Many thanks to everybody who replied, I have a little more data, if it helps. In the roof space are 2 water tanks, a small one which supplies the rads, boiler and heating coil in the immersion tank. This tank has 2 vents returning to tank, the other tank is larger and supplies the immersion tank with water to heat for the taps, this tank has only one vent. when using the open fire with the back boiler on draw, the water will boil in the back boiler after a couple of hours and the steam can be heard venting in to the tank, So I wonder whether the valve is letting hot water/steam back to the tank, or if the valve activated, would steam come out of the ring of holes? Would it need a pressure valve if open vented to the tank? Really not looking forward to touching anything without a plan. When using the back boiler, it heats the water cylinder by convection current.