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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. 
1. Flow hoods aka blowometer - measurements will vary depending on the location you put the flow hood on the register.  Experienced technicians will take the average of several readings at different locations. The flow hood will give you the velocity and you need to input the size of the duct (area) to get the flow rate.  Flow hoods costs range from $1000-$2500 but do not include different fittings for different size registers.
2. Anemometers - using an anemometer in front of a register gives the revolutions per second, then find your area of the register boot.  Multiply the area of the register boot by the revolutions per second for your flow rate. Anemometers costs range from $40 to $200. 
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

References: http://www.homeenergy.org/show/article/id/59

https://homes.lbl.gov/sites/all/files/lbnl-51551_0.pdf
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

Next Section

3a. Applied diagnostics and troubleshooting
  1. Application of measured air leakage test results 
  2. Understand building shell/envelope leakage as a function of pressure difference and the size of holes in the air barrier 
  3. Apply fundamental construction mathematics and unit conversions 
  4. Calculate building tightness levels (minimum ventilation requirements) 
  5. Calculate heating degree days and cooling degree days 
  6. Identify proper appliance and combustion appliance venting 
  7. Ventilation calculations and strategies 
  8. Proper methods for identifying / testing fuel leaks 
  9. Blower door setup, accurate measurement and interpretation of results 
  10. Combustion Appliance Zone (CAZ): depressurization, spillage, draft, Carbon Monoxide (ambient and flue) 
  11. Carbon Monoxide (CO) evaluation: ambient 
  12. Proper applications and use of temperature measuring devices 
  13. Pressure pan and room to room pressure diagnostics 
  14. Recognize contributing factors to comfort problems 
  15. Inspect for areas containing moisture or bulk water in undesirable locations 
  16. Understand and inspect for basic electric safety (e.g. frayed wires, open boxes, etc)