Mysteriously overflowing cold water storage cistern..

[Aquafix]

Hi can anybody shed light on why the cwsc in the loft space keeps over flowing, it has had a new part 2 float valve fitted and seems to be in working order yet it still keeps overflowing, it is a open vented system, venting into the cwsc but the problem doesn't only happen when there is a call on the heating, so not down to venting.
Any help appreciated

 

[Mike Jackson]

Is the header tank higher than the cold water cistern? Could be a split coil in the cylinder. Alternatively a mixer tap or mixing valve on tank fed hot, mains cold letting back. Run off some hot water and then feel the cold feed, if you feel the hot back up it it's likely one of those problems.

Mike

 

[bobpape]

Check if it does it continuously
or when a certain appliance or tap set is used,