My neighbour in the flat below has a combi boiler vented through an outside wall to the back of the flats . Earlier today I noticed that directly below the vent there was a reddish brown wet stain runing down the wall to the ground . It starts immediately below the vent . I have never noticed this before so it must be recent . Is it possible ( and in what circumstances) for water to come through this vent . I dont know how combi boilers work but do they need to be drained -presumably there might be occasions when the CH system has to be drained but again presumably,there would be an outlet for this water into an existing drain . Is it possible that someone has drained the system and put the draining hose out this vent to get rid of the water ??-cant see how THAT would be possible as the boiler will be sealed to the vent -won't it ?? . Stuart -- "YESTERDAY is history,TOMORROW is a mystery,TODAY is a gift That is why it is called the present "