So i find myself knee deep in my new found hobby. My latest addition is a 6inch fliter and fan out the roof. Along with a loft spot for seedlings and small clone. At this point I’m thinking this should keep any and all smell down all the way through drying. Is there anything else i should be doing to eliminate odor?
3 Likes
I grow in a 2 bdrm apartment with just a 4 inch filter. Im sure you’ll be ok.
2 Likes
Your seedling and clones will not have much smell, if any.
The only way to contain the smell in a tent is to maintain at least a slight negative pressure in the tent. If your tent is in a negative pressure there is no reason too use something like a HEPA filter on the intake. Since there are many tiny holes in the tent due to the zippers, tread stitching holes, and wiring penetration openings. Tiny particles will be pulled in through these hole by the negative pressure. All you really need on the intake is a thin cut to fit filter, or an end of duct type filter.
If you are venting to the outside where nobody can smell the exhaust you may not need any odor control at all. Just insure the duct is sealed well on the outlet side of the exhaust fan,(Between the fan and the roof penetration). If you want to insure there is little or no odor being exhausted you will need to use a carbon filter somewhere in the exhaust line.
I find it easier to dial in a slight negative pressure by using both an intake and an exhaust fan, and putting each it’s own speed control. This will also allow you to ramp up or down the air flow through the tent while maintaining the needed negative pressure.
Sorry, I should have said. All you really need on the intake is a thin cut to fit filter, or an end of duct type filter, or just use the tent’s built in screened openings.
Not sure I’m following. You added the intake fan to reduce the negative pressure created by the exhaust fan?
Wedry xl
Yes, just adjust both fan speeds so that the exhaust fan is moving a little more air than the intake fan. This is very easy to tell when you are at a slight negative pressure by tent walls. Speed the intake fan up until the tent walls start to bow outward from the positive pressure and then slowly dial down it’s speed until the walls start to bow inward. You will pick up on whats going on with this technique.
Bottom line on controlling odor is a quality carbon filter. Maintaining negative pressure inside grow space. I like to use a couple sets screws after sliding filter on, also tape around intake side of Exhaust fan so you are not pulling Unfiltered air.
Happy growing 
Maybe I am not being clear about intake filters here. The type of intake filter is determined by the desired tent pressure. The tent pressure is determined by the grower’s desired grow room conditions. If you desire is odor control then you will need to maintain a negative pressure in your tent. The negative pressure will hold the odor molecules in tent until they can be exhausted, and outside air with the tiny particles will be invading the tent through the small holes in the tent that I spoke of earlier. Since the tent has the tiny holes in it, then there is no reason to use an intake filter that is finer than the holes in the tent.
If on the other hand, if your desire to have a very clean grow room, and do not need odor control. Then a HEPA filter in to intake is way to go. But the tent needs to maintain a positive pressure. The positive pressure will prevent the outside air with it’s tiny particles from invading the tent. But the positive pressure will push the odor molecules out of the tent through the same tiny holes in the tent.
1 Like
Why wouldn’t they just run the filtered intake fan, passive exhaust, & size everything to maintain necessary CFM & stay in positive pressure?
What’s wrong with one exhaust fan & tuning the passive intake area?
My original concern was filtering each tint. I guess my real question is now that i have the room sealed other then my fresh air in and filtered air out to outside the building. How necessary are the tent filters?
1 Like
I use a 6" filter and fan in a small room and no smell at all outside. I have it vented through the roof @TeNNWisKeY. You shouldn’t need to filter the tents individually.
My room is sealed also
@TeNNWisKeY,
In my flower tent I use both an intake fan and exhaust fan with both on speed controls. Pulling intake air from a closet close to floor level in the finished side of the basement, and pushing the exhaust into the adjoining garage. I no longer worry about the smell, and the garage does have some smell when the plants in the flowering tent are finishing. The only carbon filter that I presently use is in the exhaust of the veg tent, and this helps to scrub some of the smell in the garage but doesn’t remove it completely.
In my clone and veg tents I use exhaust fans on both venting out of the tents into the garage where they are located, and just open the tent’s inlet screens. This works fine with just the screens filtering the air, and I get little or no smell from these test as long as they are in veg.
