Many thanks to the gang at Faber Hoods for this very helpful guide on the technical side of kitchen ventilation. True kitchen ventilation (rather than the cheap and usually ineffective method of hanging a vented microwave over your cooking surface) is an idea that's catching on again. All ventilation uses a measure called Cubic Feet per Minute to indicate how effective the blower motor in a ventilation system is at circulating air. Few topics can confuse people as quickly as CFM ratings. There is a mistaken belief, that like most everything else in appliances, bigger is better. Not necessarily.
Using a hood with higher CFM (above what you need for your cooking surface) means more air is being pulled out of your kitchen and your home than needed. Therefore a lot of cooled or heated air is being pulled out your home, which would lead to higher heating and cooling bills.
Also, a situation of negative pressure can also occur when too much air is being pulled out of the home and isn't being replaced by air from the outside. Homes built today are increasingly air tight and when too much air is pulled out of a home, you need to sometimes make up for that lost air by pumping outside air into the home. There are all kinds of rules of thumb regarding make up air and it's best to consult with an HVAC specialist before you install a high-powered ventilation system in a newer home.
When you're choosing a hood for your cooking surface, one that has too many CFMs won't be energy efficient and too few CFMs won't provide adequate ventilation. The more CFMs, the more energy they use and the more noise they make. The key is to buy the right hood for the job at hand. Somewhere there's an ideal CFM count to match your needs.
So buy a hood that can remove the heat, steam, odor, smoke and grease produced by your cooking properly while at the same time not overdoing it. This diagram below shows a good way to estimate how many CFMs you need for your kitchen. In this kitchen, the ceilings are ten feet tall (Z). The walls are 10 feet (X) by ten feet (Y). So Z x X x Y = 1,000 cubic feet. If you have a 500 CFM rangehood in this kitchen, in 2 minutes you will have completely exchanged all the air out of the kitchen (or 30 exchanges in an hour). The National Kitchen and Bath Association recommends 8 to 15 air exchanges in an hour for proper ventilation, so in this example, we're at double the recommended level. Rules of thumb like this can get you started but the amount of heat generated by some cooking appliances throws a wrench into the works. Heat is measured in British Thermal Units, or BTUs. There are additional calculations that need to be worked out when it comes to using professional-style ranges so be sure to consult with a professional kitchen designer before you commit to buying anything.
So even though the example above has us at twice the recommended CFM, using a four-burner gas cooktop will put you 100 CFMs under the required 600 CFMs for use over gas. If you're upgrading to something more substantial, a 48" range top for example, you're going to need at least 1,000 CFM. In the opposite direction, because induction cooktops generate so little radiant heat, a 300 CFM ventilation hood over it would work out perfectly. Confused? Don't be.
Calculating the volume of your room is helpful and knowing the heat output of your cooking surface is helpful too. Combining the two and coming up with a satisfactory CFM takes a bit of judgement and experience, but that's why I'm here. Me and a whole bunch of compatriots who like nothing better than to figure stuff like this out.