Air-water heat pump including ventilation system vs. brine heat pump including ventilation system

Hello dear forum community,

we are currently planning our heating system for our new build (135 sqm living space, KFW 55). So far, we thought we wanted to get an air-to-water heat pump with central ventilation with heat recovery. KfW is applied for and approved for the air-to-water heat pump.
Now we had a conversation with a heating engineer who warmly recommended the brine heat pump to us. It all sounds logical to us… better annual performance factor, since deep underground there is never a risk that it gets too cold… thus the "risk" that the electric heater has to kick in with the air-to-water heat pump and the annual performance factor becomes very bad is eliminated.
Price-wise, the two systems are about the same due to the subsidy (the brine pump is then 1500 euros more expensive after BAFA).
With the brine pump, there would be the option of a decentralized ventilation system that only centrally extracts air from the kitchen, bathroom, guest WC, and utility room and supplies fresh air through mechanical openings at the windows (this variant is even 1500 euros cheaper than the air-to-water heat pump).

Now to the questions:
1. Does anyone have experience with the heating systems and can tell me why they decided on one of the two variants?
2. Do I have to change the KFW application if I decide on the brine system, even though it has a better annual performance factor?
3. We definitely want a ventilation system with heat recovery, but the decentralized solution sounds somehow strange to me… does anyone have that? Or is the central solution better?

Thank you very much for your answers :-)
Bauer2018

We are still in the planning phase, but ground source heat pumps are basically the better system where they can be used and drilling is affordable (so with you, according to your offer). The efficiency is not primarily better because of the heating rod but mainly due to the closer temperatures at the warm (heating water) and cold (brine or outdoor air) ends. But whatever, with an effective price difference of €1500, it pays off relatively quickly. And it's also nicer because the annoying outdoor unit is eliminated.

The choice of ventilation system is completely independent of the chosen heat source; you seem to be mixing that up in your head...

Through the decentralized supply air (without heat recovery!!!) you are destroying the gained efficiency again. Although only seemingly, but that would have to be calculated. However, my assumption is probably close to the truth.

By using decentralized supply air (without heat recovery!!!) you are destroying the gained efficiency again.

… moreover: Who would want to have the house wall drilled through in every room where there is a decentralized ventilation system so that the device has space. This solution is also not really attractive visually.

… besides: Who wants to have the exterior wall drilled through in every room where a decentralized ventilation system is installed, just to make space for the device. Visually, this solution is also not really attractive.

I would have expected a window rebate ventilation instead.

Price-wise, the two systems are quite similar due to the subsidy (the brine system is then 1500 euros more expensive after deducting the BAFA subsidy).

If that is indeed the additional cost compared to the air-water heat pump and the conditions are met, I wouldn’t hesitate for long. However, the extra cost seems a bit low to me. How much does the drilling cost for you? How many meters do you have to drill? Or are you talking about horizontal collectors?
One more addition regarding the brine-water heat pump. Please also check the regeneration capacity of your soil beforehand. If you have a high clay content in the soil, that could potentially be an issue. Otherwise, if everything fits, I would invest the 1500€ immediately – at least for aesthetic reasons alone.
Regarding the controlled residential ventilation. I think going decentralized to save money is saving on the wrong end.