Will increasing the exhaust port size allow my inline fan to draw more air?

R

Relic79

Well-Known Member
Good day! Here goes the first post!

I have a 4x8x9 foot grow area in a 8x8x9 room. It has two 8"x8" square passive intakes, and a 402cfm 6" AC Infinity inline fan for exhaust.

The exhaust ducting is 6" rigid ducting until the last 4' where it goes out of my house through the existing 4" rigid ducting that was there for the old laundry room.

When my door is closed and the fan has to pull air through the two 8"x8" passive intakes and push it out through the 6" to 4"
reduced exhaust, my room gets about 1 to 2 degrees too hot. If I open the door so the fan can pull air in through the door, the temp drops to the ideal temp range.

Based on the door being open fixing the problem, it seems like I have an intake size/restriction problem more than an exhaust size problem, however I can't cut any more intakes or enlarge the ones I have, but obviously do need to move more air through the room.

I want to enlarge the exhaust port to use 6" all the way out and remove the reducer, which I hope would allow the fan to pull harder, but am concerned this won't help the fan pull more air when the door is closed.

If I increase my exhaust port from 4" to 6" can I expect my fan to pull more air through the existing intakes without enlarging them? Can I expect more air flow? I worry that enlarging the exhaust will simply lower the speed of the air coming out of the exhaust but will have no net effect on air flow through the room.

I know I could just try it, but enlarging an existing hole is a pain, and will need me to cut vinyl siding as well, so getting it right is a one shot deal so I want to make sure it will be worth it.

Any HVAC people out there that can help with the calculations on this?

Thanks!
 
Nizza

Nizza

Well-Known Member
Yes the reduction to 4” was impeding your air flow a lot, you will be surprised the difference it’s going to make.
Perhaps the best plan is adding a whole new 6in exhaust port if possible maybe in a closer spot to the fan . You can then have the 4” hole capped from the inside and only use it as a fresh air inlet if you need it to solve your issue
Remember you want around 2-3 feet of straight duct before making any 90’s if possible, and to make offsets it is better to offset with two 45s instead of two 90s. It’s great you are using hard duct for velocity
 
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Renfro

Renfro

Well-Known Member
Passive intake area should typically have 2 - 2.5x the area of the active exhaust. Your two 64 square inch intakes are more than adequate for a single 6 inch exhaust. The big restriction is indeed the 4 inch wall vent. Going up in size will help tremendously A 4 inch hole has 16 sqin and a 6 inch has 36 sqin! Makes a BIG difference..
 
cannabineer

cannabineer

Ursus marijanus
Renfro said:
Passive intake area should typically have 2 - 2.5x the area of the active exhaust. Your two 64 square inch intakes are more than adequate for a single 6 inch exhaust. The big restriction is indeed the 4 inch wall vent. Going up in size will help tremendously A 4 inch hole has 16 sqin and a 6 inch has 36 sqin! Makes a BIG difference..
Click to expand...
For circular duct, you need to multiply those numbers by π/4 which is about 0.78.

The choke point has an area of 12.6 square inches. Opening that to 6 inches should have a large improving effect ...
 
Nizza

Nizza

Well-Known Member
Area of a circle=3.14*diameter
When it comes to moving air though in hvac, my rule of thumb is a 4" duct typically moves a little less than half the air of a 6" duct.

If you really want to get into calculations try out a ductulator , but you have to know alot of things to use a ductulator correctly

The OP is concerned by increasing size it will only make the noise quieter and still deliver the same CFM, which is possible.. but I think that it will make a huge improvement and will let the fan run better because it is being restricted from it reducing at the end.

To me it makes more sense to add just a new 6" hood and leave the 4" so if he does need more fresh air at some point it will be easy
 
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R

Relic79

Well-Known Member
Thanks to everyone!

It sounds like I should see improvement but to clarify, my largest concern was that when I open the door to the room effectively making the intake something equivalent to 12sqft of open space, the inline fan seems to move enough air to cool the room through the existing 4" duct.

