I have my carbon filter ducted to my light with ducting out the other side to my fan, keeping the fan the last thing in line so that it's pulling air through the system, these fans pull air much better than they push it. I don't pull any outside air or dump it outside
Just a little advice I posted this elsewhere.
I see many people placing their fans at the end of their exhaust which is slightly less advantageous.
Heres why
The reason the can fan should be after the charcoal filter but before the light fixture.
The fan drawing air through the charcoal filter.
Will create negative pressure in the chamber space. Ensuring that all air gets drawn through the charcoal filter for scrubbing.
Ahead of the fan it creates positive pressure through the air cooled light fixture then ducting into the room.
This does 3 things.
1.) It creates a higher mass of air flowing over the bulb since its positive pressure (more air density). This means the air will have a higher heat capacity.
Combined with the pressure of the air acting on the bulb to aid in heat transfer.
As well as being a denser charge of air
It will carry out more heat.
2.) If anything after the fan (lighting fixture/ducting) has air leaks. The already scrubbed air would leak back into the chamber to be re-filtered through the charcoal filter.
3.) The fan will be exposed to lower temperatures being before the light increasing its life expectancy.
If placed after the light fixture or at the end of the exhaust stream. It would work opposite to all 3 of those variables.
1.) The negative pressure would create a lower mass flow of air (lower density).
It would be lower pressure meaning lower heat capacity.
It would transfer less heat from the bulb due to lower force of air pressing onto the bulb.
And also due to a lower density of air flowing through.
All this would reduce the efficiency of drawing the heat outside.
2.) If there are any ducting leaks before the fan and after the charcoal filter(ducting/light fixture).
The negative pressure created by the fan will draw unfiltered air (bypassing the charcoal filter) out of the chamber and into the room. Leading to your neighbors finding out.
3.) The can-fan will be exposed to higher temperatures reducing its life expectancy.