Too Much Negative Pressure...

So, after i just installed my 8" exhaust fan, turns out my 6" intake can't keep up...not even close. I'm thinking my intake needs to get stepped up a bit, but i'm a little hesitant to cut any bigger holes, given that it's cold here 8 months out of the year.

Any wisdom on this subject from anyone? I'm already worried that having a 6" hole will be too much cold air coming in during winter...

When i get the grow fully going, it'll be 4 vented 1000w lights in my 10x12 room, so i know it'll get hot in there at times.

Here's a couple pics of what i have going now.

My intake:
My exhaust fan: View attachment 2711282
First grow currently under way:
I have 3 mothers in a recirc dwc that i built. They seem to be pretty healthy, despite only being under a T5. I already trimmed the hell out of the left plant to make clones for my first big grow.

So, is it just the one fan? Or do you have a fan on your intake and a fan on your exhaust? What cfm is your fan rated for? Also, I see you mentioned the room is 10x12, how tall are the ceilings? Its important to know the volume of air you are dealing with, and the place to start is knowing your total cubic footage. That's also going to be important for knowing how much charcoal you'll need if stealth is a priority and you need to mask the smell.

I've heard that speed controllers really help out these types of situations. Generally, you want your exhaust to be 2x as powerful as your intake, or moving twice as much air. Even then tho, you can still have problems without adding speed controllers to really get the air flow dialed in. It takes a little messing around with, but once you get it perfect everything's gravy.

BTW, those mother hens are looking good!

Edit: about your question with the holes, I don't see the need to make any more holes. The six inch intake port and 8 inch exhaust should be fine; its a matter of getting the right fans in there and moving the right amount of air through your ducts.

Aaand, edit again!: I guess it would matter how much cubic feet you're working with, tho. Get that ceiling heighth so we can get some calculations.

Usually for adaquate negative pressure to be maintained without causing issue your intake should be 10-20% less powerful than your exhaust.

What exactly is the problem you're experiencing.

The only thing you actually mention is that your intake cannot keep up?


Is it a temp problem you're experience?



J

Ok, theyre 8.5ft ceilings, so roughly 1,020 cubic feet. I have an intake fan, buts its some wimpy home depot 6" inline fan. Rated for 250cfm, but theres no way. The outgoing cfm is 675cfm on the 8" MaxFan.

The issue is that I think I'm not using the full capability of this exhaust fan. My room is in a sealed shed and when I open the door, its jumps inward about 2 feet!

This is my first grow room, so I'm just feeling it out as I go and trying to get to get to know it.

So you think if I just get a stronger 6" inline fan, that'd solve it? I think a speed controller might also work, but I don't really want the cfm's lower. It gets hot and humid during the day and I'd definitely like to change out the air quicker.

Also, what kind of speed controller would work with my max fan? I keep reading that you have to buy their $200 controller bc the cheaper ones will burn up the fan...

And ty Thor! Those are some Critical Kush that I grew from seed. They definitely grow like an indica. I had never grown dwc before, but holy hell do they grow quick!

im growing in a 4x4x7 tent x2. i have a 6" 250cfm intake fan and an 8" 550 cfm exhaust fan. im having perfect neg pressure w/o any speed controllers. all my fans run 24/7. iirc speed controllers do not work with the booster type fans, only the inline fans.

Here is how I would personally work out fan power etc for your room and lighting.


Room dimensions.
LxWxH=cuft

Cuft x 1.25 = fan power with the addition of a carbon filter.

This equals 1275cfm for exhausting and scrubbing your room.

We then need to take 5% of 1020cfm for each air cooled light.
51cfm extra per light.
51x4=204cfm extra for air cooling 4x1000w lights.

1275+204=1479cfm

1479cfm fan in order to adaquately cool/ventilate and scrub the air.


This is based on an air exchange of once per minute.

So realistically if no AC is being used then you need a fan and filter capable of around 1500cfm of air movement in order to keep temps in check and exhaust stale air and replace with new.


Information found here.
http://urbangardenmagazine.com/2010/07/planning-your-grow



There is also another way to do it but that would involve a sealed air cooled lighting rig whereby no grow room air passes through your lighting system.
Then your 8" could easily scrub and exhaust the room adaquately enough to keep temps etc in check.




J

So, when i was planning out how to design the venting for my room, i used this page http://www.weedguru.com/forum/viewtopic.php?t=26677

Seems to be roughly the same calculations, but the important difference is this page says you want an exhaust fan that can clear your room at least once every 3 minutes. The fan i got seems to do pretty nicely at only 675 cfm, i don't know why i'd need a 12" exhaust fan. Also, i currently only have a T5 running, but when i put in my other lights, they will be on a sealed intake and exhaust.

