What you need to know about Exhaust Fans

I have seen a lot of questions about Exhausts and Intakes......Should I use exhaust or intake fans? Should i have a bigger fan for supply and smaller for exhaust? let me help clear some of this up....After 20 + years of HVAC work you learn a lot about airflow.

The exhaust and intake air will be the same regardless if one is more powerful or not... Unless you have a fan that is designed for High Pressure applications, which i promise you will not find or afford, the cfm is controlled by the size of the opening of both the exhaust and supply holes....Air travels to the path of least resistance (law of physics) . when you have resistance you then create staic pressure. static pressure is measured in inches of water column. It takes a lot of pressure to move water in a small tube and exhaust fans and intake fans cannot overcome this pressure so it creates a backpressure effect and becomes much less efficient.

Example: If you have a 100 cfm exhaust fan and a 50 cfm supply fan, you will have 50 cfm of air movement. The only way to overcome this is to have a fan capable of creating high pressure,which no fan available to the typical consumer would have.. Now of course you can create a slight vacuum effect by having a larger exhaust and smaller supply, which would be good for odor staying in the room and not spreading all over the house, but your air movement will always be the same as the smallest opening in your room.

If you have just one fan, make it an exhaust... It is harder to create air pressure than remove it. Whatever size fan you have, make the intake opening twice as large as the exhaust opening.

A good rule of thumb to follow if you dont have an air calculator is:

4 inch opening is good for up to 50 cfms.
6 inch opening is good for 100 cfms.
8 inch opening is good for 200 cfms.
10 inch opening is good for 400 cfms.

All of these numbers are assuming you are at 1.00 inches of water column, which is basically every exhaust fan, oscillating fan, and home heating and air conditioning fan.

with a few exceptions, this is the standard rule....however there are fans that can overcome this pressure slightly such as the cone or bell shaped fans you see.

For instance there are little 4 inch fans that say they can move 150 cfms or more.

This is misleading, let me give you an analogy: If you hold a bowling ball close to your chest, it is a little heavy but, you could hold it with no problem for a long time. If you take that same bowling ball and hold it out away from your body you can only hold it there a short time because it is much harder to hold even though it is the same weight. That is the same principle behind static pressure, when you add ducting to a fan you are holding a bowling ball further away from your body, the more ducting you add, the harder it is to move the air. the harder it is to move the air, the higher the static pressure and there goes your cfms out the window. It will still move air, but not nearly as much as the rated cfm on the label.

Another rule of thumb is....keep your intake low and your exhaust high...Just the heat alone will help move air out of the room and bring fresh air in.

You do have the option of spending hundreds of dollars on little fans but if you follow the general guidlines of HVAC you can achieve the same air flow at a quarter of the cost.

Just remember....if you have one fan, make it an exhaust, keep it high, and make your intake twice as big as your fan opening. You can move just as much air with two 15 dollar bathroom fans from homedepot as you can with that 200 dollar fan at the hydro shop with only a slight increase in energy consumption.

The reason you need a supply opening twice as big as exhaust is: unless your room is completely air tight, atmospheric pressure fills the room and your fan has to overcome this pressure first and then begin to draw air in, so your room has now become a part of the ducting. If your supply opening is twice as big, your exhaust fan doesnt have to overcome any resistance.

This of course DOES NOT APPLY TO COOL TUBES OR DUCTED LIGHTING...ALWAYS USE A PUSH CONFIGURATION ON LIGHTING... There are too many variables to know when your fan is going do die from overheating in a pull configuration..

There is also one more thing that i need to mention....

When you do run ducting to your fans..try your best not to oval the ducting if possible.. such as stuffing a 6inch round duct thru a 4" x 8" square hole or something similar.. The air speed/velocity will increase but cfms will drop and heres another analogy to prove why:

If you hold a styrofoam coffee cup, fill it with water to the very very top and then gently squeeze the cup what will happen? It will overflow...

Theoretically it should not overflow because the volume in the cup is the same...no matter how much it is ovaled out... it is still the same.

It overflows because you have created pressure inside of the cup...

Another analogy would be: putting your thumb over the end of a garden hose...When you put your thumb on the end of the hose the water is put under pressure and the result is the water shoots out much further and faster than without your thumb...By doing this you are also decreasing the volume of water moving thru the hose.. This is how a pressure washer works..

