I would have thought this comes down to the type (e.g condensing?) and
possibly even the individual boiler rather than a common spec. Try
e-mailing the boiler manufacturer to be sure
I know when we had a new combi, condensing boiler there were vertical
and horizontal specs and we had to check the plans when we considered
putting the boiler flue out of the roof below a window. I can't
remember the manufacturers spec but the window opening is about 80 cm
above the roof and the flue would have been 50cm horizontaly away from
the wall and that was OK.
There may be other issues if the gasses are being vented over/towards
someone eles property.
Also check building regulations. The distance required between
a condensing boiler flue and a boundary was recently increased,
although IIRC this is measured along the axis of the flue opening,
so angling it up can be used to reduce the horizontal distance.
If it was installed by a CORGI registered plumber then it should comply with
the manufacturer's instructions and the building regulations. If it was a
DIY installation (illegal) then it's anyone's guess. The flue gases should
be harmless outdoors in a well ventilated area. Boiler flues are often
positioned near opening windows but there are minimum distances. The
building control department of your local council should be able to advise
you on whether the position of your neighbour's boiler flue is safe for you
The exhaust from my combi condensing is evry visible, warm and moist. I
certainly wouldn't want it being directed at my window or onto my
driveway from close range which is why they have the specs. The
distance along the flue axis makes sense as well as the exhaust is
visibly projected out 2-3 ft on a windless day.
Mine is about 2' horizontally from the boundary but the axis is
parallel, it's also about 8' off the ground.
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