Size of fan?(cfm?)

First off- I live in a A-Frame. Makes things very interesting to say the least I have a growlab 80 that I use as a veg with a 400w in a cool tube. I have made a makeshift room in the top of my a-frame because 2 plants in the tent were more than it could handle. The second room is about 10x8x8...but keep in mind that it's in the shape of an A. I have a fan for the tent(170 cfm), but would like to run just one fan and have it pull air from both rooms. THe other light is a 600w under a hydrofarm radiant hood. What size fan should I use to pull enough air from both rooms, but not have too much negative pressure in the tent? THoughts?

find the cubic footage for both rooms and add them together then divide by 5, that is the total cfm you need to exchange the air 5 times in a min.

Hey hey... Im trying to figure out ventilation for a small cab right now, so maybe we could work together. I put my post in the newbie place, ya bad move.

Anyway, a room thats 10x8x8 is 640 cubic feet, assuming its a perfect triangle cut that in half so its like 320 cubic feet. Dimensions for the growlab 80 are what? Im guessing 2.7x2.7x5.11, so we'll just go with 3x3x6 to make it easier and account for an allowance due to leaks and what not. So the GL 80 has like 50 cubic feet. So to be safe I always round up a little. 350+50=400 cubic feet. That said I would not get a fan under 500cfm, theres gonna be some resistance on ducting and what not. I wouldnt worry about negative pressure, I doubt the make shift room is completely air tight.

On a side note here, what are your temps now? Do you have cooling hoods for the lights? Are you pulling cold air in from somewhere?

Im not all that familiar with A frames, but Im thinking it probably gets hot in that top room. Be very careful with where you choose to expel that hot air. If the man comes around, thats the first thing they look for... hot air coming out.

oOBe RyeOo said:

find the cubic footage for both rooms and add them together then divide by 5, that is the total cfm you need to exchange the air 5 times in a min.

Click to expand...

This formula will find a fan that will exchange the air once every 5 minutes, which you should use as your minimum guideline. You'd be better off going with something a little bigger than you need with a fan controller knob. You can cycle it down in the winter and crank it up in the summer when it gets hot again. I imagine you can cuild up a lot of hot air in the top of an a-frame.

My grow box will be 2.67 x 2. 67 x 4 = 28.5cf If I divide this by 5 I have 5.7. I am planning to run a 400w HPS light and an inline carbon filter on this 160 cfm fan with a passive air intake. Is this over kill? can I put an inline dimmer switch on the fan to regulate its speed? Any drawbacks to that idea? Should I be looking at something else for ventilation?

The dividing by 5 is not correct. Cubic feet per minute, you multiply by 5 if you want to exchange air 5 times a minute. Like freddie said, dividing by 5 will exchange every 5min. To calc proper cfm, you must provide more details. What is your temp now? What size light? Air Cooled hood? Intake CFM? Water cooled bulb?

As a general rule, 3.2*wattage/temp increase= fan size. So 3.2*400/x=160, x=8, so expect an increase in temp of atleast 8 degrees. Assuming you do not have an air cooled hood. To answer your question, 160cfm wont do very much to combat a 400w HPS. If you already have the 160, use it as in intake and get a bigger exhaust fan. Keep in mind, every room needs to be tinkered with because there are just so many variables. Set everything up and see what the temps are before you plant.

By the way, you prob should start a new post when the question your asking doesn't pertain to the original posting member's question. Some people get pissed. Just an FYI for the future.

Thanks Rudy for the response. I read freddie's post, but it did not sink in. I am currently planning my grow room and what I will need to purchase. The 400w HPS light I am looking at is not air cooled. The grow room will be in my basement which gets quite cool during the fall/winter when I will be growing (House is in the NE and is heated with wood with the basement running in the high 40's to low 50's). Planning a 32" x 32" x 48" grow room made of 1/2" plywood. Based on that info should I be looking at a larger fan?

I have a similar set up. Ive got a cab that's roughly 1.5ft by 3ft, 5 ft tall. Im in the middle of switching to 250hps, the LEDS just weren't cutting it. Im def getting an air cooled hood. Right now my temps are at like 74-77. Thats with 2 150 cfm axial fans for intake, and 50 cfm for bathroom fan for exhaust. With the 250hps I'll take out the 50cfm exhaust and add a 6in 250cfm duct fan for the air cooled hood, and will just pull in air from the grow room. I'm only dealing with roughly 25 cubic feet, and you can see all the air I will move through the cab, in hopes on minimizing a temp increase.

Ventilation can be tricky, again there's a lot of variables. No calculation will give you the perfect solution. Sometimes you just have to set everything up and go from there. Getting an air cooled light will help a great deal in controlling temp. Remember, at one point that light will be off for a solid twelve hours. So if thats your only heating source in the winter, you may need to consider something else. Your def going to have to go higher than 160. With temp swings like that, I can only assume that it prob gets hotter than you expect during the summer months as well. This summer has been brutal for backroom growers. In my opinion, 400w is a little overkill for that height, 4ft. Lumens intensify as the light is closer to the tips of the plants. I also suggest you SCROG. Again just an opinion.

Is this soil or hydro? Generally soil handles temp swings much better than hydro. When you screw up in hydro, you know it, fast. Very unforgiving.

I cant stress enough, set everything up and run it for a week or so before you plant.

Here is a chart, someone provided to help me. See how high the Lumens are at 400w range? You going to have either a foot of dirt or water too, so factor that in. You might want to consider 2, 150w's. Also take into consideration lumen's per sq ft.

Cant get the chart to show correctly... just go to this thread and look at it. https://www.rollitup.org/marijuana-plant-problems/361898-plants-taking-forever-finish.html

Rudy Rudiger said:

Here is a chart, someone provided to help me. See how high the Lumens are at 400? You might want to consider 2, 150w's. Also take into consideration lumen's per sq ft.

Click to expand...

What's up with the picture? It's not clickable... Did you attach that or link it?

If you linked it, then you linked us to the thumbnail.

Right now, I am only planning on growing during the Fall/Winter. Planning on a soil grow. I know temp. with lights off might be a problem-thinking about a reptile heat rock from pet store if needed. The 400w is essentially the same price as the 250w HPS. It puts out 50,000 lumens and with 7 sf of grow that is 7,000/ft which seems about right. Still in the planning stages so I could go with the air cooled light. I have read a bit about SOG, but I need to read more. Had been thinking LST. Either way it is down the road. Need to settle on a box design with lighting and ventilation as my seeds will be here in a week and I hope to get going.

Ya 400w will work. SCROG it.

Thanks for the responses all. No worries about jacking my thread THe more the merrier. Not sure how to figure out the actual space because of the A-frame. It does hold heat very well at the top and that is why I'm going down to a 600w instead of my 1K. I was running the 1K with a 260cfm fan and couldn't get temps below high 80's-low 90's. I don't have that much space and that's why I was thinking of trying to run just one fan. I'm thinking of something in the 440cfm range should pull from both. Was only worried about the neg pressure in the tent, as my makeshift room is def. not air tight, ...barely light tight.