Gunnar,

An unusual but great question. You can run both vents up but I really don't know about running both down. The question that I'd have is would the warm air in the furnace room sink into the cold (cooler) crawl space? I'd probably be asking my local gas utility their opinion. Personally, I don't think I'd want that arrangement in my home. But then again, the vent on my water heater is undersized and allows for a little spillage. (Shoemaker's children)

__________________
Eric Barker
Moraine Woods Consulting, Inc.
Barrington, IL