There are usually three types of filters within an air handling unit:
Primary filters are typically panel filters that have a pleated design. These pleats increase the filter’s surface area, allowing it to catch more dust. By collecting dust before it travels further into an AHU, primary air filters also prevent dust buildup from occurring on mechanical components such as motors, fans, and cooling coils, and in ductwork.
Where primary filters act as the first line of defense against dust, secondary filters are used after them to capture finer particles like bacteria and pollen, as well as any remaining dust. Secondary filters are generally filters that protect the higher-cost final filters and have an efficiency rating of MERV 11 -16. The higher the MERV value, the more efficient the filter will be at trapping airborne particles.
Both primary and secondary filters protect higher-cost final filters used when having clean air is critical. Final filters are designed to trap microscopic airborne particles and contaminants from an air stream. You can expect final filters to be HEPA, or ULPA filters, which all serve the same function but deliver varying levels of filtration efficiency and performance.
As filters become filled with dust, or dust accumulates in the system’s ductwork, pressure resistance increases. This resistance causes the HVAC fans and motors to work harder, wasting energy and leading to wear on the mechanical system. The relationship between air filters, energy efficiency, and HVAC mechanical components is a delicate one, so your filters must not fail. Here we examine some of the most common reasons that filters underperform and ways you can avoid unnecessary problems.
Common reasons why filters fail
1. Selecting the wrong filter for the intended application or HVAC system.
When selecting a filter, it helps first to define the overall goal you are trying to achieve. Say you are trying to provide clean air for patients and employees. That would likely require using a filter with a Minimum Efficiency Reporting Value (MERV) of 14 or higher. The higher the MERV value, the more efficient the filter will be at trapping airborne particles. Great, you’ve narrowed that down. But there is often a tradeoff with using higher MERV level filters. While they produce cleaner air, they may also require a stronger fan and more energy to push the air through them. High-efficiency particulate air (HEPA) filters have a high filtration efficiency and are most often used in hospitals and medical facilities. Always consult with your system’s manufacturer to determine the pressure difference across the filters in your AHU. Specifying high-efficiency filters will help create a lower pressure drop, making it easier for equipment to overcome these demands.
The bottom line is that you should specify filters that meet required filtration levels and system configuration needs. Choosing the right filter will yield a lower overall cost of ownership and require less frequent change-outs.
2. Not paying attention to the environment as it relates to filter construction.
You have to know what elements your AHU filters might be exposed to at any given time. In agriculture buildings, for example, filters need to withstand extreme temperatures, wind and snow. In a commercial kitchen, humidity would be a factor to consider. Having this knowledge will help you determine what frame construction (paper vs. metal vs. plastic) or media technology (fiberglass, washable, antimicrobial, etc.) will be most appropriate.
3. Getting the size, seal or installation wrong.
HVAC filter size is measured using length, width and depth. While all manufacturers offer standard sizes, some will create custom sizes with dimensions to fit individual units or filter banks. Using incorrectly sized filters or filters that are improperly installed with faulty seals or gaskets can cause gaps. Even small gaps that may seem insignificant can permit a bypass of dirty air, decreasing the performance of the rated filter and the HVAC system. Always install a filter according to the air flow direction indicated on the frame. Check to be sure the filter is properly seated in the housing, and pull any clips or fasteners tight to ensure a proper seal. For filter banks, make sure the mounting is structurally sound and will not collapse or weaken the filters under stress.
4. Improper handling and storage.
The way a filter is handled and stored prior to installation matters immensely. Filters have fine structured media fibers, so care must be taken not to drop, touch or puncture them. A bent frame can mean the difference between a good and bad seal. The correct way to store air filters is in the upright position, with the pleats running vertically. Never store air filters flat on the ground or in a damp environment.
5. Irregular changeouts.
Regularly scheduled filter maintenance is crucial to achieving desired IAQ, reducing additional equipment maintenance and downtime, and extending the life of HVAC components. After selecting and installing the proper air filters, they should be monitored to ensure pressure drops are within established limits and to address any other filter issues that could overtax the system. Whether you track filter change-outs manually or use a computerized maintenance management system (CMMS), keeping accurate service records can assist with HVAC system warranty claims.