A Buyer’s Guide to High Efficiency Air Filters
An Overflow of Options: Energy-Efficient HVAC Filters
MERV Ratings Explained
MERV stands for the Minimum Efficiency Reporting Value of a filter. It is a method of stating the efficiency of a filter based on particle size and is determined by testing the performance of the filter when exposed to particles of a known size in the air stream.
Although we refer to air filters as being MERV rated, MERV is not a rating. Instead, it can be used as a guide to selecting filters for specific contaminants within a range of particle sizes.
For instance, a filter designated with a MERV of 12 would be most efficient at capturing particles between 1.0 and 3.0 microns in size. These would include contaminants such as legionella, humidifier and lead dust, milled flour and auto emissions.
High Efficiency Pleated Filters are the standard when it comes to most HVAC systems because they are easy to install, come in standard sizes and have a low initial cost. Unlike regular flat filters, high efficiency pleated filters contain media that is folded like an accordion, which allows for greater surface area and lower resistance to air-flow. They are available in standard size pleats or mini-pleats and offer MERV ratings ranging from 11-14.
Advantage: Energy Efficient and Environmentally Friendly – Because of their low air flow resistance, pleated filters are a good choice for applications that require high efficiency with a low initial pressure drop. In addition, many pleated filters are available as an Environmentally Preferable Product (EPP). According to the EPA, environmentally preferable products "have a lesser or reduced effect on human health and the environment when compared with competing products that serve the same purpose."
Common Applications: Commercial, healthcare, municipal, government, hospitality and residential.
Rigid Cell Filters come in a box-style format and contain either a synthetic media or fiberglass. However, synthetic media is preferred over fiberglass since the fibers can mix with air flow during operation. Rigid cell filters look similar to pleated filters but have a depth of 6-12 inches deep and are surrounded by a galvanized metal or plastic frame. They can offer up to 95% efficiency and are available in MERV ratings 11-14.
Advantage: Durability – Galvanized steel frames and diagonal supports make rigid cell filters capable of withstanding tough conditions such as variable air volume (VAV), without having a negative impact on performance.
Common Applications: Commercial and industrial HVAC systems, VAV systems, high humidity applications, clean rooms, healthcare, food service and manufacturing.
Pocket Filters (also called bag filters) are constructed of a plastic or galvanized steel frame and contain fiberglass or synthetic media. They are called pocket filters because the media is stitched or sonically welded to form pockets ranging from 10”-36” in depth. They offer MERV ratings up to 14 and efficiency up to 95%. Pocket filters are usually used as final filters in commercial applications or as pre-filters in pharmaceutical or hospital applications.
Advantage: Long Service Life – Pocket filters offer a long service life and can maintain a low pressure drop for over 8,000 hours. While service life depends on MERV rating and environmental factors, it can be extended even further with the use of a pre-filter. (4)
Common Applications: Commercial, educational, healthcare, government and manufacturing.
HEPA Filters are often used in environments where biological, chemical and radioactive particles pose a hazard, but they can also be used in commercial, industrial and residential applications. To qualify as HEPA, the filter must meet the standards set forth by the U.S. Department of Energy and be periodically tested by the Institute of Environmental Sciences and Technology (IEST) and UL for efficiency and performance. HEPA filters are not MERV rated because they exceed ASHRAE standards and must offer efficiency of at least 99.97% and up to 99.99% of airborne particles 0.3 microns in size.
Advantage: Highest Efficiency – HEPA filters are so efficient that they work better with continued use. That means that the dirtier they get, the more particles they are able to filter out. This happens through impaction – when particles that are larger than the spaces between the fibers (0.3 microns) embed themselves on the fiber as they try to make their way through the filter. As these larger particles become embedded, the space between the fibers narrow and the HEPA filter becomes more efficient.
Common Applications: Clean rooms, medical facilities, laboratories, airplanes, residential and many others.
Determining the Life Cycle of Your Filter for Greater Efficiency
Pressure Drop is a term that refers to the air pressure directly before and after the filter. A high pressure drop means that there is increased resistance to air flow, so that the air pressure on the output side of the filter is considerably lower than on the intake side.
Even with a high initial resistance, the pressure drop may eventually decrease as air pressure builds up on the intake side of the filter and your system equalizes. However, this will cause the HVAC system to work harder and longer, resulting in increased wear and tear and wasted energy.
Paying attention to the amount of pressure drop is important because “increasing efficiency without regard to pressure drop can result in dirtier air and increases in energy usage/cost. Additionally, there is an increased risk of system malfunction, compromising overall HVAC system performance caused by reduced airflow.” (3)
Life Cycle Costing: Filtering Out the Facts
The cheapest filter isn't necessarily the lowest cost filter. Life cycle costing is a tool that can help you determine the actual cost of a filter. There are 3 factors that go into the life cycle cost, initial investment and maintenance, energy consumption and disposal.
Initial Investment/Maintenance – Initial investment and maintenance account for about 18.5% of the cost to operate a filter. This upfront cost includes the price of the filter and as well as shipping costs, damage or loss during shipment, storage and the cost of labor for maintenance and installation.
Energy consumption – At 81%, energy consumption comprises the bulk of the operating cost. Therefore, using a high efficiency filter that offers low initial resistance can help reduce pressure drop and result in both cleaner air and lower operating costs.
Disposal – This is the least expensive factor within the life cycle at only 0.5%. Disposal is associated with the cost of labor to either throw away or incinerate the used filter.
Considering that energy consumption makes up the bulk of life cycle cost, purchasing the cheapest filter doesn't always make sense. A more expensive, but higher efficiency filter can help cut down on energy usage. Higher efficiency filters may also require less maintenance and the labor costs for even one less filter change per year can make a difference, especially when changing multiple filters. With higher energy costs and labor costs, that can really add up.
- Filters and Filtration Handbook by Kenneth S. Sutherland
- Life Cycle Costing of Air Filtration By B. Dean Arnold, Member ASHRAE; David M. Matela; and Alan C. Veeck, Member ASHRAE