I was therefore concerned that even if I increase the exhaust size, the air flow won't increase unless I also increase the intake since opening and closing the door (adjusting the intake) alone allowed me to hit my target.

Now that I think it out, if I was trying to blow through a straw with one end smaller then the other, I suppose it doesn't matter which end I blow through, it will be equally difficult. Is that sane logic when it comes to air flow?
 
R

Relic79

Well-Known Member
Renfro said:
Hmmm....
Click to expand...
Not sure if this is what you were questioning but...

I have 2 - 8"x8" square holes in my wall (no ducting, just holes which installed dark room louvers into): https://www.adorama.com/dkl8.html <- light tight louvres

The air is pulled through them from my open unfinished basement sits around 67 to 68 degrees. It then goes out through my AC infinity 6" which is directly coupled to 6" rigid ducting that goes out through the roof and then through about 15' of rigid ducting to where it is reduced and sent outside through a 4" dryer vent.

I really should read up how to share pictures and things safely so I can post a pic without any personal/location info. It would make it way easier to explain!
 
Renfro

Renfro

Well-Known Member
Relic79 said:
Not sure if this is what you were questioning but...

I have 2 - 8"x8" square holes in my wall (no ducting, just holes which installed dark room louvers into): https://www.adorama.com/dkl8.html <- light tight louvres

The air is pulled through them from my open unfinished basement sits around 67 to 68 degrees. It then goes out through my AC infinity 6" which is directly coupled to 6" rigid ducting that goes out through the roof and then through about 15' of rigid ducting to where it is reduced and sent outside through a 4" dryer vent.

I really should read up how to share pictures and things safely so I can post a pic without any personal/location info. It would make it way easier to explain!
Click to expand...
I was questioning cannabineer
 
Renfro

Renfro

Well-Known Member
Relic79 said:
I really should read up how to share pictures and things safely so I can post a pic without any personal/location info. It would make it way easier to explain!
Click to expand...
Dude, it's easy.

Take the images on your PC, right click, properties, details tab, click "remove properties and personal information"

They make apps that you can drag and drop a bunch of images on but this works fine for most.
 
PadawanWarrior

PadawanWarrior

Well-Known Member
Nizza said:
Area of a circle=3.14*diameter
When it comes to moving air though in hvac, my rule of thumb is a 4" duct typically moves a little less than half the air of a 6" duct.

If you really want to get into calculations try out a ductulator , but you have to know alot of things to use a ductulator correctly

The OP is concerned by increasing size it will only make the noise quieter and still deliver the same CFM, which is possible.. but I think that it will make a huge improvement and will let the fan run better because it is being restricted from it reducing at the end.

To me it makes more sense to add just a new 6" hood and leave the 4" so if he does need more fresh air at some point it will be easy
Click to expand...
The area of a circle is not ~3.14 unless the the radius is 1.

Pi = the circumference of a circle divided by the diameter.
 
R

Relic79

Well-Known Member
That is correct @cannabineer, DIY room, not ducting for intake, just square holes on an interior wall. I knew pi*(r^2) = area of a circle, and that my exhaust was overly restricted.

My confusion comes from the fact that opening the door does the trick, so in my mind this means the 4" exhaust is small but adequate to vent the room, and the intakes are causing the problem. If the exhaust was the bottle neck, I would think that opening the door would have made little difference since it still had to push the air out the 4" exhaust either way.

I also know air moving through a duct is more complex, and didn't want to assume my simplified logic was correct.

At the end of the day, I should have a properly sized exhaust, and if it still isn't cool enough, my next step is to put the glass back in my hoods and get the air-cooled hoods going. I know this would absolutely fix the problem, but I wanted to try to get it running without the extra fans, and glass, noise, ducting, and holes in the wall.


Exhaust.jpg
RightHandSide.jpg
Room.jpg
 
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