Have you ever wondered how much air actually comes out of your hot air floor registers, or how many cubic feet per minute your bathroom fan actually exhausts? Well the Canada Mortgage and Housing Corporation has developed a simple tool that will give you a good approximation.
They call it the Garbage Bag Airflow Test. Take a clothes hanger and open it up to a rectangle or a circle. Tape a garbage bag to it. We were using a 66 x 91 cm bag. Larger bags can be used to test more powerful fans.
Collapse the bag. Put it over the register and count how long it takes for it to fill up. According to the CMHC, if it takes 2 seconds, you have 75 cfm flowing; 4 seconds, you have 40 cfm flowing; 10 seconds, 20 cfm and a pretty poor fan. If you want to do the test in the other direction, inflating the bag fully and holding it up to the ceiling exhaust fan in the bathroom, add a second to each of those, giving you 3 seconds for 75 cfm. When your fan says 110cfm and you have 40, you have a problem that could explain why there is condensation in your bathroom despite the fan going all the time.
Check out the CMHC web site for more details, and lots of other useful information. The Garbage Bag Air Flow test can be found as one of the "About Your House" fact sheets.