Pellet boiler correct boiler size 20 or 25 KW?

I looked at the brochure; the 15 and 20 models seem essentially identical, both have a minimum output of 4.5 kW. The 15 will be more than enough, but the 20 is technically just as suitable. I think, for psychologically irrational reasons, I would simply take the 20 at a slightly higher price and be done with it, but it’s rather a meaningless oversizing. The 25, on the other hand, has far too much minimum output and would be a big mistake.

P. S.: Probably the 10 would also be sufficient in practice and even fit better, but first, the heating installer wouldn’t agree to that, and second, there’s no need to overdo it. A well-balanced underfloor heating and then the 15/20 fit perfectly.

consider your usage scenario:

we were away several times in winter for 2 weeks and had the heating set to frost protection (approx. 4°).
As a result, after returning, heating up the cooled-down building was laborious and
took more than 1 day.

The heating system was dimensioned according to DIN (here Winter-low at -13°).

If you also have this often, then I would choose the larger boiler.

I do not consider it very sensible to let a house cool down that much.
But aside from that, the question is whether the one-day heating really takes that long because the heating system does not have enough power. The supply temperature usually results from the outside temperature. This is followed by the sluggish heat transfer to the room.
The heating output only needs to be sufficient to maintain the supply temperature constantly. More power would not lead to faster warming of the rooms.

… More power would not lead to a faster warming of the rooms.

Sounds like the analogy to the car. More horsepower (power) first have to be transferred via the wheels to the road. Here probably the power of the heating system to the heating surface(s).

Hi! In 2018, we built a semi-detached house with approximately 400 sqm of heated area, also kfw55. We have an Ökofen 18kW condensing pellet boiler, which is totally underutilized. The heating technician has already reduced it to 13 kW (I can't remember the exact value, but it was around 13) maximum output because it can modulate better that way. So take fewer kW, that's my experience.

also kfw55. We have an Ökofen 18kW

And so there was no heating load calculation here either. :confused: But the OP can still do everything right.

And here too, there was apparently no heating load calculation. :confused: But the OP can still do everything correctly.

That is probably the case, however, the idea of pellets in new buildings is simply not particularly smart. For example, Ökofen only starts at 10kW. No matter how you calculate, that is far too oversized. Nothing will run efficiently, and wear will be correspondingly increased. And that with very high acquisition costs. It is simply not a sensible form of heating in new buildings, at least not in 1-2 family houses. For an apartment building, one can see it differently.

Ökofen website:

The new generation for the small output range

With the Pellematic Compact, the name says it all. In addition to its extremely space-saving design, it is suitable for almost any single-family house with output sizes of 10, 12, 14, 16, and 18 kW.

You can hardly tell bolder lies. This is consumer deception of the highest order.