What to buy boilers and boiler for a combined heating system?

The minimum boiler power is taken from the calculation of 100 W per 1 m2, in your case it is 18 kW (power line: 12, 18, 24, 30 kW). Plus, we lay a small margin in power, and take into account that part of it will be used to heat water in the boiler. It turns out either 24 kW, or 30 kW. When choosing, you must take into account that up to 24 kW the boiler can be wall-mounted, if you expect such a placement. If the boiler is supposed to be used outdoor, then 30 kW can be supplied.

The boiler capacity is usually calculated based on the number of family members. But, in order not to go into calculations, you can rely on practice: in houses of this area boilers of 150-200 liters are usually installed. It is important to take into account that the capacity of the boiler heat exchanger (it is indicated in the technical specifications of the device) does not exceed the capacity of the boiler. I would suggest options: a 24 kW boiler and a 150 liter boiler, with an 18-20 kW heat exchanger, or a 30 kW boiler and a 200 liter boiler. In the second case, the heat exchanger power can be selected in the range of 24-30 kW.

If you plan to connect a solar collector, it is better to choose a boiler with two heat exchangers - as in the photo. By adjustments, you can set the priority of solar heating. A useful additional function is an independent TEN heater installed directly in the boiler. It is useful in the summer if you do not want to turn on the boiler just for the sake of heating water, and as a backup. in case of boiler failure.