ᐅ KfW 40+ Subsidy with Photovoltaic System and Ventilation Unit

Hello everyone,

We are currently deciding which efficiency class to choose for our build.
It will be a prefabricated house with 150 sqm (1,615 sq ft) including a fireplace, for 2 adults and 2 children, equipped with an air-to-water heat pump from Stiebel Eltron (probably the LWZ 5 S).
The heating system will be provided by the prefabricated house manufacturer.

For the KfW subsidy, we received the following offer:
(Insulation is excluded)
Air-to-water heat pump + ventilation
16.37 sqm (176 sq ft) photovoltaic system on the house (9 modules each 390 Wp, 3.51 kWp south-facing attic roof)
Usable battery capacity 2.6 kWh
Manufacturer: BYD Premium HVS 5.1

Unfortunately, I have read a lot of negative feedback about ventilation systems and am quite hesitant.
Also, I see a lot of information about photovoltaic systems with many modules and a small battery.

The total package (photovoltaics + ventilation) would cost me 21,000 €.

Could you please help me?

Does this option make sense, and can I upgrade or improve the system later?
I want to be very cautious as I am not very familiar with this topic.

I would appreciate any answers.

Thomas W. schrieb:

Unfortunately, I have read a lot of negative things about ventilation systems and am quite skeptical.

I actually read almost exclusively positive reports about mechanical ventilation with heat recovery (and I can confirm these reports myself). What exactly did you read?

Thomas W. schrieb:

16.37 m² (176 sq ft) roof-mounted PV system (9 modules, each 390 Wp, 3.51 kWp, south-facing, 25° attic roof)
1 housing unit usable capacity 2.6 kWh

That’s just a token system. You should cover the entire roof.

nordanney schrieb:

That’s just a token system. Cover the entire roof.

I completely agree! 3.5 kWp is really ridiculous!

Thomas W. schrieb:

16.37 sqm (176 sq ft) pitched-roof PV system (9 modules of 390 Wp each, 3.51 kWp, south-facing, 25° roof pitch)
1 residential unit usable storage capacity 2.6 kWh
Manufacturer: BYD Premium HVS 5.1

I can understand the idea of choosing a small battery to qualify for subsidies. But as mentioned, the system size is a joke!
Fill the roof properly. Ideally, plan for photovoltaics already when designing the roof (planning extensions or dormers so that photovoltaic installation is smooth and easy).

Normally, you should be able to install around 8–15 kWp on a single-family home.
Then select the battery accordingly — approximately 30–60% of the PV system’s kWp capacity.

As already mentioned here: Fully equip the roof. It's better to get separate quotes. The ones from the house manufacturer are always much too expensive; they usually only cover basic ventilation systems.

What have you heard that's bad about ventilation systems? At most, they produce dry air. Otherwise, negative feedback is more often about air-to-air heat pumps (that is, heating via the ventilation system).

Neubauling schrieb:

As already mentioned here: Fully equip the roof. It’s better to get separate quotes. The offers from the house manufacturers are always much too expensive, as they usually only cover basic ventilation systems.

What bad things have you heard about ventilation systems? At most, dry air. Otherwise, most complaints are about air-to-air heat pumps (i.e., heating via the ventilation system).

As you already wrote, dry air. That worries me the most, otherwise there are contaminated ducts where bacteria can develop.

Thomas W. schrieb:

As you already mentioned, the dry air.

First of all, the air is always dry in winter (even without mechanical ventilation with heat recovery) and secondly, you can also choose a system with humidity recovery.

Thomas W. schrieb:

That worries me the most because contaminated ducts can cause bacteria to form.

Bacteria develop in moist conditions. But you yourself say the air feels too dry. You have to decide. Both cannot happen at the same time ;-)
Besides, the systems don’t get contaminated if you maintain them properly (regularly changing filters).