Central Heating - Expansion problem


Hi Folks
I have recently noticed that the my overflow to my central heating header tank is dripping. My first thought was that maybe the ball valve was over filling it but on inspection the cause was found to be water dripping from the expansion pipe into the header tank.
I don't know why this has started to happen. It usually stops while the heating/hot water is actually on, i.e. the pumps running, but as soon as it stops if I grab hold of the expansion pipe I can feel it heat up as the hot water rises up it.
I have tried emptying the header tank down to its correct level but by the next day it had filled itself up again.
Any ideas greatly appreciated.
Thanks Jason.
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The only reason I can think the water level rises after the heating goes off is if there is no pump overrun on the boiler, so when the pump and boiler switch off, the latent heat in the heat exchanger causes the water to boil so pushing it up the expansion pipe. If this overflows through the overflow pipe what is probably happening is the water level rises and overflows. When it cools down and the level drops the ball valve opens up and tops up the tank and so on. One way to fix that would be to lower the water level (making sure the ball valve only tops up to this new level) so that when the system expands and the level rises, there is enough of a gap between the water level and the overflow to accommodate this expansion without any water being discharged down the overflow pipe.

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Thanks for your reply Paul,
I had similar theories as you were but I have since found out the cause of the problem. The heat exchanger in the hot water cylinder is leaking. Two things confirm this, water from the hot tap is discoloured from water contamination from the heating circuit, and also if shut off the cold feed to the cylinder the water eventually stops flowing from the central heating expansion pipe. It's going to cost a packet to have the cylinder replaced but it's not something I fancy doing myself :(
Jason.

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