You would only stack fans in series to overcome a pressure problem. Because of the high back pressure the fans don't produce the maximum flow but will add their pressures together. If they were unrestricted the flow would be around the weakest fan's max CFM.

Originally Posted by uraverage

i guess i smoked way to much to underastand this......
who besides me is still lost?????

medicated--no comprendo
shall return-----
sober!
thx

[rasputin]

Redgreenry, awesome thread.

Quick question jus to be sure... flowering room is 5x5x6... I need approx. 125 cfm fan (I'm rounding up a bit for a little CYA)? I need to keep it well ventilated enough for odor to be gone and temp's to stay balanced. I was thinking of a 6" ducting fan attached to a DIY carbon filter made 12-18" in dia.

Yer thoughts?

Thanks Rasputin,

you're asking to do two different things at the same time.

To take the heat out of the room, you need roughly 3CFM per watt. If you use vented hoods or cooltubes and pass outside air through the tubes and exhaust it outside, you will need 0.3 CFM per watt.

A 5x5x6 room is 150 cubic feet. At an air exchange rate of once every 5 minutes you would need a scrubber that would pass 150cuft/5min = 30 cubic feet per minute

****************************** ****************************** *****
Let's design this room using some rules of thumb. 50 w/sqft is the normal size for HPS lighting. Your 5x5 room has 25sqft which would take 1250 watts or two 600w HPS.

Each 600w light would need 0.3 CFM for 180 CFM. If you series the airflow, the exhaust air would be twice as hot but it's going outside, not on the plants. Get a 180 CFM axial fan and a small scrubber rated to 30 CFM.

If you want to pass all the air through the scrubber you would need to double the size of the fan and it would need to be a centrifical blower not an axial fan at 360 CFM and the scrubber would need to be about 200 CFM.

[BlueHalo]

Hey guys! I just got this link and have a vent issue that I really need some feedback on, if you have time, which would be greatly appreciated.

Ok, So I have previously been using a 55 gallon rubbermaid to grow in with 25,600 lumens of CFL's but have now built an oak cab that is 7 feet tall x 3 feet wide X 19 inches deep and have it partioned in half horizontally so that I have a mother cab in the bottom and a flower cab up top. The mother cab is using CFL's while the flower cab will be, as of tomorrow, using a 400 Watt HPS Cooltube.

I am using a 5" S&P Mixedvent inline fan-TD series 125 with 192 CFM for my exhaust which is hooked up to my attic. On the bottom of mother cab I have 12 X 1.25" passive intakes and then from the mother cab I have 10 X 1.25" intakes drilled through the shelf that seperates the two rooms. At first I only had 12 X 1.25" passive intakes + (2 X 1.25") passive intakes drilled into the shelf partition and the doors sucked shut on me and the motor strained a bit. From what I understand this is due to the negative pressure inside the cab. Then today I drilled 8 more 1.25" holes into the shelf (a total now of 10 X 1.25") partition leading from the mother room and into the flower room, but now the door no longer sucks shut from the negative pressure inside of the cab. Should I plug some of those holes up that I drilled today in order to get my pressure back? As I said I have a 400 watt HPS coming tomorrow and really need to get these vent issues straightened out ASAP. Any help that you might be able to provide would be greatly appreciated. PUFF PUFF!!

Currently growing Tundra (Autoflower), WhiteBerry, Spoetnik, and Purple Lady

I added 6 more intakes to the bottom after the photo.

[BlueHalo]

Hey redgreenery I was wondering if I could increase my negative pressure inside the cab by putting the S&P on high setting? Right now I have it on low but with no speed controller. I haven't bought a speed controller because my TD 125 is a 2 speed motor and am not sure if you can actually use a speed controller on these, and if so then what kind? PUFF PUFF!!

[rasputin]

Originally Posted by redgreenry

Thanks Rasputin,

you're asking to do two different things at the same time.

To take the heat out of the room, you need roughly 3CFM per watt. If you use vented hoods or cooltubes and pass outside air through the tubes and exhaust it outside, you will need 0.3 CFM per watt.

