Measure and calculate the proper size and then verify the design It’s often taught that diagnosing a restrictive air filter is as simple as a visual inspection to see if it’s plugged. And, if the filter is dirty, replace it. Once this obvious defect is fixed, everything is assumed to work correctly. Could there be more to it?

What if the new air filter you installed is perfectly clean and still causing airflow issues? How should a technician test, diagnose, and solve this problem?

Measuring the pressure drop over an air filter reveals valuable clues to its influence on a system. This simple test will help a tech determine whether the filter is adequate for the system it is installed in.

Filter pressure drop testing enables a tech to effectively evaluate the type of filter needed to match the system. This testing will identify which type of filter to use, its surface area, and the filter rack best suited for the application. Let’s look at how to test, diagnose, and correct a restrictive air filter.


To measure air filter pressure drop, a tech will need a quality static pressure kit to obtain pressure measurements before and after the filter. A good static pressure kit should have the following test instruments and accessories in it:

• An analog or digital manometer;

• 3/16-inch inner diameter (ID) test hoses;

• Static pressure tips;

• A 3/8-inch drill bit with protective sheath; and

• 3/8-inch test port plugs.

Pressure testing always begins with a visual inspection of the equipment and installation to head off any potential issues. Always check before drilling to ensure the drill bit doesn’t puncture a drain pan or refrigeration tubing.

Once the inspection is complete, begin testing. To measure filter pressure drop, follow these five simple steps:

1. Install a test port into the duct or equipment on the entering airside of the filter by drilling a 3/8-inch test hole. This will allow the tester to execute a pressure measurement before air enters the filter.

2. Install another test port into the duct or equipment on the leaving airside of the filter by drilling another 3/8-inch test hole. This will allow the tester to execute a pressure measurement after air leaves the filter.

3. Set up a manometer there and monitor the display as the pressure is measured. If the gauge is of the analog variety, assure it is level. Attach a test hose to each pressure tap on the manometer and insert a static pressure tip into the opposite end of each of these hoses.

4. Insert the static pressure tips into the 3/8-inch test ports that were drilled on each side of the air filter. Be sure to face the static pressure tips into the airflow.

5. The live pressure drop over the filter will now appear on the display of the manometer. Record the reading to diagnose filter pressure drop. Place test plugs in the test ports when finished.


Once you’ve obtained filter pressure drop readings, determine if those readings are acceptable. To see if the filter is too restrictive, use a rule of thumb the National Comfort Institute (NCI) teaches based on the fan’s rated static pressure. Ideally, filter pressure drop should not exceed 20 percent of the fan’s rated maximum static pressure. You can find this fan rating on the air-handling equipment’s data plate, often located on the door or inside the cabinet.

Let’s say you’re testing filter pressure drop on a gas furnace that’s maximum rated static pressure is 0.50 inch of water column (wc). Multiply the maximum rated static pressure of the fan by 20 percent, or 0.20, to come up with the ideal filter pressure drop. In this example, the filter pressure drop should not exceed 0.10 inch of water column (0.20 x 50 percent = 0.10 inch of wc).

If filter pressure drop exceeds 20 percent of the fan’s rated pressure, some changes will probably need to be made to the system in order for it to work properly. If filter pressure drop is extremely low, look for signs of filter bypass or a poorly constructed filter rack assembly.


By measuring filter pressure drop, the odds are pretty high these calculations will uncover a filter issue in the first two HVAC systems that are tested.

Here are some options that can reduce the pressure drop across the filter.

The easiest option is to remove the existing restrictive filter and replace it with one that has a lower pressure drop, such as fiberglass. This can immediately solve the problem and drastically improve system performance and comfort. Be sure to measure the new filter pressure drop after the old filter has been replaced to ensure it worked as intended.

There will be instances that the solution isn’t this simple and the filter surface area will need to be increased to reduce the pressure drop. The most common ways to do this include:

• Installing a custom filter rack;

• Installing filters in a V-shaped pattern;

• Adding a return drop with an additional air filter; and

• Adding multiple return air filter grilles.

These options will often solve the problems associated with high filter pressure drop. When the filter surface area in a system is doubled, the pressure drop over the filters will typically reduce by more than 50 percent. The filter will need to be resized when employing these options, as well. This can be calculated in three steps.


To resize an air filter, the tech will need to know the recommended air velocity through it. This measurement is specified in feet per minute (fpm). The manufacturer’s specifications are the source of this information but can be difficult to find. When unavailable, use these typical filter velocities that have proven to work well in the field based on actual testing.

Typical filter velocities (in fpm) found across the industry include: fiberglass, 400-500; electronic, 350-450; hog hair, 350-400; washable, 350-450; 40 percent pleated, 300-350; electrostatic, 200-300; 90 percent media, 150-200; and HEPA, 200-250.


Calculate the square feet of filter area needed for the HVAC system using the following formula: Required equipment airflow (cfm) ÷ filter velocity = square feet of filter needed.

If the system is 3.5 ton, take the required airflow (in this example, we’ll use 400 cfm per ton) and divide by the necessary velocity across the air filter. In this example, we’ll aim to maintain a filter velocity of 400 fpm at 1,400 cfm. To figure square feet of filter area, the formula would appear as follows: 1,400 cfm ÷ 400 fpm = 3.5 square feet of filter area needed.

To achieve 400 fpm filter velocity at a fan airflow rate of 1,400 cfm, there needs to be 3.5 square feet of filter area. Next, the size of the filter/filters will need to be calculated.


Calculate the size of the filter/filters in square feet using the following formula: Length x width ÷ 144 = square feet of each filter.

Let’s say there’s only enough room to install an air filter that is 14 by 20 inches in size. The tech needs to figure out the size of the 14-by-20-inch filters in square feet. Plug in the filter dimensions into the formula and do the math: 14 by 20 inches is equal to 280 square inches ÷ 144 square inches = 1.94 square feet.

Now, the tech has the size in square feet of the filter he or she will be using. Next, this size will need to be compared against the required amount of square feet that was determined back in step one to determine how many filters of this size will be needed.


Calculate the number of filters needed using the following formula: Square feet of filter required ÷ Square feet of each filter used = number of filters.

From the information found in steps one and two, you can complete the formula in the example: 3.5 square feet of filter needed ÷ 1.94 square feet each = 1.8 filters needed.

Round up to two 14-by-20-inch filters for this 3.5-ton system. Since two filters are needed to obtain adequate filter surface area, they would need to be installed in a V-type installation or upon multiple filter grilles.


After a filter modification is completed, verify the pressure drop of the new filter(s) at the designed velocity and airflow. Remember, design is only an estimate of what should happen under live operating conditions. Test and verify to ensure the design actually does what was intended. Anything else is a guess.

Some systems won’t be repaired easily; this is a reality of working in the field. Don’t let that discourage the company from offering what customers need and want.

Publication date: 3/28/2016

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