Condensation happens in a boiler when fumes from the combustion are cooled down to a certain temperature. This temperature is called the dew point, and depends on several factors such as fuel type, flue gas pressure…
The biomass pellet dew point ranges around 45-48°C, although it may be lower or higher in individual cases.
Conventional boilers prevent condensation to avoid rust and therefore corrosion. To do so, they force flue gases to exit at high temperatures, thus wasting much of the energy not transferred to the heating water.
This energy contained in the steam is called “latent heat” or “heat of phase transition”.

Efficiency higher than 100%?

Traditionally, it was considered that this latent heat could not be exploited without damaging the
boiler, so two terms were coined for defining the fuel energy:

HHV = LVH + latent heat of vaporization

By convention, efficiency continues to be calculated referred to LVH. This is the reason why the
apparent efficiency of a condensing boiler is higher than 100% (measured in traditional terms),
whereas of course it is less than 100% relative to HHV.

A condensing boiler, even not working under condensing conditions, will always perform a higher efficiency than a non-condensing boiler working under similar circumstances.