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BPI Written Exam - Section 3 Measurement & Verification of Building Performance
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7. Ventilation Calculations and Strategies
As of 4-4-15, BPI used ASHRAE Standard 62.2-2013 for minimum ventilation requirements in homes. Past of the calculation is to measure the flow rate of exhaust fans. It is important to measure the flow rate of exhaust fans because powered fans do not always perform as expected.
Measuring the flow rate of exhaust fans
There are several ways to measure the flow rate of exhaust fans used for ventilation. It's important to remember that flow rate and velocity are two different things. We get the flow rate by multiplying the velocity by area. A high velocity will blow your hair back and move air around (think of a ceiling fan) but it won't put more air into the room. If you increase the duct size, you have the ability to increase the flow rate if your plenum is sufficiently pressurized. Surprisingly all three methods have been shown to give similar results.
3. Garage bag method - takes a known volume (5 gallon trash bag), fills it with air and times how long it takes to evacuate the bag of air in seconds. Then divide volume / time to get your flow rate.
CFM = 8.02 x Vol in bag / time
More information on ventilation:
Section 2.12 http://www.bpiexamacademy.com/12-understand-issues-involved-with-ventilation-equipment.html
Section 2.17 http://www.bpiexamacademy.com/17-understand-various-mechanical-ventilation-equipment-and-strategies-spot-erv-hrv.html
3a. Applied diagnostics and troubleshooting