how to calculate the proper size fan JUST to cool my lights

Hey everyone,

Ok, I have been searching the forum, and Google, and I can't find an answer that seems solid. No offense to anyone who has posted a solid answer, I probably didn't find it or got confused about if they were talking the same kind of setup as mine.

So, here goes.

I will be using 2 fans and duct systems. one is all figured out. That one is the one with the scrubber that goes outside. All set on that one.


The other one will be JUST to cool my lights. Pic of just this system as it will be:



Details you may not care about:
It will be the setup where this ducting system will pull from one room in the living space of my dwelling, and also feed back into the living space, at least when it's colder outside. When it's warmer, I will feed the air out somewhere else. Right now, it will be doing 2 1000W hoods. HPS if it matters. Down the line, it will be those 2 hoods and also another 400W which will be MH sometimes and HPS others. Flanges on the hoods are 6 inch.

Ok, the big question: How big of a fan do I need?

I am holding out until I have saved enough for an S&P, but they are pricey.

While I am at it, has anyone ever had a fan in a system like this fail, or the lights in the ducts explode or anything? Does anything really dangerous come out of the vented hoods and then go into the house if the fan fails or you get lamp explosion or meltdown or something? I am trying to find out if there is any danger of something malfunctioning, and killing my cats...or even me while I sleep or anything. Safety first, right?

Respect, gratitude, and rep to those who have experience and/or solid knowledge relating to this. Thanks a lot in advance.

Hold on, respect to all, man. WTF was I thinking?

No love at all. Damn.

Well bro I run 2 600watt hps on cooltubes and use a 700ish cfm 8" fan to cool them both. I can touch the lens its very kewl except where it next to the bulbs there some heat but not enough to burn me. I run 8inch ducting to Y splitters to 6 inch ducting and back to Y splitter to 8 inch ducting. I would probably run the two 1000w on a 720cfm 8 inch fan with all 8 inch ducting. I would run the MH on sepearate 400CFM fan. I would vent outside air or use a room as a lung to keep thing cooler. That would give you optimal cooling.
Good Luck
https://www.rollitup.org/grow-journals/384835-swiftgrow-journal.html

cool, thanks man!

It sounds like your adding more lights down the road is that correct? Do you want one fan for the first 3 lights or one fan for first 3 than second 3?

Anyhow this is what I would do, for the picture you did I would use a 6", yes a good S&P. I say this as I run 4 6" cool tubes using a 6" fan year round, tempertaures go from -40 to 100 and I have no issues, I have 2 90's coming in and 2 going out. I also run 4 8" C T on an 8" fan set up the same as you.

Not sure what your hunting for but this may help, each 90 reduces flow by 20% and flex line it depends on the length (don't use it if you can help it) if your looking for details like that google HVAC

The design of the ductworks in ventilation systems are often done by using the

• Velocity Method
• Constant Pressure Loss Method (or Equal Friction Method)
• Static Pressure Recovery Method

The Velocity Method

Proper air flow velocities for the application considering the environment are selected. Sizes of ducts are then given by the continuity equation like:
Alternatively in Imperial units
A proper velocity will depend on the application and the environment. The table below indicate commonly used velocity limits:
Type of Duct Comfort Systems Industrial Systems High Speed Systems m/s ftm m/s ftm m/s ftm Main ducts 4 - 7 780 - 1380 8 - 12 1575 - 2360 10 - 18 1670 - 3540 Main branch ducts 3 - 5 590 - 985 5 - 8 985 - 1575 6 - 12 1180 - 2360 Branch ducts 1 - 3 200 - 590 3 - 5 590 - 985 5 - 8 985 - 1575
Be aware that high velocities close to outlets and inlets may generate unacceptable noise.

The Constant Pressure Loss Method (or Equal Friction Loss Method)

A proper speed is selected in the main duct close to the fan. The pressure loss in the main duct are then used as a template for the rest of the system. The pressure (or friction) loss is kept at a constant level throughout the system. The method gives an automatic velocity reduction through the system. The method may add more duct cross sectional changes and can increase the number of components in the system compared to other methods.
The Static Pressure Recovery Method

With the static pressure recovery method the secondary and branch ducts are selected to achieve more or less the same static pressure in front of all outlets or inlets. The major advantage of the method are more common conditions for outlets and inlets. Unfortunate the method is complicated to use and therefore seldom used.

YES! THEORY!!

I am both scared and psyched to see that I can get an exact number on this. Scared, because I don't understand it yet, and psyched, because I can now KNOW exactly what I need now, and what I will need when I add the more lights.

To answer your question, I will be adding sealed reflectors, but not any more lights. I have all the lights (2 X 1000W and 1 X 400W) but only 2 (The 1000 Watters) will be in sealed hoods right away. Then the 400W will, too, down the road.

At that point, I can pull the trigger on CO2 enrichment. Until I get all 3 lights in sealed hoods, separating the airflow for venting of the light(s) is really only going to help me by reclaiming the heat from whatever lamps ARE in sealed hoods, and send that heat back into the living area, instead of outside (It's cold where I am). Once I have all 3 in sealed hoods, then it will do that, and also allow me to start CO2 enrichment.

The thing about 90 deg bends reducing pressure by 20%....holy shit! I knew it reduced it some, but wow...20% is a lot. That makes me really not want to use bends in the ducting path to stop light leaks (both in and out). Maybe (motorized?) dampers to stop light leaking into the room through the hood, and I will just deal with any light leaking into the living space some other way, that seems pretty minor at this point. Any suggestions to stop lights leaks without pissing away hard earned fan pressure?

It's interesting that you brought up that flexible line (is that flexible ducting?) also drops your pressure/air flow. I have believed that for a long time. it just makes sense that the tube flexing in uses up energy that would otherwise go into air flow. The people I have talked to about it looked at me like I had 3 heads when I brought it up, though.

My plan is to use the flexible ducting at first, when I have a fan that is pretty much way more than powerful enough to do the job at that point, and then when I get all 3 hoods, to switch over to some kind of rigid ducting, then dial in the fan speed with a controller to minnimize noise and electricity usage, while still keeping things cool.

I figure the flex ducting is temporary anyway, and I will have insight in my design of the rigid ducting layout if I can start out small and live with it for a little while before I decide where to pop holes in walls and floors and ceilings and where to run rigid ducting in most efficient and least obtrusive way. Thanks again for your input!

Love your avatar, btw, woods.