Bit of an unvented puzzle!

[cr0ft]

www.dripbusters.co.uk/images/tundish.mov

Anyone seen this happening on a Megaflow unvented cylinder tundish before? I serviced it a few days ago as customer was complaining of low hot water pressure, hadn't been done for ages. What they did say though is they are having to regenerate the bubble every month or so.

My thought is a dodgy mixer tap sending cold water back to the cylinder at higher than 3 bars of pressure. The mixers aren't fed from the balanced cold feed in the house.

Rather weirdly there doesn't appear to be any water flowing from the cylinder but there is air coming out of the tundish.

 

[Best]

That is weird! My first thought is maybe the pressure is just slightly above what the pressure relief valve opening pressure is and the valve is just passing a little water.
You would need to test the pressure at unit when no taps have been opened for ages.
I know mixer taps and showers can let water pass through them, so a double check valve on hot outlet from unit would prevent that risk