Normally requires a brazed heat exchangers some buffer,
when you open for the hot water , you use lot of energy ,
that energy must come from the pellets burner and boiler,
but the boiler contents only a few litres of hot water,
so you will empty you storage of energy fast.
The boiler temperature will drop, before the burner has adjusted to up the maximum output.
So if you going to use lot of hot water for long time ,
I will suggest a hot water tank on 200-300 litre.
Regards Jannich