central heating problem

[Tearle]

my central heating system is causing a problem in that the expansion tank in the loft is constantly being filled with hot water and causing lots of condensation in roof space.  The pump is on first position and the valve on the bypass pipe is closed because it leaks if opened.  Nothing has been changed for four years and this is first time we have had a problem - any ideas please

[peter.gray]

I had the same problem 2 years ago. The hot water also used to pour out of the overflow pipe. I woke up one day and thought it was raining, only to find water pouring thru the ceilings.  A plumber reckoned the coil had gone inside the copper cylinder and the water was just syphoning into the expansion tank.  I renewed the tank and also increased the height of the breather pipe and touch wood i have had no problems since.  Act now as i had to have new ceilings throughout. 

[paulgallwey]

I had this problem. It was due to the thermostat for the hot water not working with the result that the boiler just kept heating the water till it boiled.