IDEAL ICOS HE24 BOILER IGNITION BANG?

Our HE24 Ideal condesating gas boiler is now 4 years old and has started to have a loud ignition bang when the programmer demands hot water. This only occurs when the pre-set timer on the programmer hits the hot water on times and does not happen if you turn up the wall thermostat to turn the central heating on. Everything works fine except for this loud ignition bang on hotwater startup. I cannot understand why the hot water demand which is set for twice a day should be any different from the central heating demand triggered by the wall thermostat, any ideas anybody?

Padder
June 2009