420 Magazine Background

How Do I Design a Fan Powered Cooling System for my Growbox?

SmokeyMacPot

New Member
Introduction:
Every serious growing box needs cooling. Most of us use air cooling because it is cheap and very effective. The following steps are used to design a simple fan-cooled box.
This method does not cover active cooling with air conditioning systems or 'CoolTube' designs. It is for grow chambers where the walls are approximately equal to the light pattern, totally enclosed for airflow control, and do not have large radiant heat into or out of the box. Your mileage may vary some for these reasons.

I also picked sane defaults for growing conditions. The formulas diverge if you get too far out of plant growing range. You should be very safe if you are within about 40 to 150 degrees F and 20% to 90% humidity ranges (those are just guesses). Atmospheric pressure was picked as sea level and doesn't really affect anything until about 5,000 or 8,000 feet depending on how accurate you want to get. If extreme conditions apply to you, there may be other FAQ entries with the entire full blown set of pressure/temp/airflow/humidity parameters.

Design:
1) Start at the beginning and design this right! Before you ever buy or cut anything for your new project, determine the highest temp (in F) your intake air will ever be when lights run. Get a thermometer and measure it to make sure you have a good value. Call this T(inlet)

2) Use these formulas to determine difference in temp you can tolerate. 81F (27F) is about the optimal for growing, 86F/30C on the higher end.

Tdiff = 81F - T(inlet) (English)
Tdiff = 27C - T(inlet) (Metric)

3) Add up wattage for all power in your box. Lights, pumps, heaters, humidifier, radio, coffee pot, whatever. Add it all up and call it Watts. This will make your number worst-case and therefore a conservative value.

4) Compute the absolute minimum fan power you will need using the following formulas. This is the minimum fan rating you must have to achieve your temperature goals. You will have to increase fan power to compensate for duct constrictions, small inlets, carbon scrubbers, screens, or other items that block airflow.

CFM = 3.16 x Watts / Tdiff (English)
CMH = 2.98 x Watts / Tdiff (Metric)

The formulas are almost identical, due to the counteracting effects of converting airflow from CFM to CMH, and converting temp from Fahrenheit to Centigrade.

formulas can be found on this web page:

(This web site also lists the above formula and uses a constant of 3.16 as shown above)

5) If you have more than one fan, they should be mounted side-by-side rather than inline if you want to add their different CFM ratings.

For inline fans, use the lowest airflow rating of all fans in the path. A fan on the inlet and a fan on the exhaust of the box are considered inline fans. Fans inside the box should not be counted for airflow but must be included in wattage. A standard computer fan is normally right around 30 CFM (50 CMH).

The two lookup charts solve this equation for common lights. Make sure you get the proper one (English or metric). For those of you who are wondering if you did this right, here are a few numbers in English units:

Note: a 30cfm computer fan is trying to cool a 1000w HID bulb, in the 3rd from the last row, as an extreme example

If you are adding any carbon scrubbers or extensive ductwork, this is where you add to the fan size to account for air pressure losses. You have to move this many CFM, or the numbers don't come out right. The deciding factor for these items depends on your exact configuration and is beyond this discussion.

6) When your box is built, buy a thermometer and measure the air blowing out of the box (temp probe or thermometer should be in the air stream just after the fan, outside of the box enclosure) and the temp of the air entering the box (again, from outside the box perimeter). Make sure there is no direct light shining on the thermometers to ruin the measurement. DON'T MEASURE THE TEMP INSIDE THE BOX YET!!!! It's best to do this with 2 thermometers or a single thermometer with a remote probe. Cheap thermometers don't work well because they aren't very accurate. If you only have cheap thermometers, use the same one for all measurements to avoid accuracy issues.

7) Subtract your measured inlet from measured outlet temp. Compare to Tdiff from above. Is your measured difference as good or better than your estimated from step 2? If not, go find out why. Your problems are probably:

A. Heat source you didn't account for (the ballast?)
B. Your fan is overrated
C. You have blocked airflow
D. Your temperature measurement was inaccurate
E. Air leaks into the box (especially around the fan!) that ruin efficiency.

8) Once you get your measured temp difference equal to step 2, measure temps inside the box. Don't let the light shine right on the sensor, it will give faulty readings!! Use a light shield made from a tin can or something. If temps inside the box are higher than your exhaust temp at a reasonable distance from the bulb, you have air circulation problems inside the box. Get some kind of fan to stir up the air in there or look for airflow paths that allow air to travel from inlet to exhaust without spending any time in the box.

