Cleaning & Maintenance Management Online

Breathing easier

September 19, 2010

Proper HVAC air filtration is at the heart of good indoor air quality (IAQ) for the buildings and facilities you clean and maintain. Filters help keep both the HVAC system clean and the indoor environment free from dust and particulates.

When proper attention is not paid to filter installation and maintenance, air can flow through the HVAC system without passing through the filter. This condition is known as bypass air.

Bypass occurs when filter media are 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.

Bypass air can cause fouling of HVAC coils and fans, which can increase operating costs through inefficient operation and increased maintenance for your staff.

It can also affect IAQ by reducing the performance of the filters purchased, increasing the amount of airborne contaminants reaching the building occupants while increasing the workload of your cleaning and maintenance staff.

Air filters and IAQ
Some buildings, such as hospitals, laboratories and food processing plants have specific filtration requirements dictated by the activities that take place there.

In the absence of a use-specific filtration requirement, a good rule of thumb is to install the highest efficiency air filter the HVAC system is capable of handling.

Air filter performance is quantified by the ASHRAE 52.2-1999 standard from The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE).

ASHRAE 52.2-1999 measures filtration efficiency – the filter’s ability to remove airborne particles of various sizes.

A MERV (Minimum Efficiency Reporting Value) is assigned to the filter, with a MERV of 5 being least efficient and a MERV 16 being most efficient.

In most commercial office buildings, ASHRAE recommends a minimum filter performance of MERV 6.

Recent studies suggest that a more realistic minimum efficiency is MERV 8 to provide good system cleanliness and efficient system operation.

ASHRAE 52.2-1999 also measures pressure drop, or how much airflow restriction is created by the filter.

A low pressure-drop typically translates into better energy efficiency. A high pressure-drop means the HVAC system has to work harder and use more energy to move air through the building.

HVAC airflow also can be restricted if heating and cooling coils become dirty, creating turbulence.

Low-performing HVAC filters with efficiencies below MERV 4 trap less than 20 percent of the large particles that foul coils. High-performing MERV 8 filters trap more than 70 percent of these contaminants and keep systems substantially cleaner and operating more efficiently.

Effects of bypass air
Improper filter installation and poor gasketing create gaps around HVAC filters, allowing air to bypass the filter. 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 1mm gap in the installation of a MERV 15 filter can reduce its efficiency to MERV 14. A gap of 10mm can decrease performance down to MERV 8.

Because higher efficiency filters typically have a higher pressure drop, 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 percent when filters are dirty.

Even moderate amounts of filter bypass can dramatically increase HVAC heat exchanger fouling. Fouled heat exchangers have diminished heat transfer performance and increased pressure drop, leading to significantly increased energy use and decreased heating and cooling performance.

In addition, respirable particles are not appreciably removed in the filter gap, which means that bypass air is detrimental to the IAQ in the buildings and facilities you maintain.

Filter installation and maintenance
Filters will only do their job and perform as specified when they are installed and maintained correctly.

To avoid bypass air and make sure that all the air in the system goes through the filter, consider these installation tips:

  • Before installation and periodically during operation, visually inspect filters and replace ones that are damaged.
  • Install the filter according to the air flow direction indicated on the frame.
  • Make sure that all filter housings have good filter gaskets, preferably with a non-porous gasketing material.
  • Properly seat filters in the filter housing or channel.
  • 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 banks and the duct wall to prevent leaking of unfiltered air.
  • Make sure all air handler entry doors are gasketed and tightly sealed.

The pros can help
Many filter distributors in North America are members of the National Air Filtration Association (NAFA) and have completed training to receive a Certified Air Filter Specialist (CAFS) rating.

These distributors are trained to help maintenance professionals select and install the right filter.

For more information on finding a NAFA-trained filter distributor, visit

Ronald Cox is a certified air filter specialist and a member of the National Air Filtration Association. He is also a Kimberly-Clark market manager for air and liquid filtration products. He has held membership positions in the American Filtration Society (AFS); the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE); and the Association of the Nonwovens Industry (INDA). He can be reached through Kimberly-Clark Filtration Products at