I do some work for a letting agent. Got called to a property with poor heating and hot water. A few rads were just heating and the HW was barely heating. Tenant told me the last guy who was at it put in sludge remover. Ok, so he flushed the system..... No he put 1 litre of X400 in last year in December and just left it. Anyways, I suspected the pump. Usual with this... Pump valves wouldn't shut. Drained the system, water was filthy, took out the old pump which was full of sludge. Flushed the system and re-filled with new pump/valves fitted. Still wouldn't circulate, system is full with no air locks. I tried running the pump round the rads, cylinder and bypass seperately. Heat just would not get out the boiler. So came to the conclusion the boiler must be blocked. Can leaving sludge remover in cause any damage??