What is the boiler pressure when system is cold (should be 1-1.5 bar)
Does it rise to 3 bar when heating? if so, sounds like your expansion vessel may need charging or at worst replacing.
If you heat exchanger is on the way out, you would most likely either hear a lot of banging when hot of notice a pool of water below your boiler (then it has certainly failed)
You say your system is 15 months old, did you not apply for your warranty? (2 years!) Ring vokera......
T
March 2008