Gas and Solar Thermal - Questions about the Offer

Hello everyone,
I have the following description from the general contractor. I notice that the hot water storage tank is much smaller than in other examples here in the forum. Does that make sense?
Are there any experiences with this device(s)?
Is circulation piping required?
Which screed for underfloor heating?

(About the house: 166 m², approx. 9 x 11, KFW 55)

Heating system
System Vaillant:
Gas condensing solar compact unit according to DIN/EN 677, series auroCOMPACT corresponding to the Energy Saving Ordinance with stainless steel heat exchanger. The nominal heat output is approx. 3.3 to 14.7 kW. Depending on the outside temperature, a weather-controlled solar system control type multiMATIC regulates the temperature level. Heating cycles and hot water heating can be individually set via a time program.

Air/exhaust gas routing:
The air/exhaust gas routing of the room air-independent condensing units is carried out as concentric air/exhaust gas routing via the roof.

Drinking water heating
System Vaillant:
The domestic hot water supply of the house is carried out centrally without circulation piping. To supply hot water to the bathroom, guest WC, and kitchen, the previously described gas condensing solar compact unit with integrated 190 L hot water storage tank combined with three solar flat collectors type auroTHERM VFK 145 V/H for rooftop installation, collector area approx. 7.5 m², tailored to the system. A thermostatic mixing valve is installed for scald protection.

Underfloor heating:
Warm water underfloor heating according to heat demand calculation with plastic piping on carrier plate and room control (surface-mounted thermostat).
According to the contractor’s choice, a cement or anhydrite screed will be installed.
One heating circuit distributor per floor is provided, which will be installed flush with the wall.

Thank you very much,
Hannes

With 3 collectors, I would - for a south-facing roof and an inclination between 25 and 45 ° - take at least a 300 l. Or better one collector less with 190 l. Otherwise, there may be a risk of overheating in summer, if it is not a Drainback system.

From a hygienic perspective: 30-50 liters per person per day. A complete exchange per day should be aimed for. 4 persons, high demand = 200 liters 3 persons, medium demand or 4 persons, low demand = 120 liters There are also collectors that can be switched off. Or you omit the collectors and meet the legal requirements in another way. With controlled residential ventilation, definitely no problem.

Is there generally the possibility to choose a different gas heater? With just under 170 sqm and KFW55, you have an estimated heating load of 5-6kw at a design temperature of -12 degrees. Most of the time, the minimum output (3.3kw) will still be much too high, which will lead to short cycling. This affects the lifespan.

If 4 people shower one after the other, we need the 190 l. but they are also sufficient. So, the storage tank doesn't need to be bigger. Why 3 collectors, I would ask. With the small storage tank. And: Isn't the heating system a bit too powerful? I would also ask. Karsten

Nordlys, what storage temperature do you have? To what temperature is the water heated in the tank at your place?