When indoor air quality declines or energy costs rise, filters are often the hidden cause. For HVAC contractors and facility managers, diagnosing filtration issues accurately protects both performance and budgets. To make this process easier than ever, we've created a guide on how to identify the signs, confirm root causes and restore systems to optimal operation. Thank us later!

Start with the Data

Before opening a single filter bank, start with the records.

Pull recent work orders, differential pressure logs and automation trends. Look for patterns: faster-than-normal loading, unstable fan speeds or repeat comfort complaints in the same zones.

Then benchmark your system. Record each filter's clean pressure drop and final resistance at rated airflow. If no data exists, establish a baseline during your next changeout.

Finally, inspect the installation. Photograph gaskets, housings, frames and note upstream and downstream pressure before removing anything. Small details now save confusion later.

When Airflow Drops: High Pressure or Starved Zones

When airflow drops in high pressure or starved zones, here's what you'll notice:

• Fans running at maximum speed
• Inconsistent temperatures
• Whistling at grilles or doors

Likely causes include loaded filters, missing prefilters, restrictive media or iced coils in winter. Fortunately, there are some quick ways to remedy these issues:

1. Firstly, check pressure across each stage and compare against baseline readings.
2. Secondly, examine your coils for matting or frost and confirm damper positions.

This isn't an issue you should ignore and the research backs it up. According to the U.S. Department of Energy, restrictive filters allow debris to foul coils, which reduces heat transfer efficiency and drives power use even higher.

Short Filter Life

Premature loading usually stems from missing prefilters, uneven airflow or outdoor contaminants. Construction dust, pollen and seasonal smoke events are common culprits.

Our advice is to log pressure data to confirm loading rate, then check velocity distribution. If one side of the filter loads faster than the other, airflow is unbalanced. Once you've done that, adjust blank-offs or turning vanes and verify that dampers are aligned.

If performance still suffers, upgrade to a higher dust-holding-capacity media at the same MERV level. The additional surface area can extend service life without increasing pressure.

IAQ Complaints with “Good” Filters

When occupants report odors or irritation but readings look normal, you're likely dealing with bypass.

Check for dust streaks behind filters and gaps along gasket edges. Use a smoke pencil or incense stick to trace leaks. Verify that filters fit correctly - even a small gap compromises performance.

It's also worth confirming that your efficiency rating meets the space requirement. ASHRAE recommends a minimum MERV 13 in most occupied spaces and higher where infection control or vulnerable populations are a concern.

Five Fast Tests for Field Diagnostics

When you need clarity fast, these checks identify most filter issues:

• Pressure differential: Measure pressure before and after each filter stage. Large jumps reveal early loading or missing prefilters.
• Face velocity: Traverse the filter face with a pitot grid. Variations above 20% cause uneven loading and short life.
• Bypass test: Use smoke around frame edges to detect leaks. Tighten clamps and replace gaskets where needed.
• Media integrity: Inspect pleats for collapse, oil contamination or sagging. Verify that the media matches the system's humidity conditions.
• Coil cleanliness: If pressure is high but filters look clean, inspect coils. A fouled coil often mimics filter restriction.

When It's Time to Upgrade

Sometimes the issue isn't failure - it's specification. If your facility's needs have changed or air quality goals have increased, a higher MERV filter may be appropriate.

Before upgrading, measure available static pressure and consult fan curves. If capacity is limited, deeper-bed or V-bank filters can achieve better performance with minimal strain. Installing differential pressure sensors allows performance-based changeouts rather than fixed schedules - improving cost control and system efficiency.

Of course, there are a number of preventative measures that really pay off, helping you avoid a potentially costly upgrade:

• Instrument and track readings: Add differential pressure taps across each filter bank and trend them in your BMS. Set alerts just before the final resistance to prevent surprises.
• Stage your filtration: A prefilter protects higher-efficiency filters and extends lifespan.
• Seal it tight: Keep racks square, gaskets fresh and clamps secure. Bypass is the most common hidden loss point.
• Adjust for seasons: Wildfire smoke, pollen or cold weather can alter loading. Tune changeouts accordingly.
• Document every reading: Ask technicians to log clean and final ΔP during each visit. That data builds a reliable forecast for future maintenance cycles.

The Bottom Line

Filter issues don't stay isolated. They ripple through comfort, energy use and even occupant health.

By diagnosing based on measurement rather than assumption, contractors and facility managers can extend system life, reduce operational costs and maintain steady indoor air quality. Attention to these fundamentals is what separates reactive maintenance from true system reliability - and it starts with reading the filters, not just replacing them.

Upgrade to Brookaire Today

If your filters aren't performing as they should, it may be time to upgrade. Brookaire supplies contractors and facility teams with high-efficiency filters built for durability, energy savings and consistent airflow. From custom sizing to job site delivery, we make upgrades simple and reliable. Reach out today to discuss your system requirements and see how Brookaire can help you achieve cleaner air and smoother operations across every site.