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Air Ventilation/circulation 101 Why do we need air ventilation/circulation? Plants need CO2 at the underside of their leaves to "breath". CO2 is found in our air, usually around 300 ppm. A plant can benefit from up to 2000 ppm's+ of CO2 at times. If the air is stagnant plants can quickly use all of the CO2 available to them. To provide adequate CO2 (among other benefits) we exchange the air every few minutes in the grow space. Even if you plan on supplemental CO2 in your room you will still need ventilation. Air management is key to a healthy garden and bountiful harvest. It deters pests, molds, removes heat and humidity, ect. If there is one time to over do something a little in your garden, this is it. Air ventilation/circulation. Ventilation is usually accomplished through ducting and in line fans. Once introduced into the grow space the fresh air will need to be circulated. This is usually accomplished by using simple oscillating fans mounted strategically to uniformly and lightly rustle all the leaves in the space. This will blow the CO2 depleted air from the bottom of the leaf and bring in fresh air. This circulation will also help make the temperature and humidity in the room more uniform and eliminate hot spots and places with stagnant air. Good circulation will also strengthen your plants as they get ready to put on some serious weight! The necessity of proper environment can not be overstated. Many problems in a garden are entirely avoided when plenty of air movement is present. From "over watering" to pests, molds, poor growth, ect. Theres not much proper ventilation/circulation of air does not help with. Luckily it is pretty simple, and there is always plenty of experienced help here to guide you if you have questions. Fan selection: To select a ventilation fan you'll need to know the cubic feet of your room. Here are 3 easy steps to find this with a example in parenthesis. 1. Measure your grow space in feet. Length, width and ceiling height. (10'X10' with 8' ceilings) 2. Multiple those three numbers together, LXWXH, to find the cubic feet of your space. (10'X10'X8'=800 cubic feet) 3. Divide the cubic feet by the number of minutes you want the air to be exchanged in. Every 2 minutes for this example. (800 divided by 2=400 CFM to exchange the air every two minutes in our space). So we need around a 400 CFM fan for our example. CFM ratings: This means the fan *should* move a specified amount of air with NO restriction. Of course your room will be at least somewhat sealed, and your going to hang ducts from the fan, so this number is unrealistic which is why we used every two minutes instead of three to five. This will help compensate for our real world use, not a lab test. What kind of fan should I use? A good quality fan like a "Vortex" brand will move a lot of air even while hindered with ducts and carbon filters. You can buy a vent fan for $30 or so from the hardware store, say 350 cfm. If you were to hook it up for our purposes it is doubtful it would do much except make noise and consume electricity. If you purchase a good quality fan it will perform to the rated specs and more at freeload and it will pull most of its rated CFM even in a real world application. If you go to http://www.atmosphere.com/ and choose their VTX style fans they have very informative and simple graphs that show the CFM capabilities of the fans dropping as static pressure rises. The CFM the fan pulls at a inch or so pressure is what you want to look at, not the rating on the box. if a manufacturer will or can not supply you with this information, I would look elsewhere. The shorter and straighter your ducts and the more intake air capability you have the easier it will be for the fan to perform near specifications. 6" and 8" models seem to be the most efficient. A quality fan will also be "speed control capable" allowing you to slow or speed up your fan depending on your needs. Intake air Either another fan or a passive vent(s) can provide intake air. Since the grow space is fairly well sealed a fan pulling air out of the room needs a little help to achieve desired air flow. The fan is not able to overcome the pressure of a sealed room. You can install another fan of the same or lesser size to blow air into the room while your main ventilation fan pulls the air out. You can also make "passive" vents in the walls or ceiling, or floor. These are simply holes in the room where fresh air is able to flow in to replace what is being sucked out of the room by the main fan. For example, you could make a hole or two the size of a small furnace filter in the walls, then place the filter over them. You would have a passive vent. Examples Lets say you have that 10X10 room with 8' ceilings above in the example. It is 800 cubic feet. You could choose a 8" fan to pull the air from the room as these fans are most efficient PULLING air. This could be done with a simple vent near the top of the room where the hot air rises too. Then you could choose another 8" or a 6" fan for intake air. Mount this low at the opposite side of your space, blowing air in. You could also cut large holes or employ other means (a unused door) for passive vents. If you have enough fan and a little ingenuity you can rock without the glass in your shades. Then you can hook up the main ventilation fan to suck the hot air off the bulbs and exchange air in the room simultaneously. This will pull air from precisely where you want! Over the canopy and in the hottest spot in the room. You could keep your glass too, and leave one end of the shade open to suck out the air in the room. If you do not hook your ventilation fan to your lights in some fashion you will be buying a fan to cool the lights and one (or two) for the ventilation. It all has to do with your space, budget, preferences, ect. Still unsure? Post up your questions below and they will be answered promptly by knowledgeable folks that love to help!