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.
It sounds like this pipe was originally an air intake, but that since it was installed, the furnace closet has been substantially altered. Since you're in California and this involves combustion appliances, there's bound to be a code official involved at some point, and he'll tell you what an appropriate source of combustion air would be. It's possible you'll be able to remove this original air vent if the inspector approves of the combustion air coming from somewhere else.

Another option is to simply shorten the vent so it pulls combustion air from the attic, assuming the attic is a vented one that is effectively open to the outside. That's how my own furnace closet is set up, and it's perfectly code-compliant. That would allow you to remove the part of the vent that goes through the roof.