Bosch~worcester 240 - hot water tap does not fire boiler?

Can anyone help?,,,,i can only get hot water when the boiler is on.....quite often that water is still not hot enough to burn my hand. The hot water tap used to trigger the boiler and provide very hot water...can i ammend the settings in any way...or do i have an internal valve problem.....help appeciated

Regards
Mike