Duct sizing question for HVAC gurus

MacroLogos · Jul 23, 2022

RootsRuler said:
https://www.hunker.com/13417142/how-to-find-the-square-inch-of-pipe

inside radius x inside radius x 3.1415

I think where I messed it up is that it's supposed to be radius and not diameter. Always confuse the two. Macro caught that one.

"A radius of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length."

Since radius is a line from the center of the circle to the perimeter or edge that would mean the radius of an 8" port would be 4". Using the formula it should be 4" x 4" x 3.1415 = 50.264 sq/in. For a 6" port it would be 3" x 3" x 3.1415 = 28.2735

Each port vent is 72 sq/in. Now that I have the correct numbers that would mean each 8" port is a little less than 2/3 the sq/in of the vent flaps. All 3 vent flaps open would provide 113.094 sq/in of vent capacity. My tent has two 8" and two 6" round ports I can use for input air. Opening them all would give me 157.075 sq/in of vent capacity.

To approximate the same sq/in vent capacity of the tent vents I would have to keep both 6" ports and an 8" port open. This would be for a passive intake system.

I'm trying not to use a booster fan for the inlet but it looks like I may need to to get enough air in the tent.

I have a 6" pole mounted fan and an 8" Honeywell turbo force fan I hung off the roof inside the tent for air circulation. The 6" works fine as an air circulator but I think the 8" would work better as a booster. I'm going to have to rig up something to get the duct to fit around the back of the 8" fan. I guess I can set it on the floor with the duct attached to the rear of the fan and use it as the under canopy fan so that fresh air comes in from the bottom and pulls through the tent up to the fan mounted on the ceiling. Problem is there isn't much room on the floor as I plan on running six 7 gal cloth pots under a screen so there won't be much floor space to set things down on. Honeywell site says the fan can do 185 cfm at high setting so the fan should be more than sufficient at getting air in the tent. I'll need to tweek the speed of the fan to get the slight negative pressure I'm trying to achieve in the tent.

You also will need to close those flaps (or duct them to block light), when flowering, or the light will ruin your grow...

We run 2x 6" AC on a 4x4x7 tent (intake & vent).. @ 20% power, they move plenty of air to meet circulation needs, but we run them at higher speeds for heat/humidity control.

On the tent intake fan, I am running an evaporative cooler I engineered out of a carbon filter (and it must be run on the intakes port of the fan, unfortunately). That said, the coolpad has a much better air flow than carbon, so I don't take as big a cfm hit... and I am willing to make this Faustian bargain to get some "ice" from "fire" (I've gotten 37° temp drops from ambient with it this summer, 3° short of the 40° threshold -- my pump/drip system needs fine tuning).

On the tent vent port, we just duct the exhaust into a 2x4x7 sprouting/veg tent... which also has an additional 6" hooked to a second evaporative cooler I designed.

RootsRuler · Jul 23, 2022

That's the whole idea to my posts. Because I'm going to need to close those bottom flaps I need an alternative input air port that doesn't allow light in to bring in enough air to make the exchanges necessary to keep the tent within spec. As long as I run a length of duct from the port opening it should kill enough light to be acceptable.

Trying to do this with the least amount of gear needed. The tent is located in my living room with the A/C set at 73* so I have a fair measure of control over the temp. My issue will be humidity but that can be alleviated with a dehu. Until now I've used the bottom flaps as intake and they've worked as intended but I need to get ahead of this issue and be proactive rather than wait until it's time and have no real solution.

I think using the Honeywell fan will be the best bet as it can double as underside canopy air recirculation as well as fresh air intake. I may just run the fan from the 6" port and connect the duct to the back of the fan leaving the outer couple inches of the fan to recirculate the inner tent air. I get that this will lower CFM of the intake air but for now I'm just looking to supply just enough air to the tent to keep it under slight negative pressure. I'll have to test and see if it's enough or if I'll have to go with the 8" duct idea.

MacroLogos · Jul 23, 2022

RootsRuler said:
That's the whole idea to my posts. Because I'm going to need to close those bottom flaps I need an alternative input air port that doesn't allow light in to bring in enough air to make the exchanges necessary to keep the tent within spec. As long as I run a length of duct from the port opening it should kill enough light to be acceptable.

Trying to do this with the least amount of gear needed. The tent is located in my living room with the A/C set at 73* so I have a fair measure of control over the temp. My issue will be humidity but that can be alleviated with a dehu. Until now I've used the bottom flaps as intake and they've worked as intended but I need to get ahead of this issue and be proactive rather than wait until it's time and have no real solution.

I think using the Honeywell fan will be the best bet as it can double as underside canopy air recirculation as well as fresh air intake. I may just run the fan from the 6" port and connect the duct to the back of the fan leaving the outer couple inches of the fan to recirculate the inner tent air. I get that this will lower CFM of the intake air but for now I'm just looking to supply just enough air to the tent to keep it under slight negative pressure. I'll have to test and see if it's enough or if I'll have to go with the 8" duct idea.

Sounds like a plan, and you can always adjust it later now that you have some deeper insight into the collection of principles at play.

Your current equipment is totally sufficient to do what you want (with the exception of fine tuned humidity control).

If I want to open a passive vent for air flow, I'll put 4'-6' of flex duct into the vent port, and spiral it on the outside to block light. There's other approaches that work too.

MedicalHydroponics · Jul 23, 2022

Using the equipment efficiently and correctly is key. And mixing home hvac with growing hvac totally different like light leaks or duct blow outs from heat. Using ideal air duct like this is best

In growing cannabis every Watt adds up $$$$ so you want to use bare min needed.
Using timers( If not sealed room or using for air cool lights, don’t run more than you have to), dampers(Don’t need air coming in or escaping), speed controllers, (80% or less saves noises and $$$) sealed room scrubbing is best.
Flex duct is fine like I posted that is designed for growing and keeps out light.

If you use a 90° bend or 45° bend it’s best to use hard pipe for better air flow especially on longer runs. How many feet of duct matters.

Air exchanges don’t need but a few minutes per hour especially for a tent. Wasting $$$ running longer than that especially using High volume vortex fans.

If using air exchanges use hepa filters cut to fit cheapest way. Keeps mold spores out and bugs.

Duct sizing question for HVAC gurus

Duct sizing question for HVAC gurus

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