I've just had a Worcester Greenstar boiler fitted to replace my old Ideal Minimiser SE. The old boiler did not has a pump run-on requirement and so bypass was fitted. This Greenstar does require pump run-on and so a bypass was fitted. But now, when the last zone valve closes with the pump still running there's a water hammer thump as the outlet pipe from the boiler shakes and hits the inlet pipe. I guess a brace between the two pipes would help, but it won't stop the water hammer. Is this a normal phenomenon with pump run-on boilers?