indirect cylinder has a redring immersion heater situated outside of it encased in its own mini copper cylinder in the vertical position. 15 mm pipe from top of this teeing into hot water vent pipe and piped from bottom of it into bottom of storage cylinder. rang redring and because its 30 years old theres no record of it. The back boiler for heating hot water coupled with a regular boiler for central heating.The immersion heater is not working and as i cant get a like for like replacement is there anyway I can put a standard immersion heater into the existing storage cylinder or has anybody any ideas?