Header tank overflow

[Tankman]

The smaller of my two tanks in the loft has an intermittent drip from the overflow. I've changed the ballcock and valve and bent the arm to prevent too much water entering the tank. It appears to work correctly, could it be being filled from the expansion pipe, if so what could be the problem. Thanks

 

[TicklyT]

The water level in the expansion tank should only be a couple of inches above the outlet at the bottom, otherwise there's no room for expansion.

If the water isn't from the ball valve, it must be coming from somewhere else in the system. The usual suspect is a leak in the hot water cylinder coil allowing water to pass from the cylinder into the heating circuit, eventually causing the expansion tank to overflow.

 

[bathjobby]

I've changed the ballcock and valve and bent the arm to prevent too much water entering the tank. It appears to work correctly,

What does that mean (you guys don't help yourselves sometimes). Have you monitored the F/E tank with your system running and
1) Is the ballcock not opening to let more water in ?
2) Is the expansion pipe (overpumping) running into the F/E tank ?
3) Is the drip from the F/E overflow of low rate i.e. a slow drip ?

Assuming 1) is not happening and you have the static (systems not running) level set just above the outlet pipe at the bottom of the tank, is 2) happening with your system running ? (you need to go and observe this).
If so, then 2 things;
a) if the expansion pipe overflow is not filling the F/E tank to the point of it overflowing then it is probably the steam within the F/E that is condensing down your (colder) overflow pipe giving you the drip
b) what height above your F/E tank does the expansion pipe rise to before looping back down to the F/E tank ? if it is less than say about 1' (300mm) then you may well cure the overpumping by increasing the height of it to at least 2' (more room for expansion in the pipe before it overpumps into the F/E tank.

As said, you need to go observe all of this with system off and on.

If none of this applies but the F/E tank is filling and overflowing then TicklyT is probably right, but worth checking all this first.

 

[harrygreen]

Also the static head from the pump to the header tank water level should be a minimum of the pump head (ie 5metres) divided by 3 to prevent any pumping over....