I apologize in advance for the length of this post, but you clicked it -
you must be interested!
There have been many, many topics posted here
that are basically the same questions - can I use this fan? Will this fan work?
Which fan should I use? What's the best fan? Hopefully I can get all the
information together here in one place and finally "blow the lid off" this whole
Please note - I am not talking about sealed rooms with CO2
enrichment. These setups will require very specific fans and air routing, to
ensure the benefit of the additional c02 is recognized.
First off -
Lets dumb it way down - there are basically 3 uses for fans in a
1. Air Exchange - This is the process by which fresh
air is introduced to and removed from the grow room, in order to provide the
plants with fresh CO2. Most air exchange setups will have to be coordinated with
#3 - cooling and odor control to ensure optimum results for
2. Air circulation - This is internal air movement in the
cabinet/room - plants waving in the breeze. It is required to help strengthen
stems and promote air movement to make sure the C02 is actively moving around,
able to be taken up by the plants.
3. Cooling and/or odor control
- These are the fans used to power a carbon scrubber (to exhaust warm stinky
air from the room) and to blow across the lights to lower temperatures in the
Static Pressure - the fans natural
This definition from Wikipedia defines Fluid Static
Pressure, but the applicable excerpt for pneumatic duct pressure is
"In fluid mechanics, and in particular in fluid statics,
static pressure  is the pressure exerted by a fluid at rest.
situations where static pressure is involved are:
The air pressure inside
a latex balloon is the static pressure and so is the atmospheric pressure
(neglecting the effect of wind).
The hydrostatic pressure at the bottom of a
dam is by definition the static pressure and so is the pressure exerted on one's
thumb when stopping the water flow in a garden hose.
The pressure inside
a ventilation duct is not the static pressure, unless the air inside the duct is
On an aircraft, the pressure measured on a generic point of the
surface of the wing (or fuselage) is not, in general, the static pressure,
unless the aircraft is not moving with respect to the air."
^ Thx to
In other words, the higher static pressure the intake
air ambient state is, the harder it is for a fan to pull the air, the less the
output of the fan will be. Fan CFM numbers quoted by manufacturers are almost
always advertised for the flow at 0" static pressure, since this will always be
the highest number. The better manufacturers will give you ratings at various
static pressures. Ducting, bends, filters, and reductions/objects(i.e.lights) in
the intake (or exhaust) create a harder path for the fan to suck and blow
through (suck through a McDonalds straw and then through a coffee stirring
straw). Put a box fan 1/2 inch away from a wall, blowing towards you. When it is
next to the wall, the fan needs to pull air through the small gap all the way
around the fan casing, thereby limiting the amount it can blow out. Now, move it
farther from the wall (making this gap larger). The fan finds it easier to pull
air through this larger gap and will blow harder. Now put it in a window or the
middle of the room, and the fan will be working as it was intended. This same
principle of "restriction" is demonstrated by large duct runs, ribbed ducts,
bends, carbon filters, and is the single reason two fans that move the same CFM
can become vastly different outputs when dealing with a small amount of
Types of Fans
"There are three main
types of fans used for moving air - axial, centrifugal (also called radial) and
cross flow (also called tangential).
The axial-flow fans have blades
that force air to move parallel to the shaft about which the blades rotate.
Axial fans blow air across the axis of the fan, linearly, hence their name. This
is the most commonly used type of fan, and is used in a wide variety of
applications, ranging from small cooling fans for electronics to the giant fans
used in wind tunnels.
The centrifugal fan has a moving component (called
an impeller) that consists of a central shaft about which a set of blades form a
spiral pattern. Centrifugal fans blow air at right angles to the intake of the
fan, and spin (centrifugally) the air outwards to the outlet. An impeller
rotates, causing air to enter the fan near the shaft and move perpendicularly
from the shaft to the opening in the scroll-shaped fan casing. A centrifugal fan
produces more pressure for a given air volume, and is used where this is
desirable such as in leaf blowers, air mattress inflators, and various
industrial purposes. They are typically noisier than comparable axial fans.
The cross flow fan has a squirrel cage rotor (a rotor with a hollow
center and axial fan blades along the periphery). Tangential fans take in air
along the periphery of the rotor, and expel it through the outlet in a similar
fashion to the centrifugal fan. Cross flow fans give off an even airflow along
the entire width of the fan, and are very quiet in operation. They are
comparatively bulky, and the air pressure is low. Cross flow fans are often used
in air conditioners, automobile ventilation systems, and for cooling in
medium-sized equipment such as photocopiers"
^ Thx to
For our purposes, we will only be interested in the
first 2 types of fans -
Blower Fan - Commonly referred to as a Squirrel Cage fan, also technically
known as a shaded pole blower, the exhaust is offset from the intake and
requires a flange to mount to ducting - Dayton and Active Air are examples of
these. Blowers are modestly priced and can be excellent for any of the three
purposes we need them for.
Centrifugal Inline Fan - This is a
Centrifugal Fan which has identical inlets and outlets on each side, and as such
is ideal for use in ductwork. Fantech, Vortex, and Panasonic Whisperline are
examples of manufacturers. These can also be used for any of the three purposes,
although generally higher priced (and MUCH quieter) than a comparable CFM
squirrel cage fan..
