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Getting Filter Sizing Right in HVAC Systems

WeSupplyTrades Team06/24/2025


Choosing the right air filter for a system isn’t just about picking one that fits the dimensions of the return. It’s about understanding how that filter performs under the system’s airflow conditions. Oversights here can lead to restricted airflow, poor system performance, and unnecessary service calls.

Filter Fit Goes Beyond Dimensions

A common misconception in the field is that filter selection can be generalized—but airflow requirements vary from one system to the next. Each HVAC setup has a target air velocity, and filters are engineered to perform best within specific velocity ranges. Matching the filter's capabilities to the system's air movement is essential. For instance, the Breeze33 16" x 25" x 5" Replacement Media Air Filter is designed with a MERV 11 rating, suitable for systems requiring efficient filtration without significant airflow restriction.

Matching Filter Specs to System Velocity

To size a filter correctly, start by calculating the system's velocity using a duct calculator. If your filter is designed for optimal filtration at 400 feet per minute (FPM), but the return duct is pushing 800 FPM, adjustments are needed. One proven method is to widen the ductwork where air enters the filter, which slows the velocity through the media. After the filter, the duct can be reduced again, returning airflow to the proper speed for system circulation.

This approach creates a controlled drop in velocity where it matters most—across the filter face—so the system performs efficiently without sacrificing air quality.

For systems requiring higher filtration efficiency, the Purolator Puro-Green 20" x 25" x 2" Pleated Filter offers a MERV 13 rating, balancing superior particle capture with airflow considerations.

High Efficiency Can Come at a Cost

It’s easy to think that the higher the MERV rating, the better the filtration. But tighter filters can also restrict airflow, especially in systems that weren’t designed with added resistance in mind. That’s why it’s critical to consider both filter efficiency and how much static pressure it adds to the system. If airflow drops too much, equipment may short cycle or fail to reach set temperatures.

At WeSupplyTrades, we stock a range of filters that meet the performance demands of today’s HVAC systems. Whether you’re designing new installs or troubleshooting an airflow issue, we’ve got the product selection—and the technical know-how—to support your work in the field.

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Contact us at [email protected] with any questions.Â