Hot Water Keeps Entering My Header Tank.

Hi,
I would be grateful if people could help me with this. I just happened to go up into my loft this morning whilst the central heater and the hot water were both on. The shower was running so I guess the boiler was heating the heating loop.
Anyhow when I got into the loft I could see that there was very hot water going into the header tank from the overflow pipe (that comes from beloe and the airing cupboard). The water in the header tank was very very hot ?
Why is this happening, and how do I stop it ? I don't think that the steam it was generating is much good for inside the loft space.
Many Thanks
Chris
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It's called "pumping over" and usually results from the fill and expansion pipes being incorrectly connected into the system. It is a BAD thing, and needs to be stopped because it wastes engergy and draws air into the system - with air-lock and corrosive consequences. You can sometimes prevent it by turning the pump speed to a lower setting - but this may result in insufficient flow to heat the house properly.
Try that first, and if it doesn't work, draw a diagram of your system, showing exactly where the feed and expansion pipes are connected in relation to the boiler, pump, hot water cylinder, and each other. Put that on the Web somewhere, and post a link to it here, and someone will analyse the problem in more detail for you.
Roger
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