Condensing gas technology with solar system

Hello, our builder offers a gas condensing boiler (Viessmann Vitodens 200W) which is coupled with a solar system (Vitosol 200F 300l water storage tank).

Is this combination necessary? Useful?
We are a household of 3 people, wouldn’t a 150 l storage tank be completely sufficient?

Single-family house, underfloor heating on ground and upper floor, 125m2 living area, ventilation with heat recovery.

As an alternative, he offers an indoor air heat pump also with a 300l storage tank.

Best regards

300 liters is standard - usually people take more. Why should the solar systems only be able to heat 150 liters? The more, the better. Question to the other knowledgeable ones: is there even a smaller storage tank???

Hm I think if only 150l need to be heated, this requires less energy as well as smaller collectors?

The first question is whether the solar system is economically viable or only serves to cosmetically improve the Kfw or energy saving regulation certificate.

With solar, nothing is nicely calculated at first. Without using at least part of renewable energy, it is already very difficult to meet the energy saving regulations (15% rule).

I wouldn’t be deterred by the indicated liters yet. We are not quite at the point where it affects our heating technology. But: What I once heard on the side is that there are also so-called "3-part" boilers. They store a lot of water, but about 1/3 is cold, 1/3 lukewarm, and 1/3 hot. This makes the thing easily 300 liters or more in size.

Did I misunderstand something there? :)

Hello,

so the 300 liters are absolutely suitable, or even a somewhat larger one would be fine, the question is, what does your user behavior look like? If you want to use a lot of warm water for bathing, I would rather go for larger than 300 liters.

And the thing with the stratified tank is correct, with it and through an appropriate size of the storage tank, the solar system can also be well integrated. The purpose of the cooler zone is that the heat harvested from the collector can be used at a lower temperature, therefore the surrounding storage water should not be tempered higher if possible.

Best regards, Erik