Many larger exhaust fans are built in a fashion where it’s impossible to measure their performance with a balancing hood. In order to determine fan airflow, you may need to measure fan static pressure and RPM and then plot the fan airflow on the manufacturer’s fan performance table. Let’s take a look at how to measure fan static pressure under real field conditions.

Test Instrument
Select a pressure gauge or manometer that is fairly close to the pressure rating of the fan. For example, if the fan is expected to operate at .80-in. of water column a 40-in. range manometer may not read accurate enough to interpret less than 1-in. WC field measurements.

You will also need a static pressure tip that attaches to the end of a rubber or neoprene test hose that connect to the manometer. To drill the test holes into the ductwork, use 3/8-in. drill bit with a nipple at the end to avoid the drill bit wandering around and scarring the duct. Don’t forget hole plugs to seal the test holes when you’re done testing.

Be certain to use a gauge that reads in inches of water column. Do not use Pascals or other units. In the United States the accepted unit of pressure measurement for the HVAC industry is inches of water column. Reporting in other units is a tip off that you’re a stranger to the HVAC industry and your test results will most likely be dismissed.

Inline Fans
Inline fans are rated by the total of the inlet pressure and the discharge pressure of the fan. In order to measure correctly, pressure test holes must be drilled into the duct system upstream and downstream of the fan.

The test holes should be drilled adjacent to the fan between the fan and the elbow, takeoff, or turning vane to assure accurate operating inlet and discharge pressures of the fan are measured.

Measure the inlet pressure of the fan and record the pressure reading preceded by the “-“ sign indicating this is the suction pressure of the fan. Then measure the discharge pressure of the fan and write a “+” sign to specify fan discharge pressure.

Disregard the + and – signs and simply total the two absolute fan pressures. For example, a suction pressure of -.54-in. and a discharge pressure of +26-in. would equal a fan operating pressure of .80-in. This is the pressure that you would use along with the measured fan RPM to plot the fan airflow on the manufacturer’s fan table for inline fans.

Remote Fans
When a fan is mounted at the discharge of a duct system such as an up-blast or sidewall kitchen exhaust fan, different pressure test procedures are required.

The fan discharges to into the open atmosphere, so only the suction pressure of the fan needs to be measured in order to interpret the operating fan pressure. Simply drill a test hole before the fan, measure the inlet pressure of the fan, and plot fan airflow using the single pressure reading and the fan RPM.

Type one kitchen exhaust fans will require an extra step to measure the fan inlet pressure. Because the duct entering the fan is a fire rated duct, do not drill test hole into these ducts. Normally, these fans are installed with a hinge at the fan to allow grease duct cleaners access into the duct for cleaning.

To measure fan suction pressure, shut off power to the fan at the electrical disconnect and open the fan at the hinge. Using a piece of 1/8-in. copper tubing, bend the copper to lay perpendicular from the center of the grease duct, up and over the edge of the fan curb, below the lip of the fan and over to the manometer. Attach the manometer to the copper with a piece of rubber tubing.

Close the fan at the hinge and start the fan under normal operating conditions. Measure and record the inlet pressure of the fan as fan operating pressure. Use this pressure and the measured fan RPM to plot the airflow on the matching fan table.

Other Testing Observations
Other exhaust fan testing observations may include the operating condition of the fan such as the cleanliness of the fan, fan maintenance, or lack of it such as belt tension, pulley alignment, or excessive fan wear. Many fans are designed to work in concert with other fans in the system or building. The operation of related fans can be verified as elements of the fan operating sequence.

You may also verify that the installed fan matches the building design documents and the mechanical plans and specifications. Frequent air testers are amazed how often the field installed fan does not match up with the design fan diameter, horsepower and airflow specifications.

Rob “Doc” Falke serves the industry as the President of National Comfort Institute, a company that provides technical and business training to the HVAC and related industries. You can contact Doc at robf@ncihvac.com or call him at 800-633-7058. For additional information and downloads go to nationalcomfortinstitute.com.