You don't say whats wrong with it. However if you drain the system,either via a bleed cock on the system or by screwing in the round knob on the pressure relief valve until it opens you can then remove it thus: Looking from the front of the boiler with the covers removed (including the black one housing the timer etc) you will see a screw which retains the pressurerelief valve. Unscrew this and when you have undone the connection to your overflow pipe it should pull out. You may have difficulty in dismantiling the valve when out but you can soak it in scale remover to clean it.
Martyn, October 2007
