-General rule of thumb: 400CFM exhaust to 100CFM intake. Play with these numbers to see how the temperatures and air flow change. I suggest hanging digital thermometers in various locations and heights in the grow room. There are thermometers that also tell you the min/max temperatures from the day. Knowing those temperatures allows the grower to understand how their daily temps are fluctuating. Indoor grow boxes are much more efficient when they are running cool.
-Clean Green Tip: Clean grow rooms are happy grow rooms. Use carbon filters on exhaust fans and HEPA filters on intake fans. Use gas duster cans to spray away any dust on fan blades. Spider mites love to live in dust and travel through the air. Avoid spreading a spider mite infestation with a little preventative pest control.
There are a few different ways to ventilate your grow room depending on the size. Small PC grow boxes, and any box roughly 2′x2′x4′ could be ventilated with PC fans or one small ducting fan. Small bathroom exhaust fans would work but may be too loud for the application. A closet grow room will need a 4″ or 6″ inline ducting fan. The inline fan can ventilate the room and clean the air with a carbon filter. If you are using metal halide or high pressure sodium light bulbs you will want the inline fan to push the air through air cooled hoods. Below is an example.
Key to indoor gardening: Experiment with everything to see what works in your grow room.
If you understand the principles behind the practices – you will be able to make things work with what you have. Try using a bathroom exhaust fan as the grow room exhaust. Bathroom fans are rated with CFM just like inline fans. They practically work the same. Control the flow of air with quiet insulated ducting. The noisy vibrations will certainly get annoying. Look for more posts in the future about ventilation and grow room air circulation.