I have two mixers upstairs and both only work on the hot water. the house is a Barrat house and is about 10 years old and the plumbing seams to be well done.
my feeder tank is lower and the ball valve is not letting in any water in.
Sounds like it could be a mixer tap it only a tiny amount over flows and only does it overnight.
Thanks
Andy
My cold water tank is overflowing it isn't the ball valve and the water seams to be entering via the feed for the hot water cylinder. Its not the expansion tank for the central heating system.
The expansion for the central heating isn't overflowing and level is static.