My 21 year-old Gloworm 24cl needs replacing, (I know, I'm saving up), but the engineer said the fault of lukewarm water and warm/cold intermittent showering water was due to the the heat exchanger needing replacement: however the central heating works fine with radiators almost too hot to touch. How can this be? - it seems counter to logic. Any expert out there who could spare a moment to clue me up, would be much appreciated.