A 5x5x6 room is 150 cubic feet. At an air exchange rate of once every 5 minutes you would need a scrubber that would pass 150cuft/5min = 30 cubic feet per minute

****************************** ****************************** *****
Let's design this room using some rules of thumb. 50 w/sqft is the normal size for HPS lighting. Your 5x5 room has 25sqft which would take 1250 watts or two 600w HPS.

Each 600w light would need 0.3 CFM for 180 CFM. If you series the airflow, the exhaust air would be twice as hot but it's going outside, not on the plants. Get a 180 CFM axial fan and a small scrubber rated to 30 CFM.

If you want to pass all the air through the scrubber you would need to double the size of the fan and it would need to be a centrifical blower not an axial fan at 360 CFM and the scrubber would need to be about 200 CFM.

Interesting, thank you Red. That is a lot more fan and scrubber than I anticipated. Luckily, I got a bead on some home depot duct fans so I don't need the expensive vortex or whatever else.

I trust yer take on the matter, you seem very well versed and nuanced on the subject. And odor is the main concern, not temps. I lazily tossed in keeping the temp's balanced and you rightly interpreted that as a need to bring them down. I don't anticipate major temp adjustments via the fan but one never knows. I intend on having 1-2 fans in the room moving air around besides the exhaust fan but the ambient temp & RH could be a problem come the summer. Blowing hot air only helps so much.

I picked up an Inductor 6" inline duct fan that is 250 CM. I could use your first example and that should do the trick even with a carbon filter attached? Ideally I'd have the fan facing the floor in the cieling w/the filter in front of it pulling air through & out the room. Cycle the room every few minutes and with, what say you, a 8" passive intake bringing in fresh air.

This is the step up from what I got. 500 CFM. That ought to do the trick either way I choose w/a carbon filter, no? Any thought on how wide a diameter to go on the filter w/this fan? Thanks Red.

https://www.homedepot.com/webapp/wcs/...a&ddkey=Search

[hoosierdaddy]

rasputin,
You can have a fan that pushes 250cfm, but pulls very little pressure. Another fan can also be rated at 250cfm and pull loads of pressure.
The duct boosters are just that, boosters. They cannot pull very much pressure at all. No comparison between the 250cfm duct fan and a Vortex or S&P of the same cfm rating.
I have used the duct fan attached to a carbon scrubber for my cab. Thing is, the fan doesn't have much power and it takes two of them stacked together at the end of the scrubber to pull enough to keep my cab cool. The cfm is virtually the same with one or two fans, but the pressure is increased by nearly double. (still not as powerful as one quality inline fan)
I now use one of those duct boosters to keep my 400w + 150w lights cool in a tube. And it is quite enough to do that job. Two won't help much in that situation because there is very little pressure to deal with, and two wouldn't help matters in that situation.
(hope it makes sense why...red has explained it technically)

IMO, leave the boosters to boosting and doing what they are capable of doing. Get a decent inline fan to use with the scrubber.

[rasputin]

Originally Posted by hoosierdaddy

rasputin,
You can have a fan that pushes 250cfm, but pulls very little pressure. Another fan can also be rated at 250cfm and pull loads of pressure.
The duct boosters are just that, boosters. They cannot pull very much pressure at all. No comparison between the 250cfm duct fan and a Vortex or S&P of the same cfm rating.
I have used the duct fan attached to a carbon scrubber for my cab. Thing is, the fan doesn't have much power and it takes two of them stacked together at the end of the scrubber to pull enough to keep my cab cool. The cfm is virtually the same with one or two fans, but the pressure is increased by nearly double. (still not as powerful as one quality inline fan)
I now use one of those duct boosters to keep my 400w + 150w lights cool in a tube. And it is quite enough to do that job. Two won't help much in that situation because there is very little pressure to deal with, and two wouldn't help matters in that situation.
(hope it makes sense why...red has explained it technically)

IMO, leave the boosters to boosting and doing what they are capable of doing. Get a decent inline fan to use with the scrubber.

Hoosier, thanks for the info. I suspected I'd get a similar reply from Red, altho' I was hoping not. Looks like I can use the 6" for blowing air around at least or just return it. Maybe hook it up in the veg room for some air movement. Time to search for some reasonably priced inline vans.

