Oh, ?I do wish DIY doctor would publish an article in the projects section, as this type of fault appears here again and again and it is tiresome to keep repeating the same reply.
Basically, it is caused by a poorly designed system where the feed and expansion pipes are connected to the system too far apart causing a pressure difference between each resulting in either water being forced up the expansion pipe and over into the header tank or conversely, water being drawn down and air being sucked in.
Thanks guys, The Feed & expansion pipes are right next to each other & tee into each other just before the pump. So I take it they would be ok, I will try lowering the level as its an inch below the overflow pipe.
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