How do I stop water from hot tank returning to the cold water cistern via the supply pipe from cold cistern to hot water tank? The cold cistern eventually overflows because of this, but I cannot see why this should happen because the total volume of water surely remains constant?

peter jackson, June 2012

Thanks, Toptrish, you were right. I've fitted a one way valve into the hot water out pipe, to stop the flow of cold through the shower unit, and everything is now ok.

peter jackson, June 2012

p.s run shower for a couple of seconds then feel the hot pipe coming out of the top of cylinder,does it go cold fairly quickly, if so its back feeding

toptrish, June 2012

have you got a mixer shower, if so cold water could be back feeding in to the hot system

toptrish, June 2012

Thanks for your reply. The heating header tank isn't constantly filling, like you suggest, but I have now put some food dye into it so if there is a leak from the coil in the hot tank it will be revealed by colouration in the domestic hot water. The tank stat is set at 60 and the water isn't any hotter than that. I thought the top vent might be blocked but it seems to be ok. I am baffled.

peter jackson, June 2012

Either your cylinder is overheating or coil in cylinder is leaking and thus your either main cold tank or F& E tank overflow.thus other tank must be constantly filling from ball valve

k, June 2012