Motorized damper question

I wondering how airtight these things are when closed.

I have an unused AC outlet from the central air that I'd like to feed into a closet but I don't want the air conditioner creating positive pressure when the exhaust fans aren't running.

I thought I'd get a normally closed motorized damper which only opens when the exhaust is running. But only if it can contain the air pressure from the central blower.

They will handle the pressure fine but don't expect the to completely stop the air flow they aren't sealed. its like closing a normal heater vent in your house. while there is a good amount of air flowing the psi isnt that high so only a minimal amount of air is let past. when it is closed the psi where you have the damper will raise causing the air that is coming from the central air system to flow to other open vents where the psi is lower.