Ventilation rates are a little bit trickier than most folks realize. This message will deal with ventilation into and out of the grow tent. I've found that if you do this correctly, you only need a small fan to circulate the air inside the tent. Typically a 4" or 6" propeller fan will do it.
First, it is very unlikely that the ventilation rate supplied by the manufacturer of the fan is close to what you are actually ventilating at. It is likely to be choked by the ductwork or carbon filter. A good conservative estimate is 50 cfm with 4 inch duct, 120 cfm with 6 inch duct and 250 cfm with 8 inch duct.
Second, everybody is usually trying to accomplish 3 different goals with ventilation: temperature control, humidity control, and (optionally) odor control.
Temperature control: The airflow you need is dependent on your exhaust air temperature and your makeup air temperature. If you don't have exhaust ducting connected up to your lights, the temp in your grow tent is your exhaust temp. If you have ventilated lights, 50-70% of the heat generated by the lights is exhausted directly, allowing higher exhaust temps than the tent temperature. For non- cooled lights, add up total watts in the grow tent and use the following formula for vent rate in cubic feet per minute:
cfm = watts x 3.54 / [(Tin - Texh) x 1.1]
Tin = temp of makeup air in farenheit
Texh = Temp desired in tent
Example: for a tent with 400W of lights and a desired temp of 80 degrees located in a 70 degree room needs 128 cfm of exhaust. If the light is ducted to the exhaust, you only need about 50% of 128 cfm, or 65 cfm. I'd use a 6 exhaust fan unducted and a 4 inch fan ducted.
Humidity Control:
Definition- we all talk about humidity in terms of relative humidity. Relative humidity measures the % amount of moisture in the air before it cannot hold any more moisture. How much moisture air can hold is very dependent on temperature. Warmer air holds more moisture, so heating up air will lower the relative humidity, all other things remaining the same.
The ventilation required for humidity control is more dependent on the make-up air humidity and temperature than anything else. Ventilation rate isn't nearly as important here if the incoming humidity is reasonable. Typically it's going to be far less than required for temperature control if done properly. Where most people get into trouble is they exhaust their grow tent into the room and pull the make-up air from the same room back into the tent, or they locate their grow tent somewhere dank and cool, or they locate it in an attic that is not well ventilated. This means that as the water in the air builds up in the space where the grow tent is, the water in the air inside the grow tent also goes up. While the lights are on, it's not so bad (in the tent anyway) because warm air can hold more moisture than cold air , but when the lights go off, the ability of the air to hold moisture drops and condensation occurs. The only way to solve these problems is with a dehumidifier or ventilating the space where the grow tent is located.
If you can exhaust your grow tent at 6 air changes per hour (100% change in air every 10 minutes) and you still have a humidity problem, it most likely is a humidity problem from the make-up air and needs to be addressed outside your grow tent. An exception of course is CO2 addition where you can't exhaust your grow tent. In this case, the best solution is a dehumidifier at night and an air conditioner during the day.
Example: 6 Air changes per hour in a 2 foot by 4 foot by 6 foot grow tent = 6 x 4 x 2 x 6 / 60 = 5 cubic feet per minute!
Odor Control:
Depending on your plants (I'm looking at you, AK-47!) you cannot keep the odor under control inside the tent. The most effective method by far is to keep the air movement into your tent and pass 100% of the exhaust air through a carbon filter or ozone generator. all openings into your tent should have an air movement velocity of at least 50 feet per minute (this comes from the diffusion velocity of odors. It can be higher than that depending on molecule size).
Example: In my case, I have a 6 x 12 opening for make-up air (giving an effective area of 0.5 square feet). I need to pull at least 25 cfm of air through that opening for a velocity of 50 feet per minute. All the other openings probably add up to to a bit more, so I need something like 50 CFM exhaust either though a filter, or in my case, up a chimney to 40 feet above the ground.
Hope this helps.
-Q