@PhotoFinisH, There is nothing wrong with using a single fan to push or pull air through the tent as you described. But after using the dual fans on speed control I know how well this works. Yes, I first did this because I didn’t like the excessive negative pressure in the tent. After adding the second fan I found it has more benefits then just bringing the pressure up in the tent. During the winter months I had to cut the fans down to about 25% speed in order to keep temperatures above 70 degrees, and now that it is summertime I am at about 80% speed and holding at about 75 degrees. Also it is very easy to set the tent to a negative, positive, or equal pressures.
During one of my last grows of two Black Widows in soil via 5 gal fabric bags, they were going four days between waterings I tried an experiment with changing tent pressures. After noticing that the plants looked droopy just after watering/feeding them. I decided that I could simulate the rainy and dry sunny days in nature a little closer with this technique, and hopefully introduce some stress that would improve the bud quality. In an attempt to simulate a rainy day followed by three sunny dry days as follows:
Day 1 rainy day, water plant to a run off, set tent to a slight negative pressure, and reduce light output.
Day 2 start sunny cycle, set lights back to full output and set tent pressure to equal pressure.
Day 3 & 4 finish sunny cycle, full light output and set tent pressure to a slight positive pressure.
Repeated the four day technique including during flushing until harvest.
This technique produced some of the best smelling and tasting buds that I have done.
Sorry to get so far off track, but just trying to share some of I have learned.
2 Likes
Interesting experiment. I was wondering about barometric pressure & how it might affect VPD targets the other day. You’re doing experimental research which is fair. I could be wrong but generally speaking I think most people want to be in the optimum negative pressure zone, just enough to keep the smell in, but not leave much CFM on the table. If someone has a box with one exhaust fan & passive intake holes & everything is running reasonably well, but there is some excess negative pressure, adding a fan to the intake & tuning speed controllers is probably not what I would recommend. I would suggest increasing the passive intake area to reduce negative pressure ( & also see a small increase in CFM as a bonus).
Adding intake filters is fine if needed. I use some DIY ones on my passive intakes. The material itself is an obstruction so account for that if you already tuned the box with open intakes. Either open more filtered intakes &/or do ‘3D’ filters to get the open surface area back & get the negative pressure just below static pressure & not leave a lot of CFM on the table. I have to clean them sometimes. They will start to steal CFM/ add negative pressure as they get dirty.
1 Like
There are so many variables with everyone’s set ups. This is why it is so hard to give advice without a complete understanding of their situation.
As to my situation with the intake fan, I have about 10 foot duct on the intake, (the duct is necessary to get cool air) and the 4 inch fan rated at 110cfm. At first I used 4 inch flex duct, and quickly found that the 4 inch flex duct was too small for that distance. After changing it to a 6 inch duct and keeping the 4 inch fan the intake air cfm was tripled. If I was to change this intake to a passive one it would require a 10 - 12 inch duct for that distance.
If you find this hard to believe. Check out an online ductulator and see how much even a short run of flex duct kills air flow. You are correct ducts, fitting, and filters all add resistance to air flow. Of course this is due to the static pressure drop as the air passes through these devices, and the more air you try to move through them the greater the pressure drop. Sometimes it is better to use a good fan to overcome these problems.
I would only recommend a passive intake or exhaust if it can be kept very short.
1 Like
@PhotoFinisH, VPD vs pressure. From what was told by, @DrWoo, a decrease in tent pressure will increase VPD by a very little bit. So little that is not worth using pressure to manipulate VPD.
I think the plants pick up some type of weather signal from the changing pressure, saturated roots, and the reduced light. Followed by drying and increasing pressure. I believe this introduces a low level stress on the plants that in turn makes them produce more resins for it’s protection.
@TeNNWisKeY, sorry about hijacking your thread. By the way I saw a picture of your set up on another thread, NICE! I must said it was not at all like what I had envisioned from the description that started this thread.
2 Likes
The real reason. But it lets you try experiments =) . Yeah if you’re dealing with a 10 foot run to get fresh air to your intake, then you’re probably doing what you have to do in that case. A lot of compromise goes into balancing the grow space.