my comby boiler overflow pipe has a permanent drip why?

AS A RIDER TO THE ANSWER BY PHIL, THERE IS AN EXPANSION VESSEL IN ALL SEALED PRESSURIZED COMBIS. THIS IS USUALLY LOCATED DOWN THE BACK OF THE BOILER UNDER WHERE THE FLUE COMES OUT. THIS CYLINDER CONTAINS A DIAPHRAGM. WATER AT ONE SIDE AND AIR AT THE OTHER. THIS GIVES THE WATER IN THE SYSTEM SOMEWHERE TO EXPAND INTO WHEN IT BECOMES HOT. THE CYLINDER CAN DE-PRESSURIZE WITH TIME, OR IN THE WORST CENARIO THE DIAPHRAGM CAN PUNCTURE. THE WAY TO TELL IS TO LOCATE THE VALVE WHICH IS USUALLY ON THE TOP OF THE CYLINDER, (SAME VALVE YOU WILL FIND ON A CAR TYRE) AND BRIEFLY PUSH A PEN TOP OR SIMILAR INTO IT TO RELEASE SOME AIR. IF ANY WATER COMES OUT IT'S KNACKERED. IF JUST A BIT OF AIR OR NOTHING COMES OUT IT CAN BE RE-PRESSURIZED.
DO THE FOLLOWING. CLOSE THE VALVES ON THE FEED AND RETURN UNDER THE BOILER THEN OPEN THE SAFETY VALVE TO RELEASE THE PRESSURE IN THE BOILER. CONNECT AN ORDINARY FOOT PUMP WITH A GAUGE TO THE VALVE ON THE CYLINDER & PUMP UP TO AROUND 10 PSI. SCREW THE CAP BACK ON. CLOSE THE SAFETY VALVE & OPEN THE FEED & RETURN VALVES. OPEN THE VALVE IN THE FILLER LOOP SLOWLY & PRESSURIZE TO 1 BAR. CLOSE THE FILLER LOOP VALVE. FIRE UP THE BOILER. THE PRESSURE GAUGE SHOULD STAY BETWEEN ABOUT ONE AND A HALF TO TWO AND A QUARTER BAR WHEN HOT. IF THE BOILER STILL LOSES PRESSURE THE SAFETY VALVE WILL HAVE TO BE CHANGED. THEY ARE CHEAP AND ITS AN EASY JOB.
NOW THE BAD NEWS. IF YOU LIVE IN A HARD WATER AREA AND THE FILLER LOOP HAS BEEN PUTTING FRESH WATER THROUGH THE BOILER FOR ANY LENGTH OF TIME ITS LIKELY THE HEAT EXCHANGER WILL BE FURRED UP LIKE A KETTLE. IN MOST CASES IF ITS AN OLD BOILER I WOULD SCRAP IT AND INSTALL A NEW ONE. THE COST OF DE-SCALING CAN BE QUITE EXPENSIVE.

DAVE
April 2007

The probable answer is that there is some debris or scale on the seat of the water safety valve which is preventing the valve to seal properly, try turning the knurled knob on the valve quickly, it is spring loaded so will turn itself off after each turn, if that doesn't work maybe you will need to change it. Hope this has been helpful. Jeff.

Jeffrey Harvey
April 2007