have recently noticed a black substance in the hot water supply to our bath room sink only happens on this tap...have replaced the washer but made know difference...also noticed that it leaves a black line around the sink....worcester greenstar 30 si.........

George Davison, March 2008

Do you have copper pipes connecting to the hot water heater? We had this problem and it was the pipes. Water and oxygen were making copper (II) oxide from the copper pipes and we were having the black greasy stuff too. We replaced the copper pipes with some plastic ones and have been fine since.

Rob, April 2008