Step No. 1: Determine the Fan Speed Setting — To properly plot fan airflow, you’ll first need to determine the fan speed setting currently being used. Without first verifying the correct fan speed, plotted fan airflow could be off drastically. There are two basic methods used for determining most residential fan speeds.

Speed taps are the most commonly used method. Taps are typically plugged into a ¼-inch terminal on the circuit board or fan relay for cooling and heating operation. Follow the wiring schematic for the equipment and trace the fan speed tap for each mode of operation.

Dip switches are often used with variable-speed fans. Here you have to interpret the dip switch settings on the circuit board. Any time dip switches are used to control fan speeds, you need the manufacturer’s instructions or engineering data to identify the correct settings. Without this information, you’re just guessing. It’s a good habit to document fan speed settings for future reference.

Step No. 2: Inspect the Blower Wheel — After you determine the fan speed being used, inspect the blower wheel to ensure it’s clean and operating correctly. If the blower wheel is dirty, be aware it will seriously decrease airflow through the fan and render the fan tables useless.

Some blower wheels will be in such bad shape they will need to be replaced instead of cleaned. This single visual inspection can also identify the need for additional services or filter upgrades.

Step No. 3: Gather Nameplate Information and Tonnage Ratings — After recording the fan speed and inspecting blower wheel condition, you need to determine the tonnage of the fan and outdoor unit. Fan tonnage is interpreted from the air-moving equipment’s model number found on the nameplate. Don’t assume fan tonnage to be the same as the outdoor unit. It’s common to find larger fans matched up with smaller-tonnage outdoor units.

After fan tonnage is established, determine the tonnage of the outdoor unit. This will be used to calculate required fan airflow. The industry standard for cooling airflow is typically 400 cfm per ton.

By interpreting the information from the equipment’s nameplate and converting it to required fan airflow, you have a starting point for how much airflow is needed. If the outdoor equipment is rated at 2 ton, then the fan needs to move around 800 cfm (400 cfm per ton x 2 ton = 800 cfm needed).

If the 2-ton outdoor unit was paired with a fan rated for 3 ton of airflow, high speed might deliver too much airflow. Airflow should be based on the requirements of the outdoor unit.

Step No. 4: Measure Total External Static Pressure — Once steps one through three are complete, you’ll need to measure the equipment’s total external static pressure. To do this, you’ll first need to install 3/8-inch test ports (drill holes) where air enters and leaves the equipment.

You’ll need to drill into areas that could cause refrigerant or water leaks, so be careful. Use a drill bit sheath so the drill bit only penetrates the metal 1/8 inch or so and doesn’t get pulled into the equipment.

Once test ports are installed, measure total external static pressure and record it. This is the last remaining piece of information needed to plot fan airflow. Note: If you measure total external static pressure incorrectly, you may be tricked into thinking fan airflow is higher than it truly is.

Step No. 5: Plot Fan Airflow on the Manufacturer’s Fan Table — It’s time to plot fan airflow. Use the operating fan speed and measured total external static pressure to plot fan airflow on the appropriate manufacturer fan table.

First, determine the equipment size and model number for the appropriate fan-performance table you’ll be using. Some model numbers are close, so be careful as you do this. Once you have the correct fan-performance table, locate the fan speed being used and lightly circle it.

Next, locate the column closest representing the measured total external static pressure in step four and lightly circle it. Line the measured total external static pressure up with the same column as the fan speed being used. The point where the two lines intersect is the amount of air the fan is moving.

BE AWARE OF FOOTNOTES

Many manufacturers’ fan tables contain footnotes. Footnotes provide insight into how the fan was rated by the manufacturer and are an important reference when interpreting readings. 

You will find information on conditions the fan was rated at, such as:

• Static pressure limitations;

• Coil condition (wet or dry);

• Factory-installed air filter (included or not);

• Filter velocity ranges;

• Blower motor voltage ratings; and

• Included pressure drops (coils and heat strips).

If you’re in doubt about how a fan was rated, dig a little deeper into the footnotes and a lot of questions will be answered. As you become a student of fan-performance tables, you begin to see truths about airflow that enable you to solve issues beyond the skill level of your competition.

Publication date: 8/29/2016 

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