Furnace in crawl space is not recommended due to access difficulties: Like periodic cleaning, changing filters. Also if the crawl space gets humid due to rain or snow; then it may be a concern on life and safety.
I am not sure of the new building code and you may want to check with the city building inspector on their opinion.
If you do have one, you HAVE to put an ultraviolet air purifier on both sides of the furnace (supply and return side...) to keep mold from growing. These typically run $500.00 to $800.00 apiece.