# Collapsing Ducting



## greenmentat (Aug 5, 2011)

I'm running two 8" valulines to two banks of three reflectors then to a Y connector and then to a light trap made of 6" drainage pipe (corrugated, black) .... Everything is fine until I get to the Y connector and then the ducting collapses.

A potentially confusing point is that the ducting in the pictures I am showing is black- but, it's the same as regular hydrofarm 8" ducting (which I tried first but it still collapsed) except it's the insulated kind, I've just removed the insulation.

The problem here is that the collapse is restricting air flow _and_ pulling my lights out of alignment

The only thing I can think of is that it's collapsing because I'm trying to make a angle with it...... anyone have any experience with this?


----------



## 4u2sm0ke (Aug 5, 2011)

why do you have the ridged pipe in the middle of the lights?..That run should be continious  jsut my thaughts


take care and be safe:bolt::bong:


----------



## ganjaguitar11 (Aug 5, 2011)

Thanks for the reply  the rigid pipe is at the very end and forms two 90 degree elbows to make the light trap

To make the rigid pipe form two 90 degree elbows I used steel wire

From the light trap I have a y connector, then some standard 8" ducting (shown in the pics), two rows of three lights connected by ducting, and some more ducting connected to two 8" valuline fans each pulls around 750 cfm

I hope this explanation brought some clarity

I hope the fact that I'm posting as gangaguitar doesn't throw anyone., my computer is down for a while and this is a username I created on my iPhone which I will log out of and never use again as soon as I get my comp working again


----------



## 4u2sm0ke (Aug 6, 2011)

:stoned:

threw me for a loop


----------



## The Hemp Goddess (Aug 6, 2011)

Can we get pics of your whole ventilation setup?  I am not following your written description.


----------



## terky (Aug 6, 2011)

its collapsing because your putting negative pressure through it.

get a speed controller and turn the fan down.

I have found that 100 cfm cools a 600W light. 8" duct can move about 150 cfm without collapsing too bad.

I would run each of those ducts from your hoods, independently, to at least a 10" duct.


----------



## dman1234 (Aug 6, 2011)

Its got to be crappy ducting, i couldnt make mine collapse unless i stood on it.


----------



## The Hemp Goddess (Aug 6, 2011)

I'm kind of with Dman here.  My ducting, which is insulated, was expensive, but it is quiet and really really tough.

You want negative pressure through your ducting.  I use a 6" 448 to cool 2 600W.  There is no way I could personally begin to cool both my 600s with a 100 cfm fan.


----------



## Growdude (Aug 6, 2011)

It sounds like there is a restriction to the air flow, this is putting to much negative pressure on the duct.

Make sure the intake side is free flowing.


----------



## stevetberry (Aug 6, 2011)

From your pictures it does not look like the ducting is collapsing all the way.  Have you tried loosening the clamp and turning the ducting a little because the ducting should try to to collapse length wise but not from side to side unless it is twisted.  Hope that made sense!!!


----------



## ganjaguitar11 (Aug 6, 2011)

Steve, thanks for the tip.  THG, Dman; do you have elbows in your ducting? I have a lot in mine I think the elbows might be what are causing the collapse because theres more pull on the inside of the ducting at the elbows via friction.  Well, I'm going to give it another go and I'll update later.


----------



## terky (Aug 7, 2011)

You are just simply trying to pull too much air through that 8" duct. I did a google search for "8 inch valueline". The first one that came up is 745 cfm. I suppose you have that one.

You need at least a 16" duct to move that much air without any negative pressure.

The cheapest thing you can do is get a speed controller and slow the fan down.

Or build your exhaust system something like this.

http://www.marijuanapassion.com/forum/attachment.php?attachmentid=173234&d=1311948611

Also, the rating that they give fans is when its not connected to duct. If you connect that fan to an 8" duct it would maybe only move 250 cfm. The 8" duct would cause enormous friction loss and your static pressure in the duct would be very negative. This would of course collapse the thin flex duct which is basically a slinky.