Its the same with air....a smaller opening will give you faster air movement but your air volume decreases..

great information in this thread. i am currently working on my closet/cabinet air exhaust and have questions. hope it's OK to ask here...

my box is 46"w x 38"d x 75" tall, not air tight, but the biggest opening is 1/4" at most down at the bottom of the door. yesterday i installed a range hood on the top with an 8 x 10 opening through the top of the box to the fan. turned the fan on for an hour, came back to check temperature and it had shot up to 102... why would an exhaust fan cause the temp to go up? do i need an intake hole? if so, should i put a fan in it?

This is really good information. Wish I had when I set up my room.

Could be a few things.....and i realize this may sound stupid but here goes.... first, make sure you have the correct path of air, meaning make sure you are not blowing air into the room instead of sucking air out.

Second, a rangehood is only gonna be good for between 50 and 100 cfms if your lucky....so if you have a 1000 watt light in there, it will not be enough air movement to cool it down.

Third, make sure you have the correct wiring on the fan....If it was for commercial use it may have a bidirectional motor, meaning you could be spinning backwards.

About the intake opening.....try to make that as big as you can without interfering with your dark period....that is definitely a major deal...

After you have it set up and running make sure that when its running you can feel plenty of air coming from the exhaust portion of the hood and make sure it is warm...

If you dont feel a big difference in the air coming from your exhaust from the air that is in the surrounding area, then you could have a bad fan motor that isnt spinning freely or the fan housing inside of the fan could be damaged or your intake opening is too small or it is installed backwards...

After scrolling back up and reading your message again, I just realized you said a 1/4 inch opening at the bottom.....lol. My fault, yep that is the culprit... Try to get an intake opening as big as possible an you should be fine.

If you have an 8x10 opening, which is 80 square inches, you would want atleast 80 square inches in your intake opening. Bigger would be better though..

I have a couple of small rooms about 2ft x 5ft x 8ft tall and I have a 15 dollar bathroom fan from home depot for an exhaust... this fan only 50 cfm, has a 3 inch round outlet on it but I have a 4 x 24 inch opening for my intake and it never gets over 82 degrees. But I only have 3- 150 wat HPS lights and 4-40 watt flourescents in each room... so its not a lot of heat to deal with....

I cant even imagine how loud that range hood fan must be dude......not to mention the extra power it takes to run it...

These little fans i am talking about at the depot only draw 50 watts...about 0.4 amps... very light, and very quiet...

But if you can get a free fan, then hell yeah.....use the shit out of it.

One more item to add........If you have a scenario like Ritchiesworld.....or something similar, you can gain some efficiency by making a box for your fan to sit on..

For instance> If I had a 3 x 3 grow room lets say 6 ft tall and i had a range hood for a fan.. I would make a 3 x 3 box with four sides...no top and no bottom. I would make it about 12 inches tall.

then I would cut holes about 1 or 2 inches in diameter evenly over the entire roof of my 3 x 3 grow room. Then I would take the little 12 inch tall box and attach it to the top of my 3x 3 grow room.

Then i would mount my range hood on top of that and seal all of the gaps where the fan attaches.. Now you have a plenum between your grow room and your fan.

Instead of pulling air from one small location, you now have air being pulled evenly throughout your entire room.

This happens quite a bit.... people have a rectangular or square room and they put an exhaust fan on one side and the supply air hole on the same side. Now what you have is air being pulled in and shot right back out of the exhaust. You must have cool intake air mixing with hot air in the room for heat to be exchanged.

Also remember, you need a small fan for air movement inside your grow room... two fans inside would be ideal... I keep a small 6 inch fan blowing on my lights 24 hours a day and a small 8 inch blowing on my plants.

with the 6 incher blowing on the lights, I can rest my hand on the hood reflector of my lights.. It barely seems to get warm...but if i cut that fan off...I can light a joint from my reflector it is so hot.

just something to keep in mind.

the range hood is mounted on the outside of the box, on top. it's not too loud at all, when i wired it up before installation it worked fine. air coming and going in the right places... there are 2 - 8" non-osc. fans in the box, the air crisscrosses between 'em. these fans blow 24/7. my lights won't be on for 8 hours, so i cant open her up right now. i will cut an intake hole and post some pics then... thanks for the help sir rance!