I have a problem with my CWS tank i went up to check it out when it split and found out after changing it that some how the tank is getting hot water in it and I dont where from the vent pipe goes into the tank but I dont think its the vent at fault I felt the cold water feed pipe to the water cylinder down stairs and this was very hot. The tank is so hot that the overflow runs with hot water that you could shower in ??
I had a similar problem some years ago. Hot water was getting into the cold tank and causing it to overflow just as you describe. After much thought I concluded that this can only happen if there is damage to the indirect coil in the hot tank. I installed a new hot tank and to prove my theory cut the tank open and there was a split in the inner coil.
Hope that this will be of some help tp you. I spent months going to the cold tank and lowering the water level with a bucket just to stop it overflowing past the kitchen window!
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