Filters should be changed immediately if they become wet, if microbial growth is visible, or when the filters collapse or become damaged to the extent that air bypasses the media.
A differential pressure measurement device — such as a Dwyer Magnehelic® Pressure Gage — can be installed across the filter bank to identify the appropriate change-out times.
Photo courtesy Dwyer Instruments
Proper HVAC filtration is at the heart of indoor air quality (IAQ), and provides an important service in keeping indoor environments clean and free from the dust and particulates that may otherwise reduce productivity, affect the health of building tenants, or cause other potential problems.
It's important to keep in mind that filters will only support good IAQ and perform as specified when they are maintained correctly.
Proper filter maintenance is 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 it can become a breeding ground for bacteria and mold. This is especially the case in HVAC systems with acoustical duct liners, which are frequently used in air handler fan housings and supply ducts to reduce sound transmission and provide thermal insulation.
For all of these reasons and more, it's imperative to establish the appropriate filter changeout frequency. However, filters should be changed immediately if they become wet, if microbial growth is visible, or if they collapse or become damaged to the extent that air bypasses the media.
Filter Monitoring
After specifying the filter media and selecting a filter, it's important to pay close attention to proper filter installation. The goal is to avoid bypass air, which causes contamination in housings, coils, fans, and ducts. Do this by making 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.
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 bacterial growth is found.
Filter Monitoring
After filters are installed and operating, monitor and maintain them, so that they provide maximum filtration, while not overtaxing the supply fan capability, which can lead to "blow-out" situations with no air infiltration.
Regularly scheduled maintenance, or monitoring, can spot many potential problems.
For example, 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. In addition, normal eyesight can only see particle of about 40 microns in size. Therefore, a filter rated at 10 microns can look dirty, yet still have a useful service life.
As a filter loads up with particles it becomes more efficient at particle removal, but increases pressure drop throughout 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, bypassing of unfiltered air will lead to clogged coils and dirty air ducts. Flow capabilities of the system fan may also be affected. Filters also 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 bacterial growth is found.
Make the Job Easy
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 making the job of changing filers easier is to place labels on the housing units with information such as the number and type of filters, date changed, and 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, quick release and hinged access doors for maintenance are more desirable than 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 reduce the air flow to the unit, creating a negative impact on the unit's heating/cooling efficiency and energy efficiency.
Protect the HVAC System and Occupants
If schedules allow, time the filter changeouts 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 odors or emissions that may occur.
In some facilities, it's nearly impossible to time filter change-outs so they occur when the facility is unoccupied. It's therefore 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.
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.
Dave Matela, CAFS, is market manager, Kimberly-Clark Filtration Products. He may be reached at [email protected].
Give us your feedback on this article at [email protected]