I'd like to know the answer to that. If it is a prohibition, by extrapolation of what I've seen, there are likely several hundred thousand installations in this area that violate such a prohibition. When I see it, I'm careful to check for back-drafting at the natural-draft water heater - at the draft hood. It usually back-drafts for several seconds until the flue heats-up, and then all is good. The techs around here accept this, as do I. (Sometimes, the back-drafting does not, at all, stop, and of course this is flagged.) By the by, what's a natural draft "AWH" water heater?