9) Always monitor the temperature difference between inlet and outlet temps every time you water the plants. If it varies much more than a degree or two, find out why. I use digital indoor/outdoor thermometer. It tracks high and low for both locations, outdoor probe is on a long wire, $14 at Kmart. No part of the thermometer is inside the box, just in the measuring air blowing in and out from the outside.

Please note that conversion values have been slightly rounded off to make this easy. Using the metric and english formulas will yield slightly different answers if compared. The difference should be less than one percent and can be ignored.

You can use the two load graphs attached if you prefer to do calculations visually rather than using the formulas listed above. Find the line for your light wattage and ignore all others. Each axis is logarithmic, make sure you count along each axis properly. The formulas listed in step 4 were used to make the graphs.

Sidenotes:
You can measure your fan airflow very accurately if you use a standard trouble light with a 60 or 100 watt bulb in it. These are very good test loads for calibrating things.
Just put it in and work through the formulas using a good thermometer to determine airflow. If you doubt the accuracy of your bulb and are really anal about it, you can calibrate the bulb against your electric meter over several minutes. You could also stick in a different brand of bulb at the same wattage and compare results. I haven't tried this, but I would just trust the bulb until proven wrong.

Testing and measuring duct losses:
Ducting losses are hard (to measure) because they rely on knowing your duct material coefficients. You can measure the losses in the duct after it is built and running, if that would help. You could measure a test section to calibrate that material, then extrapolate. Here's how:

Take a known fan (or the fan you will be using) and blow it into a plenum that has a heat source and some of your sample duct mounted to it. To do this, you need a trouble-light or other low wattage known source and a cardboard box to put it in, then mount the fan on the box and stick the duct on the other side.

Calibrate the box by measuring temps without the ducting, then compute CFM. Add the duct, measure the new temp, compute the new CFM. The difference is duct loss. Basically, use temperature and wattage to measure airflow and compute duct loss.

If you have an existing room, just measure inlet and exhaust temps, add up the watts, and then compute effective airflow. I just did this for my box and it's pretty much dead on. I think it varies by about +/- 0.2 DegF for 150 watts and two computer fans.

Once you have the value for your ducts, you can estimate loss by adding up the length. We would have to come up with adjustment for going around corners.

I once saw a Mech Eng. book that had different shapes of pipe listed (Tee, 45 Deg bends, 4-way branches, Y-branches, etc) and then gave an equivalent length of straight ducting they add for flow resistance.

Author: ranger2000
 
Last edited by a moderator:

c10dores

New Member
Help, I bought a grow cabinet (about 30 sq feet) with outside room temp about 75 degrees and inside the cabinet I am about 86 degrees. Water is running about the same (whatever the inside temp is). First is that OK and if not what should I do? and second what will CO2 (which I am adding) do to help with temp.. Also using auto flowing, if that makes a difference.. First Timer here. PS... What I see in front of me is about 2' and looking nice, just a little yellow on a few edges...
 

Skybound

Well-Known Member
full


Here's what I'm working on and I think can solve your issues.
 
Last edited by a moderator:

blownvaliant

New Member
Wow!! Waaaaaaaay to much info for this lil black duck to comprehend...

So its basically , measure both IN and OUT temps for the fans on outside of box and they should be within a degree or so of each other , if not then you need a bigger exhaust (im guessing exhaust) to displace the hotter air inside grow room faster?
 

LEDBud

Well-Known Member
To summarize

You want your intake and exhaust temperatures to be the same or within about 1 degree
Use a common digital temp gauge with probe for accuracy

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

That is for grow cabinets & boxes , for rooms they suggest you to change the air every 3 minutes , if you have a
10x10x8 room = 800 cu.ft divide by 3 = 266 cu.ft
To get that 260 cu. ft of air exchange you will need a fan rated for about 500 cu.ft , as fans are commonly rated by free air movement , not pushing or pulling threw the duct work.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

My 12 volt 2.25A 6" fan is rated @ 250 cu.ft although when it meets its full working pressure in a duct its rated for 125 cu.ft per minute.

