pressure on the hot water tap?

I have pressure on my hot water tap when the heating and hot water is first turned on. When the tap is turned on it releases the pressure and you can hear the pipes all knock together then it is fine. This happens every day when the heating and hot water come on while i'm at work, obvioulsy very concerned this will blow a pipe somewhere. What could be causing the pressue?

Shazk
March 2011