Quote Originally Posted by sleestack
where is the best place for the passive air intake vents? i have a romm , 5x13 split into 2 rooms , veg 5x5 and flower 5x8 , i plan on to share the output fan, bewteen the rooms, and was wondering ,about how large do the passive intake vents need to be? and should i put them in the waLLs? or use the ceiling in between the rafters? ,
Most grow rooms I have seen have the intake low and the exhaust up high or in the lights.

Why? First, physics. Heat rises. It is more efficient to exhaust from up high and take out the hottest air.

Second, the goal of bringing in fresh air is to give the plants fresh co2 to breathe. You want to bring it in down low and allow it to flow around the plants.

So I would put the inputs at floor level or just above if it were me. The commercial grow tents that I have bought do exactly this. The intake air slits are in the sides at about 6" above the floor. Can't tell you for sure yet, but it seems to be working well for the grow I am attempting. I have also noticed in other's pictures of grow boxes and tents that they do something very similar to this as a general rule.

It makes sense to me too not to share the airflow between these two rooms for several reasons. Imagine the time you want to have a male isolated in the separate room while your girls flower in the other room. Seal these rooms and put a Y on your exhaust fan... and then suck the air from the top of each room individually. If you have an effective filter after your fan, you have just eliminated the chance of contamination between the two areas.

As far as size... it depends on your fan, the size of your room, and how well sealed it is. You don't want too big of a hole, so that you can maintain negative air flow into the room as well as keeping your air flow out only at the exhaust so you can filter it. A smaller hole however will create a faster airspeed of the incoming air and not allow as much humidification and heating inside the room until it slows down. It's a fine balance as to how fast to replace the air in the room against how much you have to adjust it. This is a big reason I decided a few weeks ago to go with a commercially made grow tent rather than trying to engineer a climate box of my own... the airflow through the thing was a critical problem and one I did not have the carpentry skills to be able to solve.

Emmie