Ah! I just caught your mention of using the duct boost to cool your lights. I could use this 6" fan to connect to the air cooled hood and let it fan the bulb. Then I can get a better fan to hang in the room w/a filter as a scrubber or exhaust.

EDIT

Alright, I had to come back here. Got into a little discussion with an acquaintance about CFM's and fans and air circulation and he's adamant, lo and behold he has an architecture degree! oh my, so in his estimation I was "fucking stupid" for listening to people I don't know on-line tell me that the 250 CFM duct fan isn't enough to vent a room. How it came up in discussion was innocent so no worries that I'm discussing my op with anyone who doesn't need to know.

If that isn't funny enough, when I tried telling him how a duct 250 cfm is like a Vortex is 250 cfm but pulls different amounts of pressure he couldn't understand it. Now, truth be told, I'm a little torn on it but I understand it. A bathroom fan versus a computer fan or even an oscillating bedroom fan aren't exactly the same kind of fan even if their CFM rating is similar. He doesn't get that. I don't know an easier way to explain it to him and frankly, I'm tired of having this discussion with him. He doesn't know I'm growing but he's a friend of a good friend of mine so he's often my friends house. And lately we've been casually discussing building and whatnot, it's a fairly typical topic over there so it doesn't raise any eyebrows, and so my friend and I discuss all this under the premise of building a music room in the basement.

He's chimed in the last few weeks and it's grown to a point where I just want to say, hey look you've never had to do any of this before and I'm pretty sure your degree didn't teach you shit about removing the dank odor of marijuana plants out of your grow room. But for good reasons I'm not going there.

Originally Posted by BlueHalo

Hey guys! I just got this link and have a vent issue that I really need some feedback on, if you have time, which would be greatly appreciated.

Ok, So I have previously been using a 55 gallon rubbermaid to grow in with 25,600 lumens of CFL's but have now built an oak cab that is 7 feet tall x 3 feet wide X 19 inches deep and have it partioned in half horizontally so that I have a mother cab in the bottom and a flower cab up top. The mother cab is using CFL's while the flower cab will be, as of tomorrow, using a 400 Watt HPS Cooltube.

I am using a 5" S&P Mixedvent inline fan-TD series 125 with 192 CFM for my exhaust which is hooked up to my attic. On the bottom of mother cab I have 12 X 1.25" passive intakes and then from the mother cab I have 10 X 1.25" intakes drilled through the shelf that seperates the two rooms. At first I only had 12 X 1.25" passive intakes + (2 X 1.25") passive intakes drilled into the shelf partition and the doors sucked shut on me and the motor strained a bit. From what I understand this is due to the negative pressure inside the cab. Then today I drilled 8 more 1.25" holes into the shelf (a total now of 10 X 1.25") partition leading from the mother room and into the flower room, but now the door no longer sucks shut from the negative pressure inside of the cab. Should I plug some of those holes up that I drilled today in order to get my pressure back? As I said I have a 400 watt HPS coming tomorrow and really need to get these vent issues straightened out ASAP. Any help that you might be able to provide would be greatly appreciated. PUFF PUFF!!

Currently growing Tundra (Autoflower), WhiteBerry, Spoetnik, and Purple Lady

I added 6 more intakes to the bottom after the photo.

Hey BlueHalo, your plants look lovely. I think they will like their new home once we get it fixed up. Who say's you can't grow in a Rubbermaid - eh!

I started looking at the calculations to open up the airflow and I realized that your fan is quite a bit oversized. A bare 400W HPS needs about 120 CFM with a 10*F temperature rise. With a cooltube the airflow can be cut by about 1/3 so 40 CFM is all that's needed.

I was going to tell you to cut a 6.5" square hole in both the shelf and the floor but knowing you have too much fan I wouldn't change anything until you get the lights in.

I'm thinking about the health of the plants. They won't grow well in a wind tunnel so it would be best to install your lights and see how the temps look and drill holes as needed. 40 CFM would give you 1 Air Change Per Minute.

I would install a 8" darkroom vent in the shelf to lightproof both sections and drill holes in the mother floor as needed.