When 1 of the 2 showers is used the header tank in the loft overflows, the ball valve is new on the tank and set correctly and is working , the hot water supply to the showers is from the combi boiler and cold supply from the tank , can you shed some light on the problem.

Hi Jeg111 and welcome to The Plumbing Page at ANHD.com.

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When 1 of the 2 showers is used the header tank in the loft overflows,

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Does this only happen whit the showers or does the tank overflow when a cold water draw, such as a flush. Is made? Are you sure something's not hanging up the float ball valve? Back to you, Tom