Possible immersion cylinder air gap failure? Pigletjohn please...

[PodgyManatee]

Any help on this would be wonderful. Problem is water leaking intermittently through tundish of unvented cylinder. TPRV has been replaced, as has the pressure reducing valve - so it's not them. On heating, pressure on gauge rises above the 2.1 bar it is set at and water comes through the pressure relief valve even though it has not exceeded the 4 bar limit of the valve (noticed it dripping when the gauge said 3.1 bar). Have tried reinstating the air gap several times - the last time it cured the problem for a few days and then it started again. I am guessing the air gap might some how be failing and then activating the pressure relief valve - but can't understand why the valve would activate below its 4 bar limit. Perhaps this is faulty too? Any ideas?

It's a Megaflow I suppose? Other brands now seem to have an external (white) expansion vessel instead of the air bubble. Plumbers argue (constantly) about which is best.

If the pressure rises excessively when heated, it's probably the expansion that is failing, so your bubble may have gone, and the cylinder vents the excess pressure until bubble is reinstated. I don't know why you're losing the bubble. Presumably your heating engineer is experienced, and G3 qualified to work on unvented cylinder? I wonder id the relief valve could be incorrectly set?

Sometimes, with an intractable problem, it's worth getting the manufacturer's agent to come and have a go. They should know the design and be able to diagnose and fix it. Their charges can be high, though. The manufacturer should also have a help desk you can call.