Boiler exhaust Fumes from flat below are passing my window and I have to keep it shut?

the regs state the flue must be a minimum 300mm from any opening in the building ie windows or vents. it should also be 300mm from a neighbours boundary so as long as its 300mm from your floor level theres not allot you can do. the gases you see are extremely unlikely to be carbon monoxide, new boilers run very clean and give of allot less harmful gases than older types. what you can see is plumage (water vapour or condensation, hence condensing boiler) and is pretty harmless. its just a visible nuisance but looks worse than it is. in humid conditions (summertime) there will be very little visible. if it still bothers you could ask them to fit a plume displacement kit to re-direct the fumes, but as long as they have the clearances they have no legal obligation to fit it. so ask nicely!! hope this helps!