Ventilation help required! Ducting

G

GP Rust

Member
Hello all

I am aware of the negative aspects behind bending ventillation ducting, but I need help.

I am setting up a centrifugal inline fan for my grow tent. The fan will be positioned directly outside the tent and will have minimal bending (basically none) between the tent exhaust port and the fan.

My problem comes in after the fan has sucked in air and is blowing it out. I need to blow the air into the attic and down into the room next door - then straight out a window. I don't think venting into the attic is too smart from what ive heard and I cant vent the grow tent in the same room.

My question is: do bends in ducting matter after the fan has already sucked the air in? Ie for the exhaust end. It will be moving about 3metres with bends (in the attic) before exiting out a window. Is this a problem?

I am really hoping to get some solid advise.

I coud do a secret vent out the windows in the grow room - but I dont want to do this if I can avoid it, because the windows are facing a "busy" part of my appartment complex.
 
LEDandCoffee

LEDandCoffee

Well-Known Member
It will in an essence, yes. It's still creating more work for the fan to push air. I just throw my carbon filter on top of my tent. I don't worry too much about where it's getting vented, as it's getting scrubbed regardless.
 
Z

Zero_OS

Well-Known Member
Bends on either side of the fan will reduce its ability to convey air (bends on the intake restrict the draw, bends on the exhaust restrict the push). The smoother the bend, the better, so sheet metal duct bends are better than bends in flexible ducting. Larger radius bends are better than sharp bends, especially with flex. Flex ducting also has those ribs, which disrupts laminar flow, so shorter lengths are preferable. That being said, 3 meters is not that long...set it up, and test it. If it needs a boost, use a cheap inline plugged into the same controller.
 
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ANC

ANC

Well-Known Member
You never reach the numbers on the box anyway. Those CFM ratings are for perfectly smooth, straight, round ducting.
 
2011rex87

2011rex87

Member
My .02 is that everyones answers so far are exactly right. I want to add that you could use adjustable variable power supplies on the intake and exhaust fans to dial in your system. For example, you may want to maintain negative air pressure to ensure that anything leaving your grow space is forced out through the scrubber. This cuts down on smell. However if you are worried about contamination from outside, you may need to scrub the intake as well or even adjust the fans for positive air pressure. Either way, I think if you use those bends in your HVAC, you will be just fine - so long as you have the CFM to overcome the bends and maintain a good air exchange rate. I think most growers try to exchange all the air in their space at least once or even twice a minute. I personally exchange air much more than this. Keep an eye on humidity and temps and you will figure out what a good exchange rate is for your space. You should be 100% good to go as long as you maintain the exchange rate and the negative or positive pressure that you want.

If you do have to vent out that window... privacy is a concern but you can actually vent through a pretty short slit as long as the opening is wide. If your window is 24 inches wide, then you can get away with opening it only 1" or 2" and you can build a custom box or stealthy setup around the slit to make it impossible to see inside.
 
Z

Zero_OS

Well-Known Member
The problem with tents is that if your CFMs are too high, the side suck in too much unless you have side bars, so maybe you want to use flex tubing to lower the effective CFM of your fan and minimize negative pressure in your tent.

A note about fan speed controllers...those cheap $15-20 Pulse Width Modulation variable speed controllers suck when used to control fan speeds. At anything less than 50%, they usually induce a noticeable growl in the fan motor due the pulses battling inertia. You can actually hear the strain in the motor. It is better and far quieter to use a variable transformer (variac), which outputs a smooth sinewave, and does not induce growl in the fan motor.
 
G

GP Rust

Member
Thankz for the reply guys. My tents is 4x4 feet and my fan is about 250cmh on medium setting. (Sorry i dont know cfm)
 
2011rex87

2011rex87

Member
Cubic Meters per Hour? Your Tent is how tall? WE can do the math for you but we want to know the cubic meters or cubic feet of your tent and without a height I can only do square feet or square meters.
 
Z

Zero_OS

Well-Known Member
250 cmh is about 8830 cfh. At 4x4x7, vol=112cf. At your specified medium fan setting/capacity, you'll get 79 airchanges every hour, which is more than adequate. If your ducting and filter reduce capacity by 30%, then you'll still get 55 changes per hour. Your setup should be fine from an air change standpoint.
 
G

GP Rust

Member
Zero_OS said:
250 cmh is about 8830 cfh. At 4x4x7, vol=112cf. At your specified medium fan setting/capacity, you'll get 79 airchanges every hour, which is more than adequate. If your ducting and filter reduce capacity by 30%, then you'll still get 55 changes per hour. Your setup should be fine from an air change standpoint.
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Thanks bro. Sorry for my inadequate information, but you geussed well - it is roughly 7 feet (is 2m).