PumpedUpKicks said:

So, when i was planning out how to design the venting for my room, i used this page http://www.weedguru.com/forum/viewtopic.php?t=26677

Seems to be roughly the same calculations, but the important difference is this page says you want an exhaust fan that can clear your room at least once every 3 minutes. The fan i got seems to do pretty nicely at only 675 cfm, i don't know why i'd need a 12" exhaust fan. Also, i currently only have a T5 running, but when i put in my other lights, they will be on a sealed intake and exhaust.

Click to expand...

If you look at the link I supplied (which is an indoor growing magazine) they talk about 1x-3x per minute for room exchange. If you have to go above 3x then AC should be considered.

This way of designing rooms has served me and other growers very well.

If you had a sealed lighting rig using another 8" fan to run just the lights your current 8" fan will suffice to exchange the room air.

Then you could utilise passive screened vents in order to have enough airflow.


Like I said though this is how I would setup the room
And how UGM would also setup the room.




J

The 1500CFM fan I mentioned was for both cooling and exchange to control temps.

If you use a seperate fan for the lights then one exchange every 3minutes is fine.



J

PumpedUpKicks said:

Seems to be roughly the same calculations, but the important difference is this page says you want an exhaust fan that can clear your room at least once every 3 minutes. The fan i got seems to do pretty nicely at only 675 cfm, i don't know why i'd need a 12" exhaust fan.

Click to expand...

I thought that post made it about as clear as it could be.

You may not agree with the info, as you have read conflicting info and made a purchase based on it, but the reason is set forth fully and clearly.

To defend and protect your decision/purchase whilst attempting to solve a problem may inhibit you from finding the most viable solution imo.

Bigby said:

I thought that post made it about as clear as it could be.

You may not agree with the info, as you have read conflicting info and made a purchase based on it, but the reason is set forth fully and clearly.

To defend and protect your decision/purchase whilst attempting to solve a problem may inhibit you from finding the most viable solution imo.

Click to expand...

i'm not trying to be comabtive about it, 1500cfm just seemed like overkill to me based on EVERYTHING i had read before. I see now he was saying without vented lights, i'd need a 1500cfm fan, which makes a ton more sense.


Now i'm trying to figure out where i should pull more air from. After reading that article, i definitely shouldn't be pulling in ALL of my air from outside, but that would mean i need to pull air from the only adjacent room. Here's a mock up of what i had pictured. I didn't include the light venting bc i haven't gotten that far yet. I'm just trying to figure out the exhaust/intake situation currently. What are your guys' thoughts on this plan?

I should mention this is a stand-alone shed that i constructed for the purpose of growing. Both rooms are identical in size and are sealed off from one another.

Ok here goes.

I would DEFINATELY intake your air from the adjacent room.

By in taking from outside you are limited by the air temp coming in from outside. You could run too cold and then have to heat the grow room. (Thinking about those 8months of cold). So bring your intake in from the adjacent room.

This will help you maintain a more stable temperature. Also there is less chance of bugs coming in through the intake.


Reposition your exhaust to the opposite side of the current wall and good to go.


J

Ok, i can definitely make my exhaust point in that other corner, no problem. Now, how big do i need to make these passive intakes to keep up?

You ideally need them between 2x-4x the surface area of your exhaust fan spigot size.

So 2-4 x8" holes. Screened and light proofed.





J

Right on, tyvm for all of your input, Jonda. You've been more than helpful.

No worries bro.


Here's a shot of my 400w room with 3 large plants and 4 clones at day 37 of 12/12. Around 30 days actually flowering.

This rooms temp and air exchange is controllable. I can control temps between 14C-40C. I stick between 19-24C and its air exchange on full hit is evacuating 1.5x the room in one minute. Its fan speed controlled though on a temperature sensored fan speed controller.










J

wow, very nice! those ladies are so dark green and healthy looking. What strain? Also, i'm surprised you haven't done SOG with this one. It's so bushy! What are you expecting the yield to be like?

There are 2 strains.

Largest cola plant is DINAFEM CRITICAL+ along with 1 clone.

Then there are 2 large DELICIOUS SEEDS FRUITY CHRONIC JUICE and 3 clones.


If I had 4 large plants I would be expecting between 400-500g ball park.

With the addition of 4 clones and loosing one large plant space due to a germination issue I'm hoping those clones can bag me around 25g each but I ain't too hopeful. It was a small mini SOG within the room to test to see if I run 20 of them next time.


So a full room weight dry of maybe 350g.


The reason they're so green is that I use a NPK of 6-3-8 from start to finish with mono Ca and mono Mg on hand as I grow in coco.


My thread about the grow is in my signature link.





J