Intermitent problem.
When running the hot tap the boiler and the pressure gauge start to pulsate. A new heat exchanger was fitted nearly a year ago. The boiler is about five years old. Although this is a hard water area, I have an electronic scalemaster fitted. It is working well as I only have a little lime scale on my sink taps & shower head and it is easily removed. There is no problem with the heating side of the boiler.

Ade, September 2010

it may be your expansion vessel is on its way out or needs recharging

kash, September 2010