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How to Find Fan Airflow

Each furnace or air handler has its own peculiar air moving characteristics. So how can you figure out what the fan your working on it supposed to be doing? There is no single source for this information, but hopefully this article can guide you in your search for this essential data.

Fan Test Methods
When diagnosing or air balancing a system it is critical to know fan airflow. This value can be measured by totaling the airflow at the grilles or registers and adding them together if there is no duct leakage, but this method lacks accuracy because nearly all ducts leak.

If the ducting has a straight inlet or discharge of between five and 10 duct diameters, the duct can be traversed to determine fan airflow. But the easiest and fairly accurate method of determining airflow at the fan is to do measure Total External Static Pressure and read fan airflow from the manufacturer’s data.

Plot Airflow
The accuracy of interpreting airflow by using this method depends on several conditions.

Read and record the manufacturer, model and serial number of the air handler being tested. Verify that the current fan motor is the same as the original, and that the fan is clean and that the fan nut is fastened securely to the shaft.

If there are belts, be certain they have the proper tension (about ¾-in. flex between pulleys) and that the belt and pulleys are in good condition.

From the model number, determine the tonnage of the equipment. Design airflow in cooling mode typically equals 400 cfm per ton, or check you engineering data.

If the fan is direct drive, determine the fan speed setting of the blower motor by reading the schematic on the back of the blower compartment door. The schematic will help you determine what the fan speed dip switches indicate, or which fan speed tap black fan wire is plugged into.

Collect the manufacturer’s fan performance data that matches the model number of the air handler or furnace. Look in the old engineering manuals around the office, or if the system is new check current information. Many new air handlers include fan data in the installation instructions.

Measure entering and exiting static pressure. Add the two pressures together to get Total External Static Pressure.

Using the manufacturer’s fan data, determine fan airflow by plotting the equipment tonnage, the fan speed and the total external static pressure. Some charts may include fan RPM and motor size as well.

Compare the fan airflow to the airflow measured at the supply registers or return grilles to determine the performance of the ducting and the system as a whole.

Often fan performance data for equipment that we service and test is not available. When this happens, we are in a position where interpreting airflow by measuring static pressure is difficult.

When the manufacturer’s fan data is not readily available, you can contact the manufacturer and they will send you the equipment fan tables. They may send you to their website to download the information you need. More and more manufacturers will give you access to their engineering specifications online.

As a last chance ditch effort, you may use similar information from another model having the same fan dimensions and RPM or use generic information, but this is as good as it sounds, generic and may be off by 5% to 15% depending on the source you select.

Consider it a quest for knowledge. Airflow is a critical part of what you’re doing. As one of my best friends says, “If you’re not measuring, you’re just guessing.”

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. You can contact Doc at [email protected] or call him at 800/633-7058. Go to NCI’s website at for free information, technical and sales articles and downloads.

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