ᐅ Selecting Gas Heating Capacity: 20 kW Instead of 14 kW

Hello,
in our case (120 sqm (1300 sq ft) living area with underfloor heating, 60 sqm (650 sq ft) basement corridor area with standalone heaters), a 14 kW Wolf gas-solar heating system is planned. We are considering installing standalone heaters in two rooms in the basement, which is fully insulated. However, the plumbing company argues that the 14 kW system is not sufficient and recommends a 20 kW unit.
I find this a bit confusing and believe I have read that oversizing can lead to higher operating costs and increased wear (among other reasons, due to the minimum modulation level).

For now, I have declined this proposal. Presumably, it would make more sense to calculate the actual heating load instead of just "going for it." However, I think increasing the heating capacity just because of two standalone heaters is exaggerated.

Is there anyone in this forum who has an opinion on this?

Yes, there is. Definitely install the smallest heating system available. I estimate the heating load at around 7 kW at design temperature. It is equally important that it can modulate down as low as possible.

If by auxiliary heating you mean radiators, then considering the underfloor heating installed throughout the entire house, the focus should be on ensuring that the supply temperature in the house does not need to be increased because of the two additional basement rooms with radiators.

To achieve this, it might be worth considering sizing the radiators in the basement rooms and basement hallway larger, so they can operate with lower supply temperatures. This could potentially eliminate the need for a mixing valve to manage two different temperature levels for radiators and underfloor heating.

What speaks against a uniform system design with underfloor heating also in the basement? Or alternatively, wall heating?

Based on the information provided, I also see no reason why 14 kW should not still be more than sufficient. I assume this is a single-family house project, so a new build? 20 kW is enough for a modern multi-family house.

BobRoss schrieb:

If by parking heater you mean radiators, then regarding the underfloor heating installed throughout the house, I would focus on ensuring that the supply temperature in the house does not need to be increased due to the two additional basement rooms with radiators.

To achieve this, you might consider sizing the radiators in the basement rooms and basement corridor larger so they can operate at lower supply temperatures. Then it might be possible to avoid using a mixing valve for two different temperature levels for radiators and underfloor heating.

What speaks against a uniform design with underfloor heating in the basement as well? Or alternatively wall heating?

Based on the information provided, I also see no reason why 14 kW should not remain more than sufficient. I assume this is a single-family home under planning, so a new build? 20 kW is enough for a modern multi-family house.

Yes, I mean radiators. And yes, new build and semi-detached house. The basement is fully insulated (under the slab and on the exterior walls).

A uniform design with underfloor heating is simply not included in the construction specifications and scope of work, since it’s a utility basement. I don’t even want to imagine how expensive that would be...

Only the corridor needs to be heated due to the energy-saving regulations, which is why there is a radiator there. I wasn’t aware of wall heating until just now. Interesting.

We want a very cost-effective solution because these rooms are not currently intended for permanent use (usually only a few hours at a time).

If the radiator is installed solely to comply with the energy-saving regulations, then choose an affordable one and that’s fine. If you ever want to have some warmth in a basement room, you’ll just need to use a more expensive electric heater with higher operating costs.

A heating system with a capacity greater than 14kW is absolutely pointless.

Just for comparison:

I live in an older building, with a well-insulated ceiling, moderately insulated walls, and windows mostly from 1994. At -12°C (10°F) outside temperature, my 130 sqm (1400 sq ft) house requires 7.5 kW of heating power.