Can anyone please tell if it's possible for the header tank water level to rise when no water is entering through the Ball Valve flow or the expansion pipe?
When the Washing Machine is used the water level rises to a point above the overflow level, I can only assume that back pressure is causing water to enter the tank through one of the two cold water supply pipe at the base of the tank.
Firstly, is this possible? And secondly, how can I fix it?
Outlets to supply something (hot water system or cold taps / toilet / shower!
Surely, if the hot water connection to the washing is turned off, the fault can't show itself because there would be no route to the header tank from the washing machine.
Try plumbbob's second suggestion. But allow the machine to fill first, then turn off the machine and turn off hot water feed to it before disconnecting the hot fill pipe from the machine. After that switch the machine back on.
Surely, if the hot water connection to the washing is turned off, the fault can't show itself because there would be no route to the header tank from the washing machine."
Well, that actually was the point, it might STOP the fault occurring and prove what is causing it.
On your second point, if the washer is allowed to fill before disconnecting the hot hose, then I doubt if anything would happen as I believe the fault is water back flowing whilst the washer is filling. If it happened at any other time, the header tank would be over flowing continuously.
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