Bio Slim Pellet Boiler by Defro: Comparing Water Capacity & Fuel Consumption for 15kW, 20kW, 25kW

I bought a Bio Slim pellet boiler from Defro with a capacity of 20 Kw. The manufacturer states that a boiler with this capacity for 130 liters of water capacity. In my opinion, this is very much for this power, the more that the 15 kW boiler has a water capacity of 69 liters, and the larger by 5 Kw has already 130 liters, which is almost 100% more. Even stranger is that the 25 Kw boiler has a water capacity of 97 liters or less than a 20 Kw boiler.
The technical parameters of these boilers are provided in the frame: http://wojciechowski.co.pl/defro-bio-slim-20-kw-kociol-na-pelet-p-4820.html
I called the manufacturer twice to explain if this is a mistake and I received the answer that everything is given correctly and that the water capacity of the boiler does not affect greater fuel consumption. Somehow I can't understand it. If you need to heat more water, you probably need to use more fuel. After all, now heating installations (radiators) are changed to installations with less water to make heating more efficient. Maybe someone will tell me if the larger (not understood in this case) amount of water in the boiler has an impact on fuel consumption or not?

Piotr1. wrote:

If you need to heat more water, you probably need to use more fuel.

But then the water cools longer.

Piotr1. wrote:

After all, now heating installations (radiators) are changed to installations with less water to make heating more efficient

Systems are changed to smaller ones just to fill the pockets of shopkeepers, fittings manufacturers and installers. In addition, the energy industry earns when you have 5 push-ups.
It goes out in the cauldron and suddenly it gets cold.
You use the boiler on partial power, when its efficiency decreases (the question of what fuel the boiler is, can the power be regulated).
It suddenly turned out that now for the correct operation of the heating system you need a 1000 - 3000 liters tank in the boiler room. We return to the starting point where formerly there were installations of 500-1000 liters.
Do you still want to heat the house with five liters of water? This is not a passive house that can be heated with a hair dryer.

Writing the above post contributed to the statement of my colleague, who worked in a company producing CO boilers, and now he does it himself. He made a cauldron, which he now complains about burning too much fuel. He gave too much water capacity as the reason. I will only add that I have a new type of installation with a small amount of water, i.e. aluminum radiators.

Fuel consumption is mainly influenced by three factors: fuel calorific value, power obtained, boiler efficiency.
If he repeated the structure of upper combustion known for over 100 years, adding factors that do not serve the economy of the blower type and the lack of secondary air, it is not surprising. Ask better how much soot he has in the chimney.
For three years, winter has been on average around 0 ° C outside, and the boiler works at 25-50% of its capacity. It also matters. Even the boiler you presented cannot go below 30% power.

So the amount of water in the boiler does not matter here?

If you don't have enough water in the system, you don't smoke it, you will smoke half a whistle with 40% efficiency and then you will burn a lot compared to the heat obtained, unless you add a buffer of 1000 - 2000 liters and smoke at full capacity % then you will burn less

Maybe not necessarily the boiler would work at half power. The condition would have to be a high-performance pump. It can be compared by analogy to modern internal combustion engines in which the capacity of the coolant in the engine block has been reduced. The main advantages are weight reduction and rapid heating to operating temperature (ecology). In such a vehicle, when you stop immediately, the liquid begins to boil due to insufficient washing of the radiator with air.

I close the topic. The above answers dispelled my doubts.