So do you account for 30% over all bends? I dont think i will have a complete 90 degree, but a lot bit in the attic. Basically 'n' shape. Up from the one room down in to the other. With slight bends at each entry point as well.

Thanks again.

2011rex87 said:
Cubic Meters per Hour? Your Tent is how tall? WE can do the math for you but we want to know the cubic meters or cubic feet of your tent and without a height I can only do square feet or square meters.
Click to expand...
Sorry bro. I was in a rush on the way out and forgot that I don't live in a 2dimensional world :wall:bongsmilie

It is 2m / 6.6feet. 4x4x7 roughly is what zero used and im happy to use that figure (rather overestimate).
 
G

GP Rust

Member
Anyone know if it will be better for me to use 6' PVC pipe instead of ducting (in the attic / on straight sections

2011rex87 said:
My .02 is that everyones answers so far are exactly right. I want to add that you could use adjustable variable power supplies on the intake and exhaust fans to dial in your system. For example, you may want to maintain negative air pressure to ensure that anything leaving your grow space is forced out through the scrubber. This cuts down on smell. However if you are worried about contamination from outside, you may need to scrub the intake as well or even adjust the fans for positive air pressure. Either way, I think if you use those bends in your HVAC, you will be just fine - so long as you have the CFM to overcome the bends and maintain a good air exchange rate. I think most growers try to exchange all the air in their space at least once or even twice a minute. I personally exchange air much more than this. Keep an eye on humidity and temps and you will figure out what a good exchange rate is for your space. You should be 100% good to go as long as you maintain the exchange rate and the negative or positive pressure that you want.

If you do have to vent out that window... privacy is a concern but you can actually vent through a pretty short slit as long as the opening is wide. If your window is 24 inches wide, then you can get away with opening it only 1" or 2" and you can build a custom box or stealthy setup around the slit to make it impossible to see inside.
Click to expand...
The windows in the flats are all the same and may not be tampered with - so im hoping to do something like you suggested . Its about 30inch tall and can open about 5inch wide.
 
Z

Zero_OS

Well-Known Member
GP Rust said:
...So do you account for 30% over all bends? I don't think i will have a complete 90 degree, but a lot bit in the attic.
Click to expand...
Honestly, I pulled 30% from my arse, I have no idea whether 30% would represent a good reduction percent from the bends, curves and flex length once you get it set up. But even it it was 50%, that still would be close to 40 changes per hour on your medium setting, which I imagine would be fine. If you are finding slow growth and think increasing your airflow would help, click the setting to high instead of medium. Or better yet, hook the fan to a variac and give yourself greater variation in your flow.
 
gjs4786

gjs4786

Well-Known Member
This comes up a lot I've noticed. I also have questions regarding ventilation setup preference. But from a HVAC point of view, and not grow light / filtration POV, just add the exact same fan you have now moving the air, after a few of the bends. This will relieve the "back pressure" that the first fan has from the bends. The more bends and obstacles, the more the air "pushes back" on the fan. If you had a fan at the right point in the ducting, this relieves that "push back" effect on the first fan, allowing the air to move much more freely. If you don't mind buying an extra fan, that would be the quickest/simplest solution. if you're trying to find a workaround that entails you avoiding buying another fan, I can't help you much there :)

Here's an analogy if you know anything about setting up wireless networks. If you want to extend your wifi range, buy a range extender. If you're trying to do it with empty beer cans that are cut into the shape of antennas, your mileage will vary. :) Myself included, I have found that a lot of questions like this are looking for a free workaround without-really-saying-it-like-that.
 
G

GP Rust

Member
gjs4786 said:
This comes up a lot I've noticed. I also have questions regarding ventilation setup preference. But from a HVAC point of view, and not grow light / filtration POV, just add the exact same fan you have now moving the air, after a few of the bends. This will relieve the "back pressure" that the first fan has from the bends. The more bends and obstacles, the more the air "pushes back" on the fan. If you had a fan at the right point in the ducting, this relieves that "push back" effect on the first fan, allowing the air to move much more freely. If you don't mind buying an extra fan, that would be the quickest/simplest solution. if you're trying to find a workaround that entails you avoiding buying another fan, I can't help you much there :)

Here's an analogy if you know anything about setting up wireless networks. If you want to extend your wifi range, buy a range extender. If you're trying to do it with empty beer cans that are cut into the shape of antennas, your mileage will vary. :) Myself included, I have found that a lot of questions like this are looking for a free workaround without-really-saying-it-like-that.
Click to expand...
Thanks! Not necessarily free - I just want options. I think im gonna try just putting the exhaust straight out the door and hoping the cross draft in my passage takes care of the heat.

I have an AC that I am using and my temps are at 27celcium
 
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