Additional costs due to incorrectly planned ventilation system + floor-to-ceiling windows?

Because unfortunately I don’t know who the window and door builder will be =(. Maybe it can be found out. But I think they are really fed up with us already

Question about the heat pump.
So I think we will go with the Novelan. Simply because I have only read bad things about Rotex earlier.

But with the Rotex HPSU Compact 308, I have a 300-liter hot water tank.
With the Novelan LAD 5, only one of 180 liters.
Is that even enough? 2 adults and 1 child. Another child is planned. So a household of 4 people.

We also have a ventilation system with heat recovery from the company Pluggit, the Avent ventilation unit AP.
From Novelan there is a combination module, the "Luftstation," which contains a domestic hot water tank and a passive ventilation unit for supply and exhaust air with heat recovery.
Wouldn’t we be better off with that?
First, from the same manufacturer and not 2 different ones, and second, space-saving since you don’t have to install an additional device if I saw it correctly.

Yes, but you can also just save on passive ventilation and open and close the windows... it comes out to the same thing...

all right, I didn't know that.

And what about the 180L water storage? That's way too little, isn't it?

Otherwise, we will probably accept the €2138 extra cost for the Novelan and €1509 for the extension/relocation, since it apparently is significantly better than the Rotex.

and what about the 180L water storage tank? Isn't that way too little?

No, not necessarily. The storage temperature is higher than the desired temperature at the tap. That means it is mixed down. It certainly depends on the number of people and their shower/bathing habits and how long the heating takes to reheat. With gas, I wouldn't worry about that at all, and a heat pump can activate the heating element if necessary – then you basically have a continuous flow heater – hot water guaranteed – but poor efficiency or high operating costs. We currently have, I believe, 170L in the gas boiler ourselves and even with guests, we've never had problems with cold water. Despite having a rain shower.

It really depends a lot. I like to bathe very hot. When I have been in the tub and then 2 people shower for about 10 minutes, my parents' tank with 180 liters is depleted.