
Three controlled mechanical ventilation systems are definitely not practical. Two are more than enough.
I suspect the general contractor usually builds to "standard" and is now upgrading everything to 300m3 (10,594 ft³)...
We have more living space, a ventilation system calculated at up to 10 times the standard in some areas, and still manage with just one unit.