Do You Know How to Determine HVAC Exhaust Fan Airflow?

There are a bunch of specifications about exhaust fan airflow rights. Before you can figure out if you are in spec, you need an easy and practical way to determine HVAC exhaust fan airflow in the field.

First, you want to locate the exhaust fan. It is located at the start of the duct or the end of the duct? In a forced air system, the exhaust fan may be located in the middle of the duct.

While it is important to match the fan and the duct according to specifications, you cannot assume that a properly matched exhaust fan and duct will meet specifications.

Common Ways to Measure Airflow

There are several different ways to measure airflow. We think these ways offer the most practical solutions for measuring actual airflow in the field.

  • • Airflow Traverse
  • • Plot Fan Airflow
  • • Air Balancing Hood Measurement

Air Balancing Hood Measurement

If you have a grille that is flat against a ceiling of wall, you can place an air balancing hood securely over the grille. Make sure you are capturing all of the fan airflow. Then, set the balancing hood to read in exhaust mode. If the reading is within +/- 10% of the specified airflow, it is considered in spec.

Airflow Traverse

More complicated than using an air balancing hood, this method depends on you getting an accurate average air speed in the duct. You drill small test holes at different places along the duct. You use an anemometer to measure the air velocity at each location in the duct. Then, determine the average air speed. Use the average speed and multiply it by the area of the full length of the duct. The result is the exhaust air flow.

Plot Fan Airflow

To determine this, you need to consult the manufacturer’s fan performance table. Measure the fan’s operating static pressure and the revolutions per minute. You can measure revolutions per minute using a non-contact tachometer and the fan static pressure using a manometer (pressure gauge) with a static pressure tip attached to a hose or a tube. Drill a test hole in the duct and measure pressure at the test hole. Use the table to plot the fan’s airflow.