It changes my 2x4x6 Cabinets air about 3 x a minute under load with the Carbon filter in place , or 5 x a minute in free air mode with the filter removed. I have ordered a DC to Dc adjustable voltage convertor / controller to control the fan speed, as the PWM speed controllers cause DC fans to fail earlier then they would otherwise.

My temps are the same going in and going out since the fan upgrade , right now the fan is running on a 9v 1A power supply as a temporary speed control method , you can swap power supplies to vary the fan speed in a pinch.

(DC to Dc voltage convertors provide the best way to control a 12v fan speed with the fans longevity in mind)


DISCLAIMER: My first fan in the grow cabinet was a under powered 12v 0.25A fan that showed a 5 degree temperature difference between intake and exhaust temperatures.
Which gave my seedlings heat stroke / a near death experience , using 1 300w LED panel (180w) in the plywood cabinet.
 

Sulli

New Member
my plant is outside growing nice with white and orange hairs all over it...should I bring her indoors and put her on 12 12 lighting?
 

Enr0n

Well-Known Member
@Skybound

I am currently setting up an indoor grow 8x8x10 and if my calculations are correct I have 640 sq ft. So I need to have at least 214 cfm of air to change it every 3 minutes. Is a setup with both a fan forcing more air in than the fan pushing out a viable one?

I am looking at using a 11" fan mounted at the ceiling venting into my 2 car garage. In addition I am considering mounting a 20' box fan in the bottom of the door to the room to push the air inside to create positive pressure in the room. My thoughts are that if I create positive pressure in the room to prevent any unwanted pollen etc...

Both fans will be covered in potting mesh/fabric the exhaust fan on the discharge side and the 20" box fan will have the fabric on the intake side to help filter the air and keep bugs out. I know that covering the fans will reduce the cfm so I figured I will have to run fans 20% larger than needed to accommodate the restriction caused by the mesh.

Not worried about the smell since I have a MM license for possession and growing my medicine.
 

Skybound

Well-Known Member
@Skybound

I am currently setting up an indoor grow 8x8x10 and if my calculations are correct I have 640 sq ft. So I need to have at least 214 cfm of air to change it every 3 minutes. Is a setup with both a fan forcing more air in than the fan pushing out a viable one?

I am looking at using a 11" fan mounted at the ceiling venting into my 2 car garage. In addition I am considering mounting a 20' box fan in the bottom of the door to the room to push the air inside to create positive pressure in the room. My thoughts are that if I create positive pressure in the room to prevent any unwanted pollen etc...

Both fans will be covered in potting mesh/fabric the exhaust fan on the discharge side and the 20" box fan will have the fabric on the intake side to help filter the air and keep bugs out. I know that covering the fans will reduce the cfm so I figured I will have to run fans 20% larger than needed to accommodate the restriction caused by the mesh.

Not worried about the smell since I have a MM license for possession and growing my medicine.

I've found in my years that smaller cheaper fans serve the purpose just fine within reason. I now have a 9x9x8 for bloom and a 2x5x6 for veg and at one time had a single 6" 440 cfm fan drawing from all the rooms and my crop did just fine. I've since added a 2nd 440cfm fan running parallel and blowing into carbon filter in the attic, but mostly because I am paranoid about heat accumulating in the rooms.

But generally speaking, your best effort will likely be just fine, so you can scale your way in with smaller investements, then upgrade your system down the road.
 

Elvin

Banned Troll
I've found in my years that smaller cheaper fans serve the purpose just fine within reason. I now have a 9x9x8 for bloom and a 2x5x6 for veg and at one time had a single 6" 440 cfm fan drawing from all the rooms and my crop did just fine. I've since added a 2nd 440cfm fan running parallel and blowing into carbon filter in the attic, but mostly because I am paranoid about heat accumulating in the rooms.

But generally speaking, your best effort will likely be just fine, so you can scale your way in with smaller investements, then upgrade your system down the road.
I just use a single 6-inch iPower fan going into both tents; exhaust of the first is the intake of the second.
Blows the tents up like balloons but the zippers are easily strong enough.
I run QB's with the transformers outside the tents so heat is not an issue from that but I do draw cold conditioned air from the first floor of the house up to my un-air-conditioned upstairs.
If it heats up with the summer heat (it gets HOT up there) I just run the fan 24/7 because cooling just the tents is way cheaper than cooling the entire second floor.
 

