My outside overflow pipe to the small header tank in the attic has started dripping.I have had a plumber in who has confirmed that there is nothing wrong with the float valves.The dripping stops when the heating or hot water is turned on and I have noticed that the water in the header tank is slightly warm.Can anyone suggest a cause please
Its difficult to answer with certainty with the information we have about the type of system, however I will assume you have an open vented heating system, and a conventional hot water cylinder fed from a cistern if the loft.
Reading between the lines a bit I am guessing you mean the feed and expansion header tank for the heating system, and not the mains cold water cistern that supplies the fresh water to the hot water cylinder?
On "vented" heating systems, the water circulating through the rads and the heating coil in the hot water cylinder is at low pressure. It's fed from a small tank in the loft. This will not only fill and supply the system, but is also intended to supply some expansion space. So normally the ball c*ck / float valve will be set to only allow it to fill to less than half way. When the system runs, and the water heats up, it will expand and some will "push back" into this tank - raising the level above the one at which the float valve would stop filling. As the system cools and the water contracts, water will be drawn back from this tank into the pipes and rads etc.
So if this is getting too full then one of two things are happening - either the float valve is allowing it to overfill. Or, water is being introduced into the system from somewhere else.
Since the plumber has checked the valve the second option sounds more likely.
The typical cause of this is a small corrosion hole in the indirect heating coil[1] of a hot water cylinder. This can allow the water in the "domestic" side of the cylinder to get back into the heating side (normally these sides should be not be connected to each other - so the dirty primary water in the rads full of sludge does not contaminate the water you shower in). This is likely if the tank feeding the cylinder is physically higher than the CH F&E tank. So water slow trickles into the CH systems primary water. This is bad for two reasons - firstly the overflow, secondly you are introducing a supply of fresh oxygenated water into the hearting system (more corrosion and scaling), and lastly you risk contaminating the domestic hot water with whatever is floating about in the heating system.
[1]John Rumm formulated the question :
It can also be due to a badly designed system which is over-pumping, or the pomp speed is set too high.
Yup it could be - although it would require fairly tight tolerances such that pushing some water up the vent pipe lowered the level in the F&E tank enough to cause the float valve to try and "make up" the level drop.
(probably a moot point since bing a HoH post the OP will probably never see the replies anyway!)
Maybe he has a leak in his roof right over the top of an uncovered expansion tank is about the only other scenario I could think of here. Brian
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