open up the boiler and you will see a small brass valve with a red knob, twist the red knob, this operates the pressure relief valve, it is a sprung device and as you let it go it will click back, may need a twiddle though. the water that is released will exit via the pressure relief tube, probably straight out of the wall close to the prv, but do it cold and make sure it wont hit anyonyone[the water coming out]
hph services, April 2009