Exhaust into heating/air duct?

I have dreamed of growing my own marijuana since I was about 14 years old (36 years old currently.) I have tried on several occasions, but could only afford florescent bulbs, and didn't have much success. Recently, I read about carbon air filters, and this, along with lump of cash (I will have around $1500.00 to budget for the entire grow closet), has got me very excited about building my first simi-pro grow room.

My closet is 58 inches by 61 inches with an 8 foot ceiling. In the back corner is a heating/air duct that I imagine stretches from the heating unit downstairs up into the attic. This duct effectively makes my growing area 37 inches by 61 inches. I plan on building a two level grow box in this space. The grow area will be contained inside a box that is inside the closet.

My first question is about using the duct as a place to exhaust my grow box. I read a little about negative and positive pressure, but I honestly just don't get it. Will exhausting into this duct have the slightest chance of air flowing backwards into my grow box?

I have a billion other questions, but I will get to those later. Thanks in advance for any advice you can offer me.

If it's a retutn duct then you could use it... wouldn't use it otherwise.

It's best IMO to exhaust to the outdoors.

NEVER... ever exhaust your grow into a exhaust or feeder duct....You want to push your grow exhaust through your HVAC system?
Don't do it...
If you want to pull air from a passive return air duct, fine, do not exhaust into your house air system...
Now, maybe this duct does go to attic, and could be used as a space which you could get a regular exhaust tube/ducting through it...
If it's just a air duct, maybe you can use it as a "Conduit" to your attic...
Good Luck man...

Thanks for the replies. I hadn't thought of using it as a conduit into the attic. That will probably solve my problem very nicely.

Just to be clear, my original plan was to filter the exhaust with a carbon filter before pushing it into the home air system. However, I like the idea of using the duct as a conduit much better. I will have to get up in the attic and see where the duct goes.