Your duct work needs to step up to 16" Immediately after the fan and then tap off to the 8" flex duct.

I would only use one of those fans. Then I would put the fan within feet of at least a 12" outlet to outside.


----------



## dman1234 (Aug 7, 2011)

ganjaguitar11 said:
			
		

> Steve, thanks for the tip. THG, Dman; do you have elbows in your ducting? I have a lot in mine I think the elbows might be what are causing the collapse because theres more pull on the inside of the ducting at the elbows via friction. Well, I'm going to give it another go and I'll update later.


 
i have 3 elbows, 

1 where the duct enters the 1st cooltube.
1 where the duct leaves the 2nd cooltube.
1 where the duct leaves the room to meet the fan outside the room.


----------



## ganjaguitar11 (Aug 8, 2011)

I am happy to announce that the mystery of the collapsing duct has been solved.  Through intense study and a great deal of sluething I discovered that hydrofarm ducting is crap and will collapse under the slightest strain and thermoflow ducting will retain it's integrity even if you run a 750cfm fan with all kinds of twists and elbows.., and peace is restored to the land...


----------



## The Hemp Goddess (Aug 9, 2011)

Glad you got it figured out, but as a side note, elbows slow down the flow of air--you would not have more pull at the elbows.


----------



## NorCalHal (Aug 9, 2011)

terky said:
			
		

> You are just simply trying to pull too much air through that 8" duct. I did a google search for "8 inch valueline". The first one that came up is 745 cfm. I suppose you have that one.
> 
> You need at least a 16" duct to move that much air without any negative pressure.
> 
> ...


 

Completely false. It's the ducting...not the fan airflow. I rock 10" can max fans with a cfm of 1000 thru 8" duct going thru my lights. no issues...ever.


----------



## Chewbongo (Aug 9, 2011)

Out of curiosity why r u pulling the air instead of pushing it through?


----------



## The Hemp Goddess (Aug 9, 2011)

ChewbacaKz said:
			
		

> Out of curiosity why r u pulling the air instead of pushing it through?



They work more efficiently that way and it creates negative pressure which you wangt as it contains the odors.


----------



## The Hemp Goddess (Aug 9, 2011)

terky said:
			
		

> You are just simply trying to pull too much air through that 8" duct. I did a google search for "8 inch valueline". The first one that came up is 745 cfm. I suppose you have that one.
> 
> You need at least a 16" duct to move that much air without any negative pressure.
> 
> ...



I'm with NorCalHal--I also disagree.  And,  why a 16" duct--a 16" duct is 4 times as large as a 8" duct.


----------



## terky (Aug 11, 2011)

The Hemp Goddess said:
			
		

> I'm with NorCalHal--I also disagree.  And,  why a 16" duct--a 16" duct is 4 times as large as a 8" duct.



I install commercial HVAC systems for a living.

You need 16" round duct for that fan to move 750 CFM. Thats just how it works.

If you use smaller duct than that, more friction will be caused, negative pressure will increase (which does not necessarily mean that CFM increases), and your fans efficiency will drop. This is called "system effect". This is how you size duct.

If you want that fan to actually move 750 CFM, it needs to be close to the outlet. You should have 24" of straight pipe before and after the fan. The outlet should be 16" duct. The inlet side should start at 16" and you can step it down 2" after every two 8" branch runs to your hoods. Ideally you would install dampers and have access to an air balance tool so you know exactly how much air moves across your lights, like I have done to mine.

You will have no problems if you take my advice.

EDIT: Norcal, did you measure the airflow?


----------



## BlueNose (Aug 11, 2011)

you may need a 16" duct to reach the fans peak performance but that's not what the OP is trying to do. Better piping that doesn't cave like a paper bag is the right answer.


----------



## terky (Aug 12, 2011)

Yes your right. I buy my duct from the supply house, not the hydro shop. I wouldn't know about the crapy duct.


----------

