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I've always been an indoor brewer, I started out brewing small extract batches in the kitchen but soon found myself setting up a brew room in an area of the basement. It was a nice setup where no matter what the weather was outside I was still able to brew whenever I had the free time. A small natural gas range provided enough btus to do all grain full wort boils with no problem and ventilation was handled by two small window fans at opposite ends of the brew room. A fan at one end of the room pulled in fresh makeup air while the fan at the other end exhausted boil vapors and harmful fumes out the other end. This made for some hot brewing days in the heat of summer and some cold brewing days in the dead of winter, but it worked.Natural gas isn't available for brewing at my new place so the brew room is setup now to use an electric powered brewing system. After a lot of research and cost comparisons I decided to buy the High Gravity BIAB 62 quart electric system. This experimental electric brewing system came closest to fitting my brewing area, which is now located in the garage, and my budget. Half the cost of the brewing system was spent on running a 30 amp 220 volt GFIC outlet, a hoisting system capable of lifting sixty or more pounds of sopping wet grains and a good exhaust hood to vent the boil vapors outside. Depending on the length of the wort boil either sixty, ninety or in rare instances a hundred and twenty minutes the exhaust fan could have to vent up to two gallons of vapors on brew day.This is when I took out the calculator, notepad and pencil as I tried to figure out how many cubic feet per minute the exhaust fan had to be in order to remove all the moisture from the garage. According to several reputable online electric brewing sources the formula for correctly sizing an exhaust fan is to take the wattage of the electric heating element and divide it by 17.6 to get the size of the fan in cfm, in my case that came out to 4500 / 17.6 = 255 cfm. What could be easier.I also found several sites that provided formulas for sizing bathroom exhaust fans, their main purpose is to prevent the growth of mold and mildew caused by moisture from showers, baths and sinks. So far both formulas appeared to be trying to accomplish the same exact thing, prevent excessive moisture from clinging to walls, floors and ceilings and in the process prevent the growth of mold and mildew. Bathroom exhaust fans are sized to replace all the air in the room every eight minutes, it's just too impractical to hang exhaust hoods over the shower, tub and toilet.A garage measuring 20 feet by 20 feet with 10 foot ceilings contains 4000 cubic feet. Using the bathroom exhaust formula of changing all the air every eight minutes requires a 500 cfm fan, or roughly twice the size required when using an exhaust hood. The instructions that shipped with the exhaust hood I finally bought said that for best performance the bottom of the exhaust hood should be between 30 and 48 inches above the top of the kettle. The fan is rated at 760 cfm and has a built in three speed controller that can be used to increase or decrease the number of cfm needed to keep the garage air clear and moisture free. So there you have it two different formulas two different cfm requirements and in the end I bought an exhaust fan and hood that can easily satisfy both.Vince Feminella [aka: ScrewyBrewer]