Question: There are two vents going up to our roof, one is the normal vent from the furnace exhaust straight up to the roof. But right next to it there's a second vent pipe, a little larger and single wall. This second pipe starts at the top of the furnace closet, about three feet above the furnace, has no obvious function and also vents out to the roof. It is not hooked to the furnace. The roofers are not able to properly cap these two vents because they are so close together, so we would like to just get rid of that second one, but we don't know if it has some official function.

Some previous owner of this 1960's California house made modifications and we just want to get it back to normal code. The furnace closet also used to have a drywall ceiling which was apparently partly ripped away, and we don't know what the code requirement is, aside from providing the proper clearance from the furnace vent to combustible materials. The furnace closet at this time is open to the attic. The odd second pipe which we want to remove seems to be part of the original house construction, judging by the way it's fitted into the ceiling drywall. 

Combustion supply air comes from the crawl space below which is well vented.

I took a close look at a friend's house and that furnace closet has no ceiling and only the one vent that goes from the furnace to the roof. Can anybody comment on this situation and help us decide what to do here? Can we get rid of that second vent? What about the drywall ceiling above the furnace? Thanks

Answer: 
It could be an intake. Furnaces require combustion air, which can only be taken from certain sources. If the furnace is in a utility closet (or surrounded by living space), the pipe could be supplying combustion air. If this is the case, it cannot be removed.

Without more detail, it's impossible to definitively answer this question.
That pipe could also be an old sewer vent pipe that got disconnected during previous renovations but not removed. In these parts at least, it is the norm for there to be a vent pipe for the sewer line on the roof to allow noxious gasses to escape and prevent air-locks in the sewer line.

It could be an air intake for the furnace too, but knowing how leaky California houses are I doubt that would be much of an issue esp. if your furnace closet is open to the attic.

I'd have them shorten the pipe to be below roof level and beg forgiveness later if need be.