Skybound

Well-Known Member
I just use a single 6-inch iPower fan going into both tents; exhaust of the first is the intake of the second.
Blows the tents up like balloons but the zippers are easily strong enough.
I run QB's with the transformers outside the tents so heat is not an issue from that but I do draw cold conditioned air from the first floor of the house up to my un-air-conditioned upstairs.
If it heats up with the summer heat (it gets HOT up there) I just run the fan 24/7 because cooling just the tents is way cheaper than cooling the entire second floor.

in that arrangement, you'd be better off pulling the air from both tents. Blowing air in has the potential to find pin holes here or there and push stinky air out, but obviously you're well aware of all the considerable factors at your grow to know better. I just wanted to comment to say what I said, not to imply you're wrong. Heck, I've been grilled about blowing stinky air into my carbon filters instead of sucking the air through them, but I'm more aware of my own variables to contend with, as you are with yours.
 

Elvin

Banned Troll
in that arrangement, you'd be better off pulling the air from both tents. Blowing air in has the potential to find pin holes here or there and push stinky air out, but obviously you're well aware of all the considerable factors at your grow to know better. I just wanted to comment to say what I said, not to imply you're wrong. Heck, I've been grilled about blowing stinky air into my carbon filters instead of sucking the air through them, but I'm more aware of my own variables to contend with, as you are with yours.
Pulling air from both tents? Why? I have enough plenty of air flow. Airflow is what matters to stop mold and wet walls on tents.
Who knows if it would work with restrictive carbon filters.
I do not bother with those because I do not have nosey neighbors.
Pinholes are there no matter which way the wind blows so that really does not matter. I have none that are large to worry about and besides I have curtains on the windows.
Everyone is scared of stress on the zippers but if 5 lbs (at most and it is likely less) causes issues with those big zipper teeth you have one crappy tent.
Sure if there was no exhaust outlet (or a carbon filter) there would be issues about stress but that is not the case
I did the silly 2 fans to "equalize" but one day I ran across "choosing the right zipper" on the internet and even though my zippers were not the same brand it was easy to see these huge tent zippers can take a lot of stress unless they are junk which mine was not.

In fact, I have a suspicion almost all "failed" zippers (other than the no-name junk tents with crap zippers) have human intervention involved (forcing a stuck zipper) not any sort of manufacturing or engineering defects.
I can see a need for "negative pressure" if running filters but otherwise why reduce tent size by sucking the walls in?
 
Last edited:

Skybound

Well-Known Member
Pulling air from both tents? Why? I have enough plenty of air flow. Airflow is what matters to stop mold and wet walls on tents.
Who knows if it would work with restrictive carbon filters.
I do not bother with those because I do not have nosey neighbors.
Pinholes are there no matter which way the wind blows so that really does not matter. I have none that are large to worry about and besides I have curtains on the windows.
Everyone is scared of stress on the zippers but if 5 lbs (at most and it is likely less) causes issues with those big zipper teeth you have one crappy tent.
Sure if there was no exhaust outlet (or a carbon filter) there would be issues about stress but that is not the case
I did the silly 2 fans to "equalize" but one day I ran across "choosing the right zipper" on the internet and even though my zippers were not the same brand it was easy to see these huge tent zippers can take a lot of stress unless they are junk which mine was not.

In fact, I have a suspicion almost all "failed" zippers (other than the no-name junk tents with crap zippers) have human intervention involved (forcing a stuck zipper) not any sort of manufacturing or engineering defects.
I can see a need for "negative pressure" if running filters but otherwise why reduce tent size by sucking the walls in?

I assumed that you used a carbon filter and needed to get the dank out of the air. If that was true, then it would be better to suck air in through pin holes than blow out stinky air due to the tents being positive pressured. But because you don't need to use a filter changes everything and I rescind my comment.
 

Victorytoker

Active Member
1watt = 3.8 btu. 3.8 btu is about 3.8 wooden matches burned.
8f you are interested 8 can do exact calculations of delta t and temp rise but generally consider every watt as 3.8 matches every hour, this goes for all electronics so any watts that are inside your tent/room WILL add that much heat. You must know your baseline ( house temp ) and how much heat you are adding total, to properly size your extraction needs. If you extract to a closed room then you will need to deal with lowering the temp in the room
 
Top Bottom