ᐅ Decentralized Exhaust System LWA 252/100 for Semi-Detached House – Is It Worth It? Looking for Experiences
Created on: 30 Dec 2025 19:36
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BayernborsB
Bayernbors30 Dec 2025 19:36Hello everyone,
We are currently building our semi-detached house (shell construction completed) and have a common issue: a central ventilation system with heat recovery is no longer possible – ducts were not planned in the ceiling, the floor is full of KNX cables, so it’s impossible to install now without creating chaos.
The heating engineer suggested a decentralized solution with an exhaust air heat pump Stiebel LWA 252 (or LWA 100):
- Central exhaust from 5 bathrooms/toilets/kitchen (across 3 floors, basement has no exhaust) → LWA → hot water
- Fresh air through 7 passive ALD13 wall vents (Kermi x-well)
My questions to you:
1. Does the ventilation actually work for the whole house? Does the fresh air from the vents reach evenly everywhere, or do interior rooms remain “empty”?
2. Drafts/cold air at the vents? Is this especially noticeable in winter, or can it be minimized with ventilation openings directed toward the ceiling?
No decentralized heat recovery devices (Lunos, etc.) due to noise in the bedrooms.
The heating engineer says this is not necessary, but I am worried about mold formation if ventilation through the windows is not done properly (manually ventilating 4 floors is especially difficult).
Who has experience with an exhaust air heat pump + passive vents in a semi-detached or single-family home?
Thank you very much for your practical tips.
We are currently building our semi-detached house (shell construction completed) and have a common issue: a central ventilation system with heat recovery is no longer possible – ducts were not planned in the ceiling, the floor is full of KNX cables, so it’s impossible to install now without creating chaos.
The heating engineer suggested a decentralized solution with an exhaust air heat pump Stiebel LWA 252 (or LWA 100):
- Central exhaust from 5 bathrooms/toilets/kitchen (across 3 floors, basement has no exhaust) → LWA → hot water
- Fresh air through 7 passive ALD13 wall vents (Kermi x-well)
My questions to you:
1. Does the ventilation actually work for the whole house? Does the fresh air from the vents reach evenly everywhere, or do interior rooms remain “empty”?
2. Drafts/cold air at the vents? Is this especially noticeable in winter, or can it be minimized with ventilation openings directed toward the ceiling?
No decentralized heat recovery devices (Lunos, etc.) due to noise in the bedrooms.
The heating engineer says this is not necessary, but I am worried about mold formation if ventilation through the windows is not done properly (manually ventilating 4 floors is especially difficult).
Who has experience with an exhaust air heat pump + passive vents in a semi-detached or single-family home?
Thank you very much for your practical tips.
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nordanney30 Dec 2025 20:09The ductwork for exhaust ventilation is in place, but no centralized mechanical ventilation with heat recovery? Besides a new heating system that also provides domestic hot water, you’re planning to install an additional unit? Are external air inlets with 7 core drill holes necessary?
Honestly? Either go for the full setup with a centralized system or rely on decentralized units with cross-flow heat exchangers. These are extremely quiet (no noise in the bedroom – even the "standard" units are very quiet at basic ventilation levels, as I can personally confirm), and you’ll have to do core drilling anyway.
So, unfortunately, I don’t have experience with your specific question, but I wouldn’t want to install such a setup myself. Just my personal opinion.
Honestly? Either go for the full setup with a centralized system or rely on decentralized units with cross-flow heat exchangers. These are extremely quiet (no noise in the bedroom – even the "standard" units are very quiet at basic ventilation levels, as I can personally confirm), and you’ll have to do core drilling anyway.
So, unfortunately, I don’t have experience with your specific question, but I wouldn’t want to install such a setup myself. Just my personal opinion.
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Bayernbors30 Dec 2025 20:33nordanney schrieb:
Besides a new heating system that can also handle domestic hot water, are you installing an additional unit? Yes, this new unit will completely take over both the exhaust air and the household hot water production (with a storage tank and electric heating element). As far as I understand, the main heat pump will only be used for space heating.
nordanney schrieb:
Are outdoor air vents with 7 core drill holes necessary? Yes, for 5 bedrooms (upper floor/attic) plus the living and dining rooms (ground floor).
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nordanney30 Dec 2025 20:52That was meant rhetorically ;-)
It was intended to illustrate the huge effort involved for a second heat pump, while the heating system only performs one of two functions.
It was intended to illustrate the huge effort involved for a second heat pump, while the heating system only performs one of two functions.
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Gerddieter30 Dec 2025 22:38If the screed has not been poured yet, I would personally opt for a central system.
The second-best option is a decentralized system, also possible with heat recovery. However, in that case, I would make sure to choose units that only require electricity at the core drilling point and have the entire control operated via Bluetooth/Wi-Fi.
Your setup is essentially an "uncontrolled" ventilation system, and I wouldn’t recommend installing that.
Gerddieter
The second-best option is a decentralized system, also possible with heat recovery. However, in that case, I would make sure to choose units that only require electricity at the core drilling point and have the entire control operated via Bluetooth/Wi-Fi.
Your setup is essentially an "uncontrolled" ventilation system, and I wouldn’t recommend installing that.
Gerddieter
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