How To Read Fan Airflow

Fan airflow can be easily interpreted for the manufacturer’s fan performance tables if you master static pressure measurement and record the fan speed. Follow these simple steps to add airflow to any service or diagnostic appointment. Quite often, you’ll be very surprised by what you will find.

Most fan performance data is laid out in the manufacturer’s tables so you can find fan airflow by referencing live field total external static pressure and fan speed measurements. Simply find the points on the table representing these two measurements and then plot the intersection of these two points to find fan airflow. Let’s dig a little deeper and read between the lines to find all the steps necessary to assure accuracy when performing this diagnostic move.

Record Air Handler Nameplate Data
Each manufacturer fastens a nameplate to each air handler. The information it contains includes the fan manufacturer and model number, from which you can interpret the required airflow of the fan.

Next, go to your manufacturer’s published fan curve or fan performance data tables. These tables or charts are published for each fan made and is essential to understanding the fan’s performance. The information is contained in the engineering data or installation instructions.

If not available with the fan, you may call the manufacturer and they will normally be happy to fax or e-mail the information to you. Each manufacturer also gives this information to their distributors who should also be willing and able to provide you this information.

Fan performance data is also available on the internet. Most manufacturers require you to register with them on-line and will send you a password to get access to this information within 24 hours. Some manufacturers have the information available to anyone without registration.

Obtain the fan performance information however you can, and verify the information matches the exact fan you are testing. Over time, you will assemble a most valuable set of fan performance data that can be kept and indexed in a three ring binder.

Measure Actual Fan Performance
Next start the fan. This can be done by turning on the fan, or by energizing the heating or cooling equipment at the thermostat.

Read the fan performance by measuring total external static pressure and then fan RPM by following NCI testing procedures for each test. Record the measurements in the actual column on the Fan Test Report.

While you're testing, inspect the fan condition and then check the duct configurations entering and exiting the fan. The manufacturer’s fan performance tables are created under laboratory conditions and are quite accurate as long as the fan is fairly clean and in good operating condition.

Conditions that can change the fan airflow include the fan blades becoming dirty, which reduces the amount of air each blade can carry with each revolution of the fan. If the fan shaft is damaged or the set screw is loose, this can also reduce airflow. If the fan is driven by pulleys and a belt, and either are worn, loose, or damaged or if they’re not installed or aligned properly, this can also reduce fan capacity.

If you’re lucky, the fan is well maintained and installed, and interpreting airflow using the manufacturer’s fan performance data is extremely reliable.

Plot Fan Airflow
With the fan tested and good operating condition confirmed, you are now ready to plot fan airflow. If the data is presented in a fan curve format, total external static pressure is listed on the vertical axis of the fan on the left-hand side of the chart. Follow up the left side until you identify the point on the table that represents the measured fan pressure.

Mark that point on the fan curve and draw a horizontal line from left to right on the curve. This line will normally intersect the fan RPM lines. Mark the point where the pressure and RPM lines intersect. Draw a vertical line straight down to the bottom of the fan curve table to find the fan airflow that is being delivered.

Usually, the point plotted is not directly on a listed CFM, so you’ll have to interpret the graphics on the chart to find exact CFM.

Numerical Fan Tables
These tables are presented numerically instead of graphically, but follow similar rules. Often, the static pressure is listed along the top of these charts, and the fan size or horsepower is listed along the left side of the table. However the data is arranged, simply plot the measurements on the appropriate table and interpret the data to find fan airflow.

Remember, plotting fan airflow is often a preliminary air diagnostic test. It's nearly always followed up by additional air balancing testing to verify system performance with increased accuracy.

Plotting airflow is an excellent test to include in service agreements or abbreviated diagnostic testing.

As you become better at this testing it becomes an essential step in your diagnostic thought process. When this becomes standard practice for you, you’ll wonder how you ever serviced equipment and fans without looking at the air performance.

You'll find most residential fans performing at 60% to 75% of capacity, and most commercial fans operating at 70% to 80% of capacity. As an industry, we install new equipment and once it’s started up, assume it performs as it should.

Fan performance is typically fairly easy to improve. Simply changing fan speeds on direct drive fans and readjusting belts and pulleys on other fans can make a huge difference in system performance. But until we test, fan performance always “looks” good.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a generic .50-in. fan performance table and static pressure measurement procedure, contact Doc at[email protected] or call him at 800/633-7058. Go to NCI’s website at for free information, technical articles and downloads.

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