fixmyplumbing

The ball valve has probably got a slight leak that isn’t noticeable when water is being drawn off regularly.

There is always a gap between the resting water level in the water storage tank and the overflow level.

A slight leak on the ball valve gradually raises this water level but every time you run a hot tap it is reset to the resting level.

If the hot water isn’t used much e.g you go away on holiday for a week the water level continues to rise without being reset so eventually overflows.

This can be solved by replacing the ball valve. It is possible to buy washer kits to replace the insides of the valves. Personally I find its much more trouble and far less reliable fix.  Its a bit more expensive but far better to change the valve.