is it against building code to use my crawl space as a open air plentum— pump heat and air into it and then from that opem chamber to the rest of the house???

I am being told to put a duct line in it to give it postive pressure but yet i can not use it as a open air pletum?  what is the advantage of that? by bother heating and cooling the crawl space if you cannot reclaim some of the engery used?

if i do this — it appears to me that i should NOT put vents in my crawl space??

thanks