Axial Fan - PC Fans, Muffin Fans, Gable fans,
Attic Fans, Box fans, Duct "Booster" Fans, Oscillating fans - Any fan that blows
air directly across the axis of the fan is considered an axial Fan. Highly
affected by static pressure, for the most part, axial fans are only efficient
for #2 - air circulation
While a cross-flow fan could be utilized,
output, size and price are not attractive for this purpose.
much fan do I need?
In one word - more.
Note - this section is from opinion and experience cobbled together
with rudimentary pneumatic/HVAC management knowledge and common sense learned
over my 38 years. I have constructed ~20 or so different grow rooms/areas, from
150W cabinets to 5KW rooms and each one had its own problems and solutions. The
only one size fits all solution is to buy the biggest baddest fan made - hardly
This question is extremely hard to answer without accurate duct
static pressure readings, configuration, room dimensions and plans,
intake/ambient temps, desired results/temps,light wattage,acceptable noise
level, number of plants, reflector design, stealth issues ... All these
differences add up to make each growspace and setup unique in their
requirements. 2000 watts in a 10x10x10 room in Alaska is a lot different than
600 watts in a C-13 in Brazil! The smaller cabinet will likely have more heat
problems, and use a single fan for more than one purpose, hence a single
powerful fan. The 10x10x10 room you might get away with 2 box fans, one
circulating air and one pointing out a window (not very stealthy though) The
only safe answer to the question "what size fan do I need" is "a larger one".
In cabinets, for air circulation and exchange,
I like to shoot for an air exchange of at least 3x per minute. I also always put
a small fan inside the cab simply for circulation - no intake/exhausts, just to
move air, blow across the tops of the plants. Computer fans can work great for
this. Say you have a cabinet that is 2' x 2' x 6' - this equals 24 cubic feet
(LxWxH = Cu Ft. - If using inches for measurement, then divide the answer by
1728 (24x24x72= 41472 cubic inches. 41472/1728 = 24 cu ft) The minimum size fan
I would use for this box would be ~ 75cfm. That does not take into account any
ducting, cooling, or routing requirements - just air exchange and circulation.
If I was using a scrubber (and not concerned about anything else), personally
I'd double it to a 150cfm fan, either squirrel cage or centrifugal, depending on
budget and other needs. This is my starting point.
After you have your
starting point, put the lights on in the cabinet WITHOUT the plants, and take
accurate temperature readings inside, once the lights have been on for a few
hours. Note that if you leave the thermometer in the direct light, it will read
much higher than the ambient/actual temperature. Shade the thermometer probe and
locate it about mid cabinet to get a more accurate temperature reading. Check
the temps of the room the cabinet is in, taking care to note situations such as
a box inside a closet with the door closed - with no place to exhaust the air
outside of the closet - the temperature of the whole closet will gradually
raise, raising the intake temps more, which makes hotter exhaust, and continues
If your cabinet temp is less than 80 degrees, congratulations
- you are done. Get a fan the size of your starting point and go go
The rest of us will have a, in some cases, huge temperature
differential (74 degrees intake air, 110+ degrees in the cabinet in the shade
for example). Lights and ballasts put out a lot of heat. So not only does the
fan need to circulate and exchange air, it needs to remove 30ish degrees as
well. Heres where it gets tricky, and is very important to know where the
exhaust is going, what the intake temperature is, how the fixture can be cooled.
Most reflectors have fittings on either end of them, making it very easy to hook
up to ducting. My rule of thumb is an additional 75 cfm for each 10 degrees
differential. So if using a scrubber in this 2x2x6 cabinet (150cfm starting
point + 150cfm for light cooling) I would size ~ 300cfm for cooling, odor
control and air exchange. Then we would determine noise requirements, budget and
mounting options, and decide between a squirrel cage or centrifugal fan in the
desired CFM range.
Though it may seem overkill, using this formula I have
rarely ever had to resize a fan - and never cooked a plant to my knowledge. This
formula accounts for static pressure from typical corrugated duct routings,
usually 6-inch in diameter, and a couple of 90 degree elbows, and a passive
intake (or 2) of 2x the exhaust size exit. Slight adjustments should be made for
convoluted duct runs, 4-inch tubing, and any other special requirements. Temps
are almost always in control from day one, making less hassles in the long run.
Don't cheap out and buy the least expensive fan you can find, make sure its from
a reputed manufacturer with a warranty and you will be much
Complete rooms are a different task altogether, although the
same methods are used for determining ambient temps. Since the air is not
contained in a cabinet, I wouldn't be as strict on the 3x a minute exchange
rate, nor will there be the same intense heat concentrated in a small area. A
larger room will almost always be easier to keep cool and fan sizing is
generally determined by circulation and odor control requirements
can't get a new fan - what can I do?
Some people cannot afford a new
fan, or have no room, or are trying to use an inadequate fan and refuse to
upsize - whatever the reason. Theres only a couple of solutions here - keep
cabinet doors open, increase intake size (if inadequate), lower the intake
temperatures, or remotely locate the ballasts outside the cabinet. other than
that, theres not much else you can do to remove heat from the
To be continued...