Filter Maintenance

Maintain HVAC air filters for optimum IAQ and lower operating costs.

By Robert Martin, CAFS

Effective HVAC filtration helps keep indoor environments clean and free from the dust and particulates that may reduce productivity, affect the health of building occupants or cause problems in the HVAC system. However, it is important to remember that air filters will only support good indoor air quality (IAQ) and perform as specified when they are maintained correctly.

Proper filter installation and maintenance are crucial to keeping HVAC ductwork clean. If dirt accumulates in the ductwork, and if the relative humidity reaches the dew point so that condensation occurs, then the ductwork can become a breeding ground for bacteria and mold. 

For all of these reasons and more, it is important to establish the appropriate filter change-out frequency. However, if a filter becomes wet, if microbial growth is visible on the filter, or when a filter collapses or becomes damaged to the extent that air bypasses the media, you should consider changing out the affected filter(s) as soon as possible.

Bypass Air

When installing filters, the goal is to avoid bypass air, which causes contamination in HVAC housings, coils, fans and ducts. Such contamination increases operating costs through inefficient operation and increased maintenance. Bypass occurs when filter media is not properly sealed in the filter frame, when filters are not properly installed and gasketed in filter racks, or when air handler doors and ducts are not properly sealed.

For high efficiency filters, small gaps around the filter or filter housing can decrease filter performance and affect IAQ, and large gaps substantially decrease filter performance. For a 1 mm gap, bypass flows may be less than 5% of the total flow; for a 10mm gap, bypass flows can increase to 25% to 35% of the total flow. Because higher efficiency filters also typically have a greater airflow resistance, bypass air tends to have a larger effect on high performance filters.1

The net result of bypass air is a reduction in MERV (Minimum Efficiency Reporting Value – a filter rating system based on the filter’s ability to remove airborne particles of different sizes). For example, models have shown that a 1mm gap causes a MERV 15 filter to perform as a MERV 14 filter, while a 10mm gap (slightly larger than the diameter of a pencil) results in the MERV 15 filter performing as a MERV 8 filter.1

Reduced filtration efficiencies may allow harmful particles to enter the breathing air. Lung-damaging dust can be as small as 0.5 microns, making high filtration efficiencies critical to providing for safe and healthy indoor air.

Filter Installation

The best way to avoid bypass air is to make sure that all the air in the system goes through the filter. To avoid problems later on, consider these tips:

• Check for filter media damage such as rips or holes and replace damaged filters.

• Make sure media is sealed in the frame to avoid bypass air.

• Install the filter according to the air flow direction indicated on the frame.

• Ensure that the filter fasteners are in place and correctly installed, especially if filters are serviced from the downstream side.

• Check to ensure that the bank of filter frames is rigid and well reinforced to avoid collapse.

• Caulk any cracks between filter frames or between the bank of frames and the duct wall to prevent leaking of unfiltered air.

• Pay special attention to filter holding frame seals, gaskets and filters that don’t match the filter holding frame size — all of which can cause bypass air.

Filter Monitoring

After filters are installed and operating, they should be monitored and maintained to provide maximum filtration, while not overtaxing the supply fan capability, which can lead to filter “blow-out” situations with no air filtration. Scheduled maintenance or established pressure drops can be determining factors, along with specific variations in environmental conditions such as humidity and seasonal changes.

A differential pressure measurement device can be installed across the filter bank to identify the appropriate change-out times. Pressure drop switches may be used to provide an alarm input to a Building Automation System that alerts operators to change the filter. Don’t rely on a visual inspection alone, since medium- and high-efficiency filters that appear “dirty” often have not reached their optimum efficiency levels due to depth loading. In fact, filters should appear dirty; this is a good indication they are doing their job.

As a filter loads up with particles, it becomes more efficient at particle removal, but increases pressure drop through the system, thus reducing air flow while consuming more energy. All filters — if loaded to excess — will become deformed, unload dust and even “blow out” of their filter rack. When filters blow out, bypass of unfiltered air can quickly lead to problems such as clogged coils and dirty air ducts. Flow capabilities of the system fan may also be affected.

Filters should be monitored for bacterial growth. It’s a good idea to remove selected filter elements periodically and send them to a lab for testing when bacteria growth is found.

Make the Job Easy

Using an HVAC filter that has depth-loading media with a density gradient structure can help to reduce air flow resistance, enhance dust loading and prevent face loading of the filter — all of which help make maintenance easier and less time-consuming. When choosing pleated filters, select those that contain an expanded metal backing, which keeps the pleats and media stable during use, especially when the filter begins to load.

Some filter manufacturers use a two-color filter media construction to help in installation by making it easy to see that, for example, the gold side faces upstream while the white side faces downstream.

Another tip for easier filter maintenance is to place labels on the housing units with information such as the number and type of filters, date changed, as well as initial and final pressure drop. Air handlers that are located in difficult-to-access places will be more likely to suffer from poor air filter maintenance as well as overall poor maintenance. Therefore, consider making appropriate equipment changes such as quick release and hinged access doors versus more time-consuming bolted access panels when security is not an issue.

Check the Details

When changing the filter, make sure that the replacement filter is of the correct size and compatible with your housing. Review the performance value of the filter to ensure the pressure drop across the filter will not be too great, especially as the filter loads. The greater resistance will negatively impact the unit’s heating/cooling efficiency and energy efficiency.

Protect the HVAC System and Occupants

If schedules allow, time filter change-outs so they occur when the facility is unoccupied. This will help to avoid problems associated with disruptions in the HVAC system and possible distribution of particulates that may occur as dust-loaded filters are disturbed.

If it’s not possible to time filter change-outs so they occur when the facility is unoccupied, it is critical to turn off the supply fans to prevent debris from entering the ductwork downstream of the filters. Similarly, the entire filter area should be cleaned and washed down while fans are off. Use a clean rag instead of compressed air to wipe dust from the inside of the filter housing and around gasket surfaces. When removing a used filter, take care to avoid dropping contaminants in the ductwork.

Delayed Maintenance

Facility managers may be tempted to delay filter change-outs in an attempt to save money. However, the small amount of money saved by reducing air filter purchases pales in comparison to the energy and operating costs consumed due to increased air flow resistance from dirty filters. It doesn’t take long for peak energy use cost to offset any savings in filter purchases.

At the end of the day, it is important to follow the recommendations of the filter manufacturer to determine the proper procedures and frequencies for maintaining and changing filters. And don’t forget to fully document all inspections and corrective actions.

1  Ward, M. and Siegel, J.A., “Filter Bypass: Implications for Filter Efficiency.”  ASHRAE Transactions.  111(2), 1091-1100.




For further help and guidance on HVAC filter installation and maintenance, the National Air Filtration Association offers several resources, including a manual titled Installation, Operation and Maintenance of Air Filtration Systems (3rd Edition, 2012), the NAFA Guide to Air Filtration (5th Edition, 2014), and the NAFA Certified Technician (NCT) Program, which is available to service companies and in-house HVAC professionals. For more information on these programs, visit


— Robert Martin is an associate category manager with Kimberly-Clark Professional Partnership Products, where he manages the company’s product portfolio for the filtration category. He is a member of ASHRAE and NAFA and is a Certified Air Filter Specialist. The author can be reached at

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