ventilation: do ducts have to be proportional?

[Marshall]

Building a duct out of OSB. Fan is a 10" can fan, so surface area is 78.5". I want to build the box 4" x 20" It will only be 4" deep, and 20" high. It will have a 6" take off. So the air will hit the back side of the duct pretty quick.

Wondering if the ducts need to be more proportional?

 

[ak-51]

If I'm imagining this right (which is questionable) then it sounds ok. I think a drawing or diagram might help here.

 

[OrganicOzarks]

Building a duct out of OSB. Fan is a 10" can fan, so surface area is 78.5". I want to build the box 4" x 20" It will only be 4" deep, and 20" high. It will have a 6" take off. So the air will hit the back side of the duct pretty quick.

Wondering if the ducts need to be more proportional?

I was just transported back to High School where I was always to high to really understand what my math teacher was talking about. Good to see not much has changed.

 

[Marshall]

well I have always heard you want to keep the surface area the same as your fan. Basically you want the opening the same size as your fan. Since the fan is round, you have to calculate the surface area which in this case is 78.5 square inches. Now a rectangular duct should measure 78.5 inches. 4x20 is 80 inches.

hope that makes sense.

imagine a long rectangular duct that is mounted in the corner of a ceiling/wall. It is around 8 feet long, 4" off the wall, and 20" down from the ceiling. The 6" ports will be mounted on the 20" side.

The issue is how much resistance will there be when the air slams into the back of the duct so quickly

 

[Pazuzu]

on a side note ..intake should roughly be two times the size of the exhaust.

 

[Oldmac]

Building a duct out of OSB. Fan is a 10" can fan, so surface area is 78.5". I want to build the box 4" x 20" It will only be 4" deep, and 20" high. It will have a 6" take off. So the air will hit the back side of the duct pretty quick.

Wondering if the ducts need to be more proportional?

I believe you are describing an inlet 10" fan and duct work for that, since you say it will have a 6" "take off".

It will develope positive pressure (in the duct) not because of the speed of the air hitting the back side of the duct, but the 6" dia take off has 28 inches of surface area. Plus the variable you did not give, the CFM of the fan.

Possibly of a better answer exists, with more info like clearly stated use, etc. Or like ak-51 said a diagram (with flow direction arrows).

ON A SIDE NOTE; Pazuzu...you are wrong unless you don't believe in the law of physics or they don't apply to you. If you had for an example a 8" dia exhaust fan/hole and you use even as small as a 4" dia passive inlet, the airflow will just speed up thru the small inlet...it's called the "venturi effect". You'd also get some neg pressure built up in this extreme example, but air in will try to equal air out.

OM

 

[messn'n'gommin']

Building a duct out of OSB. Fan is a 10" can fan, so surface area is 78.5". I want to build the box 4" x 20" It will only be 4" deep, and 20" high. It will have a 6" take off. So the air will hit the back side of the duct pretty quick.

Wondering if the ducts need to be more proportional?

The math sounds right. How many 6" take-offs? Three 6" holes is about 83.75 square inches. But, you will probably have as much, if not more, resistance in the length of ducting than what is being caused by a 90 degree turn. If possible, and you really wanted to, you could run 8" ducting from your lights to 8" ports in the duct manifold and 8" (or even better, a 10") port and duct to the fan. The fan doesn't have to work so hard, resistance goes down and, to a certain extent, so does noise.

 

[Marshall]

its a manifold design, intake from the crawl space, exhaust to the attic. 2 rows of lights. Intake and exhaust are same size

didnt really think of the 6" ports decreasing the air flow. But it does, 2 6" ports is about 56 inches.

I am going 8" x12". That accounts for the 2x2's inside that will hold the box together, etc


I want to build it out of duct board but have no way to get duct board. The only place I know of sells it in 12' sheets and I dont have a truck. I am thinking of renting a truck just to get 2 sheets of duct board so i dont have to build one out of plywood.

sigh

 

[messn'n'gommin']

Cut it where you buy it. A razor knife and a 2x4 for a straight edge.

 

[Oldmac]

its a manifold design, intake from the crawl space, exhaust to the attic. 2 rows of lights. Intake and exhaust are same size

didnt really think of the 6" ports decreasing the air flow. But it does, 2 6" ports is about 56 inches.

I am going 8" x12". That accounts for the 2x2's inside that will hold the box together, etc


I want to build it out of duct board but have no way to get duct board. The only place I know of sells it in 12' sheets and I dont have a truck. I am thinking of renting a truck just to get 2 sheets of duct board so i dont have to build one out of plywood.

sigh

Thanks for providing the diagram Marshall, it makes it easier to understand what you are doing.

This should work great for you, with the 10" exhaust fan located in that 10" duct, drawing air thru the system. While the 6" sections are slightly restritive, the air flow out will equal the air flow in. The restrictions are equal for both pair of light's air circuts and they should run equal, but check that one pair of lights is as cool as the other.

Many times when running parrellel like this, one side may draw more flow then the other (side closest to fan usually) and the simple answer to that is to add a little more restriction to that side.

With the large manifolds on each side, it should not be a problem.

As messen'n'gommin said, problem with length, just cut it when you buy it.

OM