Does the cylinder you are buying have an "indirect heating coil" inside?

Am I too late to reply?, its a piece of piss. I've done it before. Buy a coil of soft copper . make a heat coil. tack half hard to its spiral ensure there is no steam trap capability ie; it must rise continuously. make its diameter 200mm less than cylinders. have flow and return legs protrude out at least 200mm. cut top off copper tank 50 down from weld. mark entry & exit on side of tank. braze in brass backnuts of coil pipe dia. fit coil ensuring half hard legs take coil weight on bottom of tank. flow and return should protude out backnuts, wind brass barrel nipples over f&r pipes seal with either braze or hemp to back nuts, centre coil in tank. fit crox nut and brass olive over copper coil. seal to barrel nipple 9or braze if you wish. wire wheel inside of tank lid to clean, dress out slightly, braze using flux. pressure test with air or water to 10% greater that maximum working pressure, reinsulate. run off open vent, coil will need also an expansion valve, do not fit isolating valves to coil ever.
Cheers

Hi, copper is good for uncontrolled heat source cylinders, water boils and converts to steam at 100 degrees celsius. copper melts at around 600 degrees. it is a pure metal and has been used for years. however stainless steel is a  subjective term for an alloy. the composition of stainless can  vary producing different grades   meaning  not all stainless steels are good for cylinders .
ps; heat shield copper in the fire box with steel to protect it.