Combi Boiler vent Stains On Wall

[Stuart]

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

[IMM]

Check if it is coming from the blow-off pipe, which is normally terminated below the flue to outside. If the pressure gets too high in the system the safety relief valve opens. The reddish brown is probably the colour of the water in the system. A fitter could have opened the relief valve to release pressure. Check this out.

The blow-off pipes should be terminated a few inches from the outside floor level, with an elbow facing the wall. If it blows then it hits the wall at the bottom. If it is terminated at high level it could spay boiling water on someone beneath.

If it is the blow-off pipe then get him to run it down the wall. Also get them to clean the wall.

[John Rumm]

If it is a condensing boiler (many recently installed high efficiency boilers will be) then you would expect allot of water vapour to come out of the flue. You will normally see this as a quite visible plume of steam.

When the boiler is installed the flue should be installed at a slight angle through the wall - such that it runs "uphill" from the boiler (a slope of one to two degrees is usually enough). Hence any moisture that condenses in the flue before it gets outside should run back down the inside of the flue and into the boiler.

If its is a condensing boiler, and, the flue looks like it is horizontal (or worse sloping down), then that could be the problem - although it is probably unlikely to be reddish brown itself you may find it is discolouring the wall due to its slight acidity.