Is there water coming out of the copper pipe that goes throught the wall to outside, if so the PRV is passing. if not ypu have a leak under the floor or somewhere where cant see the water. If the PRV is passing, you could drain the system(rads/boiler) remove the PRV and clean the rubber washer. But removing the PRV isn't easy, it should be in the bottom section of the boiler with a red know on it, if you give it a quick turn, it will click(It will turn and click turn and click alday long) . Behind the red knob is a black ring, try to get a pair of grips on the black ring a turn it anticlockwise. When you get it out get it a good clean a use some silicon grease to help reseal it. MAKE SURE the system is drained AND THE ELECRIC IS OFF.......ALWAY ALWAYS ALWAYS TURN THE ELECTIC OFF BEFORE PUTTING YOUR HANDS IN A BOILER.........!!!!!!!!!ELECY OFF FUSE OUT!!!!!!!
LTGF
August 2008