Chapter 8, section 804 of the RESNET Standard provides us with an onsite procedure for measuring the airflow of ventilation systems. These procedures treat the air flows into a grille and out of a register measured separately. There are 3 RESNET-approved test processes used to determine airflow: 1) powered flow hood, 2) air flow resistance and 3) timed bag inflation. Each method, as most things in life, has positives and negatives.
Powered Flow Hood
The powered flow hood method is the most accurate, but also the most expensive. The powered flow hood differs from a conventional flow hood in that there is a fan which assists air movement through the flow hood to prevent a pressure differential at the register or grill created by the flow hood. The most common is the Energy Conservatory FlowBlaster® which works with your existing Duct Blaster Fan and DG-700 Pressure and Flow Gauge. The fan is powered by a combination fan speed controller and rechargeable Lithium-Ion battery. This method may be used on either exhaust or supply systems.
Air Flow Resistance
The air flow resistance method is probably the most common and can only be used on exhaust systems (air entering grill). This method determines the air flow by measuring a pressure difference across a known hole size. The air flow (in cfm) is equal to the hole size (in square inches) times 1.07 times the square root of the pressure difference (in pascals). (Yes we are mixing units, but the 1.07 factor takes care of the conversions.) This device will give the best results when the pressure difference is less than 8 pascals – largely because the exhaust fan speed will be reduced with greater pressures. There is a commercially available “box” or flow meter again from the Energy Conservatory or you can easily create your own. (If interested in creating your own – drop me a line and I will send you the directions.)
Timed Bag Inflation
The timed bag inflation method is the least expensive of all. It can only be used on supply systems. As the name implies, a bag (typical a garbage bag) of known volume is inflated by the supply air. The time required to fully inflate the bag is measured with a stopwatch. This method takes a bit of practice to get repeatable results, but is rather simple to do. As the standard indicates, bag volume and thickness play into the accuracy of the results – so a trial and error approach is needed. Aim for a fill time of 2 to 20 seconds – the longer fill time will be easier to do, but may require a fairly large bag depending on the amount of airflow. The airflow is easily calculated by multiplying the bag volume (in gallons) by 8 and dividing by the time (in seconds) required to fill it. The Canada Mortgage and Housing Corporation has a nice write up on the method along with a table to convert to airflow.
These three procedures are the only RESNET-approved methods for measuring airflow in either whole house or spot ventilation systems. (Well, there is one exception – if an ERV/HRV manufacturer has ports installed on their device for the purpose of measuring airflow; that may be used when following their directions.)
So go measure and have fun out there…