There is no specific formula that will allow you to calculate a definite answer for the CFM of an exhaust fan; however there are some methods that will allow you to get an approximation. The best method is to use a specialised hood which can be placed over the exhaust fan, to measure the depressurisation, from which an estimate of the CMF rating can be calculated.
An exhaust fan is also known as a whole-house fan which is a type of fan commonly venting into a building's attic, designed to pull hot air out of the building. A whole-house fan can significantly lower the temperature in a building very quickly, and is much less expensive to operate than air conditioning. Newer whole-house fans can be environmentally friendly and energy efficient additions to house cooling systems. On hot days they can be turned on to circulate rising hot air out of the house while pulling cool air in. Also new models are quieter and smaller than their older counterparts.
A major issue to watch for is back drafts from open flames like water heaters. If the whole-house fan creates a lot of negative pressure it can cause unsealed furnaces to release toxic carbon-monoxide into the house.
In a whole-house fan air is drawn into the house through open windows, meaning that the air is unfiltered and may contain pollen or other allergens, which settle out on floors and furniture making the house get dirty fast. This is in contrast to an air conditioner, which mainly circulates air through a heat exchanger.
The fan can also be noisy. The noise is due to the large volume of air handled by such a fan and the speed of the fan blades. Direct drive motors spin the blades faster, producing more noise compared to belt-driven fans, which allow lower fan speeds and less noise. The noise may interfere with other normal household activities such as listening to music, watching television, or conversation.
An exhaust fan is also known as a whole-house fan which is a type of fan commonly venting into a building's attic, designed to pull hot air out of the building. A whole-house fan can significantly lower the temperature in a building very quickly, and is much less expensive to operate than air conditioning. Newer whole-house fans can be environmentally friendly and energy efficient additions to house cooling systems. On hot days they can be turned on to circulate rising hot air out of the house while pulling cool air in. Also new models are quieter and smaller than their older counterparts.
A major issue to watch for is back drafts from open flames like water heaters. If the whole-house fan creates a lot of negative pressure it can cause unsealed furnaces to release toxic carbon-monoxide into the house.
In a whole-house fan air is drawn into the house through open windows, meaning that the air is unfiltered and may contain pollen or other allergens, which settle out on floors and furniture making the house get dirty fast. This is in contrast to an air conditioner, which mainly circulates air through a heat exchanger.
The fan can also be noisy. The noise is due to the large volume of air handled by such a fan and the speed of the fan blades. Direct drive motors spin the blades faster, producing more noise compared to belt-driven fans, which allow lower fan speeds and less noise. The noise may interfere with other normal household activities such as listening to music, watching television, or conversation.
