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Add Fan Airflow to Static Pressure Readings in Four Simple Steps

Last updated: 12-31-2020

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Add Fan Airflow to Static Pressure Readings in Four Simple Steps

When you measure static pressure, duct system problems that were previously hidden become obvious. Static pressure is a foundational cornerstone of air diagnostic testing, but it can be inconclusive unless you know fan airflow. Let’s look at how you can add this valuable piece of information to your static pressure measurements and greatly improve your airflow diagnostic skills.

The good news is that once you measure static pressure, you can estimate fan airflow in less than 30 seconds with the right information. Once technicians understand that static pressure testing gets them to fan airflow, they will see why it is important to measure it on every call.

Once you have a total external static pressure (TESP) reading, there are four steps you must perform to determine fan airflow. Please note: If you skip just one of these steps, you’ll likely end up frustrated because your results won’t make sense.

You need to interpret some basic information from the indoor and outdoor unit model numbers to establish how much airflow you need and how much airflow the fan can move.

First, determine the outdoor unit tonnage from its model number. You will need it to calculate required fan airflow. It is the baseline against which you compare your measured fan airflow. The industry standard for cooling airflow is typically 400 cfm per ton. If an outdoor unit is rated at 3 tons, then the fan must move around 1,200 cfm (400 cfm per ton X 3 tons = 1,200 cfm).

Once you know the outdoor unit tonnage and the required fan airflow, determine how much air the fan can move. You will interpret fan tonnage from the air-handling equipment model number. Don’t assume fan tonnage is the same as the outdoor unit. It’s common to find larger fans on smaller-tonnage outdoor units.

After you interpret the outdoor and indoor model numbers, open the blower compartment and inspect the blower wheel. Assure it is clean and rotates correctly. A dirty blower wheel seriously decreases fan airflow and makes a fan table useless, since they assume a clean blower wheel.

Some blower wheels are in such bad shape that they need to be replaced. This single inspection also identifies the need for additional services or filter upgrades that you can make available to your customers.

While you have the blower door off to inspect the blower wheel, check the motor wiring and determine the fan speed settings. It’s a best practice to document them for future reference. If you don’t verify the correct fan speed, fan airflow could be off drastically.

Constant-speed fans use speed taps attached to the control board for cooling and heating operation. Follow the wiring schematic to identify motor speeds you are unfamiliar with. Don’t assume high speed for cooling and low speed for heating.

Variable-speed fans use dip switch settings on the circuit board to control fan speeds. In this case, you need the manufacturer’s instructions to identify the correct settings. Without this information, you’re just guessing.

Every fan table is unique to the air-handling equipment you’re testing. Each blower has a different motor horsepower, blower wheel size, and airflow rating. If you use the wrong chart, your plotted fan airflow will be incorrect. This is an easy mistake to make — I’ve done it more times than I care to admit.

Most manufacturer fan tables are found in the installation instructions or on the blower door. A quick Google search also will help you locate the fan table if it is missing from either of these locations.

Don’t waste a lot of time searching for the manufacturer fan table. If it takes more than two minutes to find it, use a generic fan table to get you close. Email me for two National Comfort Institute (NCI) generic fan tables that will help you when you can’t locate the manufacturer’s information.

Here’s how to plot fan airflow on the appropriate manufacturer fan table using the operating fan speed and measured TESP.

Locate the air-handling equipment’s model number on the fan table. Some model numbers are similar, so be careful. Once you have the correct model number, locate the fan speed in use and lightly circle it.

Next, find the column that is closest to your measured TESP and lightly circle it. You should have two points circled on the fan table at this point.

Line up your measured TESP and the fan speed being used. The point where they intersect is the airflow amount the fan is moving. Record this reading so you can diagnose the system and compare it to your required airflow.

A properly operating fan should deliver ±10 percent of required airflow near the equipment’s maximum-rated TESP. To see how close your measured airflow is to required, you’ll need to determine the percent of required airflow.

Let’s say you’re testing a 2.5-ton system, and your measured fan airflow is 750 cfm. If you use 400 cfm per ton, your required fan airflow should be 1,000 cfm (2.5 x 400 = 1,000). To determine percent of required fan airflow, divide 750 cfm by 1,000 cfm (750 ÷ 1,000 = 75 percent).

A fan that only moves 75 percent of its required airflow needs some help. Now that you have this new knowledge, go back and look at your static pressure readings to see where the highest pressure is. You’ll need to correct this to improve fan airflow.

As you begin to research blower capacity, you’ll notice some fans rated at .50 are much stronger and can deliver airflow at twice that pressure. You’ll also find some that are weaker and can’t achieve proper airflow unless they operate below the .50 rating.

You might think this seems like a lot of testing steps at once, but it is much more intimidating on paper than it is to perform the test. With a little practice, you can measure static pressure and plot fan airflow in less than five minutes.

Implement fan airflow testing just as you would static pressure testing. Help your technicians see the opportunity it provides and how it moves them forward in their careers. Make fan airflow tests part of your maintenance calls and emergency service calls. Be sure you provide a lot of practice before going live with your customers. There is a learning curve anytime you introduce a new process, so give yourself a chance to succeed before giving up.

Develop a process to track and follow up on test results. Static pressure and fan airflow lead to profitable air upgrade and duct system optimization projects. It’s your job to make invisible air properties visible to customers and help them understand how it influences their comfort and systems.

To help you start static pressure and airflow diagnostics, NCI has a free app called AirMaxx Lite. Go to the App Store or Google Play and download yours today.

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