If i'm reading this correct, you are planning on using an existing vent to provide cool/heat and not a return vent.

If you want to prevent any backup of odors you need to keep a negative pressure inside the growspace. The only real way to do it is with an exhaust fan, which means treating exhausted air with a scrubber. A 3x5 space doesn't require a huge fan or anything, but you must maintain a negative pressure. If the pressure becomes positive it will push out through any imperfections in the seals around your room and door.

Try as we all might, i really don't think there is gonna be a way around this one.

If you could run portable ac and have a 100% seal, that's a different story. Just a whole different set of circumstances to deal with at that point that i'm too stoned to ramble on about.

Do a search for "noobs guide to growrooms" or something close to that. Great info on room set up in there:thumbsup: