We sometimes get asked how much fuel can be safely loaded in to the boiler.
With manual boilers, the filing usually takes place through middle door and the bottom of the door frame should be used as a over filling level indicator.
This level is for wood pieces and not for pelleted fuel or coal as the energy contained within those fuels will almost definitely exceed boilers ability to either store or dissipate the heat safely.
Dual fuel boilers, where primary fuel is fed automatically and the boiler is equipped in fire grate for manual loading, the space can filled liberally.
The pellet bins can be filled to desired level and does not impede on boiler operation but important point to note here, once filled the fuel storage door must be sealed shut.