ok so my AC unit sucks air from outside and pushes it inside this room. And I have no exhaust anywhere so the pressure is causing my door to swing open. whatcha guys suggest.
thanks for the input.
Okay, so let's look at this again, from the beginning;
1. you have A/C from central air system pushing air into the room, causing overpressure.
2. since this is NOT a self contained A/C, you're not able to seal the room in the traditional sense.
3. positive pressure from this room is obvious when you open the door, and likely anytime you walk into the garage- after all, that air IS going somewhere, it's just not controlled.
4. I'm unclear about how or whether you're adding CO2.
First, you don't have a sealed room; by definition, a sealed room is one where the working air does not leave the room- and if you have an air intake, you're creating an outlet whether you like it or not- otherwise, you'd have no flow at all and your room would be overheating. Therefore, let's stop thinking about this as a sealed room for the time being because until you can install a cooling system that does not rely on air exchange for cooling, your room will not- cannot- be sealed.
The suggestion above to install a return duct for your room air to return to the A/C system will fill your house with the smell you're trying to eradicate- what else could reasonably be expected? The smell isn't gonna vanish on a trip thru the ductwork.
I'd like a bit more detail on your CO2 situation; are you supplementing, and if so, when and how? How this is done will make a big difference in how your room performs.
At the moment, I would install a carbon filter and an exhaust fan pulling through it up high in your room, and exhaust this waste air outside of the garage; don't worry about smell since the air will be filtered through the carbon filter. This will reduce but likely won't eliminate the smell in a garage, and it will help equalize pressure.
To create the pressure conditions you want (negative pressure in the room), you'll need to make sure the exhaust fan is pulling more air than the A/C is trying to push into the room. Adjust fan speeds, duct sizing or whatever to make this happen. Once this is properly done, the smell inside the garage should be a thing of the past.
CO2 naturally falls to the floor and is only spread around the room by fans, so place the filter and exhaust fan up high to limit this loss. Because the room is open, you're getting CO2 from the outside atmosphere- and what's more, you will likely not be able to significantly increase this percentage since you're turning the air in the room over so much.
Also, I'm betting your humidity is too low, due to the drying effects of central A/C and high air turnover. This would drastically limit the amount of CO2 the plants could take up anyway, since in dry weather they keep their stomata closed to reduce transpiration losses- which then leads to slowed growth.
To better control conditions, start by slowing the intake of A/C air as much as possible, while still maintaining acceptable temps in the room. In this case, less is definitely more. Adding humidity with a swamp cooler will help you twice; once in raising humidity, and again by the cooling action of evaporative cooling itself. This will reduce your need for A/C air as well. Seal and vent your lighting with air that is NOT coming from or blowing back into your room- this will reduce the heat load your cooling system has to cope with, making it easier to control the climate inside effectively.
To properly seal your room while still using the central A/C, look into using an "air to air" heat exchanger to shed the excess heat in your room without exchanging the working air inside. Other options are to get a minisplit, or a water chiller unit. It is at this point where you will need to ensure you're supplementing CO2, as the plants will deplete it quickly in a properly sealed space.
