I need an intake filter for outdoor air being used to cool a room. My goal is to filter out any potential hair, dust, pollen, and mold spores etc. I want to build a supersized version of the can fan intake filter Can-Fan Intake Filter 6 in
with a media cabinet and filter that are typically used for home hvac. I’m thinking of using a MERV 5-8 pleated filter 16x25x5 inchs. They are typically rated for 12 months when used in a home hvac system, cheaper then carbon filters, widely available and easy to replace. The intial setup cost will be higher as I need the cabinet that houses the filters and flanges ect. The 5 inch filters should have a lower pressure drop vs 1 inch. My questions are if there is a minimum CFM required for the filters to be effective? And if I should be aiming for a higherer MERV rating for my intake filter?
Hepa my brother, if u stick with that Merv it won’t catch what you need it to. I researched this once on commercial grows using hepa… operating room is on average 22 Merv 
More Merv more drag, I made one at one pre filter then the bigger one. 18 Merv I think it is.
What are you using for a fan? I think I may need to upgrade and get dedicated fan to get air through a hepa
Tested it with a 8” blower works fine…I use intake in the winter to cool the rooms with the amount of lights I have can’t get around it. Summer block those intakes off and run mini splits
Thank you! I will definitely aim for a better filter then the 5-8.
Ok @treetrunks relooked that up because I didn’t want to tell you wrong. Standard Merv in most grow operations is 8 Merv pleated per the national air filtration association. Operating room is 22 Merv is why I went to so high on the one I made…I use charcoal filtering cloth over the ends of my intakes in attic and change out once a month with no issues but like I said I only intake in the winter. 
I agree and one other thing…keep the bugs out, that is more important than mold or dust.
Here’s a low cfm option for 6 in ducts for anyone interested FB 6 Inline Filter Box MERV13 - Fantech but with a max cfm rating of 176 CFM this would be much to low for my setup
For anyone interested of building one of these over sized filters I will try to update as I go along. If you are in Canada you may have some trouble finding the cabinets for the media filters without going through an hvac supplier. For those in the states you will probably have a much easier time locating these https://www.homedepot.com/p/Maxx-Air-25-H-X-16-H-X-5-D-FPR-5-Air-Cleaner-Cabinet-with-Filter-AF25165/206855440 This style of cabinet and filter have a maximum CFM rating of between 1,400 - 2,000 CFM. It’s over kill for my current setup but leaves me room to upgrade should I ever need it.
The size of your fan and ducting will be your limiting factors in the actual CFM you get out of your filter. At the moment I am planning on using 6 in round ducting on each end of the cabinet. I have one 6 in ac infinity fan pulling in outside air and another venting hot air from the room. If I find that I am getting a large loss in airflow after installing the filter I will try using larger ducting between the window and filter to hopefully reduce the workload on the fans.
I plan on starting off with a pleated MERV 8 filter to keep the CFM losses to a minimum and may raise the filter ratings as I go if I’m having minimal CFM loss.
What ducting are you using?
After talking to an hvac guy I know again he convinced me to go for a window mounted design. So the new plan is to install a larger filter at the window something like 20 x 25. Im planning to attach two 6 in ducts one leading to the flower room and the other to the veg tent. I have both 6 in intake and exhaust fans for each space.
My current setup is the same except there’s no filter on the intake except for the window screen. I haven’t had any trouble with the dew point or condensation yet and was able to run it all winter. Currently I have a solid metal run of duct, some ac infinity plastic flexable duct and some of that aluminum foil looking stuff. I’ve been adding to my setup with the money I save every harvest and alot of the flexible ducting has been used for things I thought I might have to move or redesign.
20x25 won’t restrict a 6" duct fan until it gets dirty, that is plenty big. Those are turbine fans, so they create pressure and vacuum. Consider the fact that you use the same size filter for an entire house.
One I made I thinks 20x16 I think 
Hi there can fans are good but need speed control and depending on where it is you might want silencers on it or bit it in a home had box for noise cancelling. Also I use phresh air inlet filters they are good and the outside netting can be washed of dust then reused.