Passive intake question

Hi RIU!
New member, love reading all your guys' posts! I'm starting my 4th grow and this one is a bit different. It's under my house in a crawl space of sorts. So height being an issue, I dug down about 2 feet into the ground to be able to get a gorilla 4x4 shorty in there. It fully insulated down there and the humidity is quite low. I will be putting foam board around the tent to protect from the cold ground.
My question is: I was considering running a passive air intake. I know it needs to be twice the size of my inline fan (which will be a 6in ac infinity). Can I simply pull air from inside the room, or do I need to cut a hole to the outside of the house to bring in fresh air? I was also considering just attaching a HEPA filter on the end of the passive intake. I just want to make sure the plants get quality air. Can this be achieved pulling air straight from the room?

Yes, if there is decent air exchange in the room. If the crawl space is vented and a pretty good size, it would likely be fine for a small tent.
I run passive intake with just a few mesh side vents on my 10x10 and run a 10" fan that pulls up to 1000cfm through it. It definitely keeps negative pressure, but its not like caving in the sides of the tent. Thats all you really need to look at imo. Close your tent up and turn your fan up to max. There should be a slight inward flex, but not to the point it looks like your tent is sucking in on itself or going to collapse. As long as that is met, your intake is adequate. You want negative pressure so its easier to control the smell with a carbon filter on your exhaust.
I always like the idea of filter boxes when youre pulling in air from a somewhat uncontrolled place.

Thanks Assassin!

I grew under my house in a 'crawl' space for years. Pretty cool someone else is too

NeWcS said:

I grew under my house in a 'crawl' space for years. Pretty cool someone else is too

Click to expand...

Yeah I'm stoked to give it a try. It's an interesting space. I was going to grow in my loft, but I have a little girl due in April and with the huge amount of family and friends that will be coming through, id rather keep it on the DL. I'm in a legal state regardless, but better to keep this to myself