we have the old type c/heating with a cold water tank
in the loft,when the heating is on constant,water flows from the overflow pipe.
After running the hot tap for a few minutes the flow
ceases.any ideas thanks BP

peo, October 2009

problem solved, thanks to all who replied.BP

Brian, February 2010

Replace the washer as in other answer & make sure when you have finished the water level is well below the overflow pipe outlet as water expands when heated & this will raise the water level in the tank causing it to overflow, you only need about 2-3 inches of water in a central heating header tank,

bamba, October 2009

rubber washer inside the ball cock arm, needs changing/replacing

wirecutterman, October 2009