Anyone who has attempted to measure the pressure drop of a filter mounted in a filter grille will admit it’s a challenge. Some installations have a significant amount of return duct between the filter grille and the air handling equipment while others are mounted on a platform with no duct.

With all the variations, how can you accurately measure this pressure drop without going through a lot of trouble? The solution might be easier than you think. Let’s look at what you’ll need, common mistakes to avoid, and how to measure correctly.

Test instruments and accessories: To measure filter pressure drop in a filter grille, you’ll need the following:

• A manometer;
• One length of tubing with a 3/16-inch diameter (Neoprene works well);
• One static pressure tip; and
• Scratch awl or thermostat screwdriver.

Keep the filter grille closed: Many technicians try to open the filter grille access panel and place a section of tubing behind the filter to obtain the pressure drop reading. This method is very awkward and doesn’t consider the true reading across the air filter.

Unfiltered: When filter grille face velocity is excessive, the filter can be pulled into the duct system, allowing unfiltered air into the system.

When tubing is placed behind the air filter, it creates a large gap between the filter grille and the filter. The gap allows air to bypass the filter and reduces the normal operating pressure drop. Avoid using this method and keep the grille closed.

Don’t remove the filter: Another way technicians measure filter grille pressure drop is to remove the filter from the grille.

In this approach, the technician first measures return pressure with the filter mounted in the grille. Next, they take a second return pressure measurement with the filter removed. The two readings are then subtracted to determine filter pressure drop.

Push Through: Piercing a static pressure probe through the filter grille and filter allows you to take a direct measurement of their resistance.

This may sound good, but if the system uses a constant speed fan, you will calculate an inaccurate filter pressure drop. This is because removing the filter decreases total external static pressure. Remember this rule — as static pressure decreases, fan airflow increases. Once the filter is placed back into the grille, the fan must overcome the added resistance of the filter. Total external static pressure increases, and fan airflow decreases. The readings are not taken under similar conditions, and fan airflow is different.

Measure pressure drop directly: When measuring a system, it’s best to test it as found and alter it as little as possible. This enables you to get a true feel for how the system normally operates. To keep from altering system conditions, pierce a static pressure probe directly through the air filter and filter grille at the same time. Depending on the filter type being used, you may need to poke a small hole in the filter with a scratch awl or thermostat screwdriver. Don’t worry about doing this. The media typically reseals itself. This method lets you see the air filter pressure drop on the manometer display. No math is needed when the test is performed as described above.

Measuring multiple filter grilles: You’ll encounter many systems with multiple filter grilles spread out across a building. The same method described earlier can be used to measure each one individually.

The filter grille with the highest pressure drop is probably pulling the largest amount of air. The grille with the lowest pressure drop is likely pulling the smallest amount. If you’ve ever encountered a system where one filter grille gets dirty very quickly and others never need to be changed, this test can point you to the reasons why. Airflow measurement with a balancing hood is a companion test to verify the amount of air moving through each grille.

Diagnosing filter grille pressure drop: A rule of thumb we teach at National Comfort Institute (NCI) to help determine if an air filter is too restrictive is based on pressure budgets. Pressure budgets take the maximum total external static pressure for the air handling equipment and proportion it across system components.

Ideally, filter pressure drop should not exceed 20 percent of the air handling equipment’s maximum rated total external static pressure. This rating is typically found on the indoor nameplate.

Here’s an example: If the air handling equipment is rated for .50 inches of water column (w.c), which classifies most residential equipment, multiply the equipment’s maximum-rated static pressure by 20 percent or .20 to determine the filter pressure drop budget. In this example, filter pressure drop should not exceed .10 w.c. If filter pressure drop exceeds .10 w.c., changes are needed for the filtration system to work properly. This is often achieved by adding another filter grille or using a less restrictive media. If filter pressure drop is very low, you need to investigate further. Look for signs of air bypassing the filter or a poorly-built return-duct system.

Filter grille sizing: Testing often reveals the need to increase the size of a filter grille. To keep noise down and filtration effective, the maximum face velocity of a filter grille should not exceed 400 feet per minute (fpm).

An approximate rule of thumb to use when filter grille engineering data is not available is to multiply the filter grille area (height x width) in square inches times 2 cfm per square inch. This sizing method should keep the face velocity of the filter grille at an acceptable level.

For example, if you have a 20-by-20 filter grille that you want to pull 1,000 cfm through, you can use this equation to verify it’s properly sized:

20 x 20 = 400 square inches

(400 square inches) x (2 cfm per square inch) = an airflow capacity of 800 cfm

The 20-by-20 is too small to properly return 1,000 cfm. To pull 1,000 cfm through the filter grille, you would need to increase its size to 20-by-25.

25 x 20 = 500 square inches

(500 square inches) x (2 cfm per square inch) = an airflow capacity of 1000 cfm

Don’t forget the importance of duct sealing with any filter grille improvements. Air takes the path of least resistance and will bypass a filter grille anytime leakage exist between the filter and equipment.

If you’re an HVAC contractor or technician interested in learning more about air upgrade opportunities, contact David Richardson at
davidr@ncihvac.com or at 800-633-7058. NCI’s website, nationalcomfortinstitute.com, has free technical articles to help you improve your professionalism and strengthen
your company.

Publication date: 1/22/2018

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