Good indoor air quality (IAQ) depends on a number of factors, including effective and efficient air filtration. Filtration provides the primary defense for building occupants and HVAC equipment against particular pollutants generated within a building, as well as pollutants from air drawn into a building through the HVAC system. Careful attention to the air filtration system – including proper filter selection and maintenance – can help buildings achieve good IAQ as well as reduce HVAC system energy and operating costs. Moreover, HVAC contractors can create new revenue streams through programs focused on upgrading their customers’ air filtration systems.
Understanding Filtration Efficiency
While most HVAC service technicians don’t usually get involved with initial air filtration system design and specifications, it is helpful to have a good understanding of air filter efficiency.
Basically, filtration efficiency defines how well the filter cleans indoor air by removing airborne particles of various sizes. Low-efficiency filters are typically used to keep lint and dust from clogging the heating and cooling coils of an HVAC system. They typically offer up to 25% efficiency at removing particles in the 3 to 10 micron size range. Medium- and high-efficiency filters are typically used to remove bacteria, pollen, soot and other small particles. These filters can remove up to 95% of particles in the 3 to 10 micron size range.
Dust collection efficiency is the primary performance indicator for HVAC filters. The ASHRAE 52.2-2007 HVAC filter test standard quantifies the efficiency of filters at various particle sizes, assigning a MERV (Minimum Efficiency Reporting Value) based on the filter’s efficiency in each of three different particle size ranges (0.3 to 1 micron, 1 to 3 microns, and 3 to 10 microns). A MERV of 5 is least efficient, while a MERV of 16 is most efficient. In typical commercial buildings ASHRAE recommends a minimum efficiency of MERV 6, but recent studies suggest that a minimum MERV 8 provides good system cleanliness and efficient system operation.
Another important filter performance factor is airflow resistance, which is proportional to the energy consumed by the filter and thus affects the filter’s cost-in-use. A low airflow resistance typically translates into higher energy efficiency. A high airflow resistance means more energy is required to operate the fan unit. Keep in mind that while the purchase price and maintenance of filters account for less than 20% of the total annual costs to operate the filter, energy expenditures account for just over 80%. That means that upgrading to new, high-performance, energy-efficient filters provides sustainable costsavings over time.
Is it Time to Throw Away the “Throw-aways”?
Armed with a good understanding of filtration efficiency, HVAC service technicians are in a good position to notice when a building is in need of a filtration upgrade.
Consider recommending a filter upgrade for any commercial buildings using panel filters (often called “throw-away filters”). These filters were invented back in the 1930s, primarily to prevent lint and dust build-up on heating elements and reduce the risk of fire. Later, such filters were used to protect the cooling coils of air-conditioning systems from airborne dust, which caused fouling. Fouling reduces airflow through the HVAC system and heat transfer in the coils, which can add up to a significant increase in energy costs.
Panel filters today are made in much the same way as they were 75 years ago (a simple cardboard frame with a thick bat of spun glass filtration media and an open metal backing for support). Their simple construction and low cost results in panel filters still being a prominent filter form today.
For years, it was believed that panel filters provided adequate filtration to keep HVAC systems running cleanly and efficiently. However, a recent study1 found that panel filters often do not provide adequate protection to HVAC equipment. In fact, it found that even MERV 5 to 6 filters — a higher MERV than typical panel filters — provide “insufficient cleanliness improvement.”
Panel filters may also have IAQ shortcomings. Most panel filters have efficiency values less than 20% for particles in the 3 to 10 micron size range. That means that more than 80% of these relatively large particles plus virtually all of the smaller particles (0.3 to 3.0 microns) in the air pass through without being captured.
One of the easiest ways for HVAC service technicians to help their customers improve their HVAC performance and sustainability via superior IAQ and reduced HVAC system operating costs is to upgrade from panel filters to high-efficiency pleated filters. Because pleated filters have an extended surface area, filter efficiency can be increased (higher-quality pleated filters are available with filtration performance up to MERV 12) and filter life can be extended without a significant increase in airflow resistance.
Upgrading from panel filters to pleated filters can also provide life cycle cost-savings advantages to customers, once you get them to look beyond the higher purchase price of pleated filters. Routine HVAC maintenance costs are reduced because there is a less frequent need for fan and coil cleaning. Energy costs are also reduced, because cleaner HVAC system components help the overall system to maintain efficient operation, thereby preventing energy and maintenance costs from rising.
Effects of Bypass Air
When upgrading from panel filter to pleated filters, or when upgrading and/ or changing filters in general, it’s important to pay close attention to proper filter installation to avoid unfiltered bypass air. Bypass air is a condition in which air flows through the system without passing through the filter. It occurs when filter media is not properly sealed in the filter frame, when filters are not properly installed (see sidebar) and gasketed in filter racks, or when air handler doors and ducts are not properly sealed.
Bypass air can also cause fouling of HVAC coils and fans and can affect IAQ by reducing the performance of the filters purchased, increasing the amount of airborne contaminants reaching the building occupants.
Improper filter installation and poor gasketing creates gaps around the filters in HVAC systems. On the surface, the gaps may seem small and insignificant. In reality, even small gaps can have a surprising effect on filter performance. For example, a mere 1 mm gap in the installation of a MERV 15 filter can reduce its efficiency to MERV 14. A gap of 10mm can decrease performance all the way down to MERV 8. Because higher efficiency filters also typically have a higher resistance to airflow, bypass tends to have a larger effect on high performance filters. The amount of dust built up on the filter also has an effect on bypass flow. The smallest bypass flow occurs when a filter is clean and can increase by as much as 10% when filters are dirty.2
Even moderate amounts of filter bypass can dramatically increase HVAC heat exchanger fouling 3, and fouled heat exchangers have diminished heat transfer performance and increased resistance to airflow, leading to significantly increased energy use and decreased heating and cooling performance.4
The Maximum Benefit