Fix for scent leak on closed environment system

I am currently designing a sealed environment system and would like experienced opinions, critique, etc.
One problem I forsee and am trying to desgin out of the system is scent leakage due to neutral pressure.

Room size 7'long 4'wide 9'high
Sun system Magnum XXL 8" enclosed hood reduced to 6"
Incoming light ventilation will be from outside air coming in via 24" square box with air filter (typical hvac cold air return) to 6" insulated ducting >> hood >> outside.
Carbon scrubber system will draw via 4" fan thru phat filter from room and exit back into room.
Window AC through wall will draw from room and exit back to room to control temp.
Dehumidifier in room to control RH not controlled by AC.
Sentinel Co2 monitor distributing from tank.

My idea to resolve the issue of neutral pressure is to build into the system a valve that can be opened/closed on the ventilation line of the reflector. I plan to install a piece of 6" pvc midline between the light and the exit. On the 6" pvc will be a 6"x4" tee with a 4"x1" mip reducer, then a 1" threaded nipple, then a 1" gate valve.

By opening the valve I could allow an adjustable air leak into the system to create a negative pressure and prevent any possible scent leaking to surroundings.
To determine when I have the desired neg pressure I'm thinking of installing a vacuum gauge on the exterior of the room with an airline that has a pickup is in the room..therby verifiying the desired negative pressure.

Hypothetically speaking, this perfectly resolves the issue of scent leaks from a sealed environment....but I could be missing something. Any and all advise appreciated!

I ran a 2000w setup just like yours for the last year. I ran a 6" 50lb carbon filter recirculating and in full flower there was a smell leak. I first ran a 4" fan and filter and it did almost nothing so I upgraded to the 6". I added an cap ozone jr into the room (I know it's supposed to kill your plants, ruin the taste, etc. but it didn't and doesn't) and that took care off all the smell. I was growing og's and they stunk and not one wiff outside the room.

The gate valve is a good idea in theory but you'll lose alot of your co2 so it kind of defeats the purpose of a sealed room.

Good luck with your grow....

That's a good point...it will be steady sucking co2 out....hmmm?
I'd have to test it out to see how much it drops the co2. That means I'd have to really restrict the vacuum down to the absolute minimum so as to avoid losing too much co2 and still privide "some" neg pressure.
Was talking with a friend and he suggested adding a valved line for the fresh intake with a backflow preventer. Otherwise my "air thight" room is going to give at the weakest link...be it the door seal, a caulk line at the floor etc.

Here's the filter and fan I had planned Phresh 4" carbon filter $82, 4" Vortex $125
My room is 252 cubic ft
The 4" vortex is rated 172 cfm. Allowing for 10% reduced flow with filter I get 154 cfm...so I get a complete cycle in 1 min 40 seconds or so.

You thinking that's not enough?

No I don't think that fan and filter will be enough. I had a 4" fan and 4" x 24" active air filter and it didn't do much at all. I have this filter http://www.hydrofarm.com/pb_detail.php?itemid=8485 and a 6" valueline fan http://www.plantlightinghydroponics.com/valueline-inch-435-cfm-centrifugal-inline-fan-p-2260.html and it works pretty well, almost no smell. I just use a fan speed controler to slow the fan a little for sound.

I think the neg pressure thing is going to make you lose alot of co2. A tiny hole drawing only a little air would not create enough pressure to really do anything. If you are using a timer your co2 goes on for 15min then off 15min during lights on so your co2 levels will be up and down giving you no benefit. Just my opinion though...

If you can vent the room do that instead of the co2. I can't vent in the winter so I have to use co2 but when I don't have to and can vent (spring and fall) I do.

I've decided to go with the 6 inch...thanks in part to you sharing your experience. Gave ya + rep for your help.

I ordered a Sentinel Controller since I can't vent. The situation is way too hot right now and will be far too cold in winter. I'm stuck with an artificially controlled environment.