I doubt biomass is to blame, assuming the thing's working and heating up the buffer tank okay. Was the system working before they put it in? Were any pumps or stuff changed on the heating system side or was it just hooked into the buffer? Most likely there's airlocks around the place stopping the circulation, dodgy pipework levels, missing AVs or AAVs in hidden places which have seized shut, dislodged crap from the draining/refilling blocking pipes, the usual stuff you get on old systems.