Combination of ventilation system and solar system for domestic water?

Hello,

we want to build a new house and have decided on a one-and-a-half-story single-family home, approximately 140m². We would like to achieve Kfw 70 and therefore want a solar system for domestic hot water and a central ventilation system with heat recovery. A gas condensing boiler will be used for heating. The ground floor, plus the bathroom on the upper floor, should be equipped with underfloor heating. Only in the bedroom and children's rooms do we want to use radiators, as underfloor heating is too slow for us there.

My first question: Is this installation sensible or is there some flaw in the concept?

The second question is somewhat more general.

At the moment, I can't really imagine the hot water circuit: that is, solar system, gas condensing boiler, underfloor heating, radiators, shower water, dishwashing water. Underfloor heating operates with a low supply temperature, usually around 28°C. How then must the radiators be designed, or how do you achieve hot water for dishwashing or for the bathtub/shower?

I hope you can help us.

Hello,

...We would like to achieve KfW 70 and therefore want a solar system for domestic hot water and a central ventilation system with heat recovery. A gas condensing boiler is to be used for heating.

A solar thermal system would not be an option for me, as it is usually not economical in a single-family house. However, this is sufficient on paper.

...The ground floor, plus the bathroom on the upper floor, should be equipped with underfloor heating. Only in the bedroom and children's rooms do we want to use radiators, as underfloor heating there is too sluggish for us.

Sensible according to which criteria? There are competing demands! Radiators are ideal when rapid heating is required, i.e. ad libitum use. Depending on the building's insulation standard, savings can be achieved here. With better insulation up to a transitional range, this advantage disappears but reappears, e.g., in a passive house. A mixed system is rarely advantageous as it cannot fully exploit the benefits of either type of heating surface. The flow temperature is determined by the radiators because their heating surface is smaller. For the underfloor heating, an additional heating circuit (mixer) including components (additional investment + consumption) is required. Technically easily feasible but overall a compromise.

...?Underfloor heating is operated with a low flow temperature, usually around 28°C.

I would like to see that with a KfW 70, preferably at a location in sunny southern Europe! By the way, underfloor heating feels cold at that temperature! There are no warm feet. Moreover, it is unlikely that gas condensing boilers can ensure reliable operation with this flow temperature! Heating and hot water, despite having a common heat generator, are separate consumption circuits, which alternate and are operated with different flow temperatures!

Best regards,

NB: All heating surfaces as well as hydraulics are dimensioned according to the room heating loads. A heating load calculation according to DIN 12831 must be available; otherwise, it will be nonsense!

A solar thermal system is nowadays already offered as standard by most construction companies. The question is whether KfW 70 can still be easily achieved without a solar thermal system. Okay, 28°C as the flow temperature may be a bit low. However, I forgot to mention that we are considering designing the underfloor heating in such a way that it might be possible to switch to a heat pump in the future. Based on your statements, I still come to the conclusion that it is probably more sensible to decide completely in favor of underfloor heating. Does underfloor heating get along with cork flooring? And then again the question about the domestic hot water circuit. If the gas condensing boiler has its own circuit for the domestic hot water, what speaks in favor of a separate circuit for the radiators?

1) 2 systems are probably not economical enough.

2) Cork floors are supposed to be suitable for underfloor heating (probably a bit better than parquet because not as thick).

3) With better insulation it probably works. We decided against it because the insulation in the attic takes up too much height.

...A solar thermal system is nowadays already offered as standard by most construction companies.

That is correct, because it is simple and convenient - for whom?

...The question is whether KfW 70 can still be easily achieved without a solar thermal system.

It can be done; whether it is easy depends on the specific boundary conditions of the construction project.

..., to design the underfloor heating in such a way that one might switch to a heat pump in the future.

That is not fundamentally wrong.

...Does underfloor heating go well with cork flooring?

The respective manufacturers provide information about the material suitability. On the other hand, the more insulating the underfloor surface covering, the less efficient the underfloor heating becomes. Also more expensive. Ideal are surfaces with good thermal conductivity, e.g. tiles, granite, polished concrete....

.... If the gas condensing boiler has its own circuit for domestic hot water, what speaks in favor of having a separate circuit for the radiators?

There are generally only two externally guided supply options (heating, domestic hot water). Internally, on the boiler side, even only one, followed by an internal three-way valve which, depending on demand, is controlled by the regulation and alternatively switches to heating or domestic hot water.

Best regards.

There are usually only two possibilities for external supply lines (heating, hot water). Internally on the boiler side, there is even only one, followed by an internal three-way valve, which is controlled by the regulation according to demand, alternatively set to heating or hot water.

v.g.

How do towel radiators actually work in this context? With about +/- 30° supply temperature and the moderate surface area, I suppose they heat very little?