In other words, manufacturer's have found a way to create revenue,
Room seals have to take into account situations where the air pressure outside the building in the area of the flue terminal is considerably higher than the air pressure inside the building. In the fan sucks directly from the air intake and not from the room sealed interior cavity of the boiler the outside / inside pressure differential does not affect the room seal.
If the fan does suck air from the room sealed cavity which is connected to the outside via the air intake then the room seal has to be able to withstand the outside / inside pressure differential and not leak under any waether conditions ( gales blowing directly onto the flue terminal ).
Not well worded and no doubt "experts" will have their opinions.
Yes I know Bernard, it sucks!
what I also meant by having no negative pressure inside a combustion chamber, or its associated flue system, a fan would create a slightly low pressure such that air and POC are sucked out of the combustion chamber and fresh air continues to be is drawn in, into the system and therefore any gasses within the CC will have no chance of escaping or popping out of your cover seals, at worst if your cover seals were leaky, they would actually draw air into the system rather than push out any through the leaky seals, therefore a fan guarantees this as well as helps avoiding any explosion from any raw gas that may start accumulating if not drawn out that could otherwise get ignited with bad consequences.
On my vaillant VCW221 Gb boiler it does not have a fan as it uses a permanently lit pilot flame to avoid raw gas accumulating, it has to flame monitoring device called a thermo-couple to switch gas valve off if the flame died, as it often does from time to time, though very very rarely. As far as I know my thermo-couple has only ever gone off a dozen or so times in its 22 years service and is still the original as it has never been replaced by anyone to my knowledge. My RGE don't even tinker with it accept when they are conducting a full service every few years when they may clean the tip of it. It has been serviced recently and I was there and saw it for myself, RGE commented how good these boilers are and newer ones need regular service due to condensate problems.
I also believe in some boilers the air is sucked in rather than sucked out, if forcing air into the boiler, it creates a positive pressure rather than a negative, so the forced air is pushed through the system and out through the CC, and then into the exhaust flue rather than being sucked out , so where the air is drawn in by a fan, and forced to go through CC, to be pushed out, there are more chances of a poorly made seal allowing air and perhaps other POC to escape into the room ..