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issue with a old gravity system approx twenty years old ,owner complained about noisey pipes and heating /hot water not coming on .Gone out and their is a load clanging noise coming from pipework when switch on demand for either dhw/ch ,flow pipe gets hot but no heat on return and obviously boiler gets hot then cuts out .just for note checked the 3 port and and flows out to system and hot water fine.Plenty of air came out the system from the aav in cupboard and system has not gone dry judging by f & e tank . They have also mentioned that hot water came out overflow yesterday on first issue of this happening although not seen it myself yet can feel the cold feed pipe and vent get very warm. Tank in loft seems plenty high enough and pump accordingly located to stop pumping over which makes me think a blocked return piupe with sludge??