I recently changed my old indirect cylinder because it had a split heating coil. five years ago we had fitted a new glow worm 12hxi to our open vented system. the system is acting up kettling etc. after trying various additives and flushing i have come to the conclusion the problem is in the heat exchanger. the heat exchanger is stainless steel. my question is, is the stainless steel used non magnetic? i would like to try a magnet test to see if there is any magnetite in the exchanger. also is anyone aware of the design of this exchanger? i now run the system on pump speed 2 and regular venting of the rads. produces hydrogen? which will light with a match. i am a retired heating installer. retired 15 years ago. many thanks for reading 