Side entry float valve, passing when closed?

[candyfloss]

I have an expansion tank in loft feeding c/w to a downstairs gravity fed shower. this has worked without any problems for about 10 years. Recently the tank has been overflowing, replaced the rubber washer and it still overflowed. Purchased a new Part 2 float v/v still overflowed with the ball float below the water level in the tank. The ball float is OK? Tried various online forums and found dozens with the same problem, including calling a plumber who as it turned out couldn't fix it? Final solution was replace with new plastic extendable arm Sky Flo valve, it still didn't work. My only hope is to call a priest to exorcise the poltergeist! Can anyone help?

 

[ShaunCorbs]

No the ball shouldn’t be below the water, should be at least 1/2 above normally change float valve and ball together

 

[snowhead]

Is the gravity cold fed shower connected to a combi or unvented cylinder for the hot?
If so it's most likely higher pressure hot crossing over into the low pressure cold via the shower mixer or possibly a mixer tap elsewhere with L.P cold and H.P hot.

 

[candyfloss]

Is the gravity cold fed shower connected to a combi or unvented cylinder for the hot?
If so it's most likely higher pressure hot crossing over into the low pressure cold via the shower mixer or possibly a mixer tap elsewhere with L.P cold and H.P hot.

Hot is from combi-boiler, but the overflow happens when the shower not in use?