Trouble with overflow pipe that keeps dripping

I know that this question has probably been asked before but weve tried everything and the overflow pipe keeps dripping. We've been in our house 12years and had that same shower and taps since moving in so we haven't installed anything new, but just recently in the last 3 months the overflow pipe has started dripping sometimes a little and sometimes a lot, and then for no apparent reason it can go several hours with no dripping. We have been in the loft and determined that it is the small cold water tank, so I have changed the valve,washer,ball & arm although this seems to be slightly better this morning it was still dripping quite a lot. The water level is slightly under the the outlet so know we are scratching our heads and thinking what else can it be, we've asked a plumber in our local DIY store and he seems to think we've done everythink right, does anyone else have any ideas. if not how much will a plumber cost to sort this out, It can't be that difficult can it ??

could be coil in cylinder passing water from domestic side to heating system, water level in small tank set too high - causing overflow as system is heated and expands, or pumping over - often caused by a dirty system.

Mrblue1 said:

The water level is slightly under the the outlet

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twgas said:

water level in small tank set too high

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I think TWgas has got it in one. But first you need to check for pumping over - water coming out of the vent pipe hanging over the tank.
If this is happening you may have a blockage or the pump is set too high.

You can check the feed pipe is not blocked by draining some water from the tank via a drain cock on a radiator - tie up the ball valve first. If the water level drop, you do not have a blockage.

If the level is just below the overflow, there is no space in the tank for the water to expand when it heats up, apart from down the overflow. You will also be diluting the inhibitor in the system.

The water level in the small tank should be about two inches above the outlet.

1. Drain some of the water from the tank, via a drain cock on a radiator, until the level is 2" above the outlet.
2. Put a can of inhibitor (eg, Sentinel X100) in the tank and add about the same amount of water.
3. Drain the tank so the water is once more 2" above the outlet (This gets the inhibitor into the system)
4. Adjust the float so the valve shuts off at the correct level.