Look under boiler base all the pipes except the hot water to the taps will have a valve fitted or there will be a slot to turn with a screwdriver. You want a 15mm pipe likely to be only two, possible a 3rd which is the safety valve dump pipe to outside which is where your water is coming from.Draining down will not make any difference as you have a plate heat exchanger failure but you can drop the pressure from any radiator vent.Before you really hit on this as a problem you have to eliminate the filling loop and make sure this loop is not passing water into your boiler accidentally, it should only be used to top up then isolated. Once you prove the filling loop then you have to shut off the water to the boiler and vent the radiator to bring the pressure back to 1bar. I am assuming that you had high pressure initally. If this water is allowed to leak inside the boiler serious damage could be caused also dangerous electrically.Shuting the valve on this line will stop the combi functioning for hot water.
Guytawe, June 2010
