Fans are everywhere, whether you see/hear them or not. They're in your stove, microwave, bathroom, computer, laptop, and more. Whether used to push/pull or vent/supply air, they all have an important job to do in keeping our tech working.
Learn the important basics when buying an exhaust fan for your home:
What is a CFM?
CFMs, or Cubic Feet per Minute, is a measure of volumetric flow. In the world of fans it's used to tell how powerful a fan is. The higher the CFM, the more air that fan pushes or pulls through. Depending on the application, there are specific requirements for how effective a fan must be.
How Much CFM Do I Need?
For a major appliance such as a gas range, Abt recommends 1 CFM per 100 BTUs. For example: a GE $1,000 gas range has five burners, totaling 47,000 BTUs. That implies you'll need a exhaust vent with a fan rated at 470 CFM. That's assuming, though, that you have all five burners on at the same time. In the bathroom, you might want to upgrade your fan if your stomach doesn't process burritos well.
What About Noise Levels?
Common sense might imply that the more powerful the fan, the more noise it will make. However, that's not always the case. Better motors, blade size and shape, and other features make small contributions to the total amount of noise that a fan makes. What you should look for in a fan is the specified noise level, generally listed in sones. If you're escaping into the bathroom for some peace and quiet, the last thing you want is a loud fan buzzing in your ear.
Other Factors
It might sound simple, but make sure your fan fits! Many high powered bathroom fans are designed to slide into 2x6 or bigger ceiling joists, so if you don't have the space you might have to settle for a lower power model. Also consider when buying a range hood for your kitchen that the space above your range might not match exactly. If you have a 30" free standing range surrounded by cabinets, the upper cabinets on the sides might have end panels that eat into your 30" of available space. Also be careful to note where the fan exhausts (it's not always straight up, sometimes those stacks are cosmetic) and make sure you have a spot to run your exhaust outside.
(Images: XPhantom/Shutterstock, Michael Higginson/Shutterstock)
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Another factor to consider is the length and size of the duct run on the discharge of the fan. All fans are rated at a certain CFM for a certain pressure drop, and, if you exceed the pressure drop, you will get less CFM.
Note that too powerful a vent hood (>1000cfm) can suck air down your flue or up from your furnace.
"If the house doesn't have enough random air leaks around windows, doors, and mudsills, the makeup air is often pulled backwards through water-heater flues or down wood-burning chimneys — a phenomenon called backdrafting. Since the flue gases of some combustion appliances can include carbon monoxide, backdrafting is dangerous. In some cases, it can be life-threatening."
http://www.greenbuildingadvisor.com/blogs/dept/musings/makeup-air-range-hoods
Really really appreciate posts like this! Keep them coming!
ps: maybe another topic could be the pros / cons of counter depth refrigerators?
Yes, Robotropolis, backdrafting is a great way to get carbon monoxide poisoning! What gets sucked out will be replaced by what can get sucked in indiscriminately: air pressure, like the Honey Badger, don't care!
Somewhat related question: are range hoods a necessity or luxury/accessory type of appliance?
@GABY - I'd check if your cooktop/range requires ventilation and what the codes are in your area.