Know The Design Intent
These days, a growing range of sources offer information on air filters for commercial and institutional buildings. This growing attention is understandable, given the importance of IAQ in the design and operation of today's buildings and the major role that filtration plays in maintaining an acceptable indoor air environment.
Technical organizations and HVAC equipment manufacturers are devoting significant resources to IAQ in buildings where people live and work. Engineering and maintenance managers must understand not only the importance of filtration on IAQ but also how to apply available filtration technology in the most cost-effective ways for their buildings. Energy and total costs for filter replacement often are $0.70 to $1.80/person/month, according to one industry source. This statistic is valid for a wide range of filtration efficiencies, and the cost is relatively insignificant compared to the costs associated with building occupants, such as salaries, rent, and health insurance.
So it pays to know the original design intent of the filtration design and to follow that design intent as closely as possible. If significant changes occur in the use of a building, managers must re-evaluate the design intent and change or adjust it to meet the needs of the building and its occupants.
GETTING STARTEDIt is important for managers and building owners to communicate and agree on the objectives of a filtration system in a building's HVAC systems.
The engineer's responsibility is to design a building's filtration system in accordance with current codes and standards. A building manager is entrusted with maintaining a building's filtration system based on the original design intent.
ASHRAE Standard 62 on IAQ mandates a minimum efficiency for filters in most buildings. Appendix S of Standard 62 requires that filters with a minimum efficiency reporting value (MERV) rating of no less than 6, or 25% efficiency, be used upstream for all cooling coils or other condensate-producing devices.
In health care applications, the American Institute of Architecture guideline or the new ASHRAE design manual for hospitals and clinics establishes minimum filtration efficiencies for these and other health care facilities. These minimum standards are established to create an acceptable indoor environment for staff, workers, patients, and visitors and to protect HVAC equipment and ductwork. Proper filter selection and maintenance reduces accumulation of dust, dirt and other material in the HVAC system, and reduces the likelihood of microbial growth in the HVAC systems.
Again, it is the responsibility of the hospital management, purchasing authority, and maintenance staff to ensure the filtration systems are maintained in accordance with the original design intent.
Managers in all facilities should be aware of industry standards and codes which establish the design criteria for the engineer who designs the filtration system for specific physical dimensions and configurations, pressure drop, and efficiency criteria. Without knowledge (or with improper knowledge) of these codes and standards, a building manager will all too often make inappropriate decisions when selecting and purchasing filtration systems after the original design and construction.
As stated above, improperly selected filters can lead to excessive dust and dirt build up on coils and ductwork in the HVAC systems.
Dirty HVAC systems can lead to poor IAQ, or even worse, "sick building syndrome." In addition to the cost of clean up and maintenance of dirty systems and reduced energy efficiency, improper filter maintenance can lead to IAQ complaints (or dreaded lawsuits) by building occupants.
An engineer must decide on which filter-pressure drop to use in static-pressure calculations when sizing a fan motor. Some engineers use an average pressure drop between clean and dirty, while others use the dirty pressure drop. This can add anywhere from a half inch to an inch to the fan static pressure. If the dirty static pressure drop is used in the fan static calculation, there should be a means to adjust the airflow when new filter media is installed.
Again, these are specific design criteria and factors the building manager or owner must be aware of when maintaining the filter. A good engineer should leave the building manager or owner with a clear understanding of the design intent for the filtration system and the necessary knowledge required to maintain that system. Additionally, the owner's maintenance staff should receive proper training on the filtration system in order to ensure proper service.
WHAT YOU DON'T KNOW CAN HURT YOUToo often, building operators don't know or ignore some or all of the above criteria and other factors when ordering replacement filters. And in some facilities, the purchasing department orders air filters without regard to the performance criteria for the original filter design.
For example, some buildings report a policy under which purchasing agents responsible for acquiring filter media receive bonus pay based solely on money saved from the previous year's budget. In such a case, the organization established no minimum standards to guide the agents in acquiring the filters, so they purchased filters that were significantly less efficient and that had a lower standard than filters for which the system was designed.
In one case, a 30% filter replaced a specified 90% filter. And, a 12-in. deep, 90% cartridge filter was purchased with one-half the media square footage called for in original specifications. Obviously, this generated false savings, because the filter had less than one-half the life of the originally specified filter. Lower filter efficiency results in higher particulate being circulated in the conditioned space where the building occupants live and work.
Facilities sometimes use inexpensive, 1-in., flat, throwaway filters to replace filters originally specified as 2-in.-deep, pleated media with 30% efficiency. Usually, savings generated from such substitutions are not truly realized.
The less expensive filters often require more frequent changes, have higher pressure drops than the specified filters, and result in more frequent cleaning of coils, fans, and ductwork. Often these lower quality filters are not changed as frequently as they should be, mostly because these filters are unrealistically held to the same expectations of a higher-quality filter.
Relegating filter maintenance to the less experienced members of the maintenance staff will often result in inappropriate filter selections simply due to lack of knowledge of how the filters integrate with the overall IAQ criteria, the specifics of the HVAC system.
TYPES OF FILTERSParticulate filters are just one type of filtration used in today's buildings. Particulate filtration is not the only issue that must be considered in the design of today's building filtration systems. Off gassing of volatile organic compounds (VOCs) from carpets, paint, and furniture and bio-effluents generate contaminants that frequently must be addressed.
Also, engineers and managers must be aware of criteria for locating outside air intakes for HVAC systems and the related filtration issues. If outside air intakes are located adjacent to loading docks, parking garages, alleys, boiler stacks, or other such structures, the design will need to accommodate gas-phase filtration to address the resulting contaminants. Where outside air intakes are located such that objectionable odors and contaminants are drawn into the building's HVAC system, gas-phase filters (such as activated-carbon filters or potassium permanganate filters) will be used to control these contaminants.
Again, communication between designer and building manager/owner is important to ensure long-term maintenance of both particulate and gas-phase filters.
Where it is part of the design intent to reduce the amount of outdoor air drawn into the building for proper IAQ, electronic filters may be used. Electronic filters or filter-enhancement devices, which charge the air particles, include bipolar ionization or high-frequency, electrical-field devices. Each of these devices requires expert assistance for proper application but might be necessary in many buildings.
Research continues in synthetic materials used as filter media. Several companies have introduced charged media, which promotes filters with higher efficiencies and smaller pressure drops. However, some controversy accompanies this new media. One current issue faced is the length of time the charge lasts and the filter efficiency after the charge has dissipated. ASHRAE 52.2 is being revised to test filters with the new charged media. The 52.2 test will introduce dust particles that neutralize the charge on the media, so specifiers can compare filters on an equal basis.
COMMISSIONINGWith the complexity of today's systems, an increasing number of buildings are undergoing a formal commissioning process. An independent commissioning agent leads the commissioning effort. Commissioning is the systematic and quality-oriented process of ensuring that facilities and their systems and equipment perform according to the design intent and meet owners' needs and expectations.
Documenting that the filtration system is properly installed and operating is the first step in a successful maintenance plan for a building. The commissioning agent will also be proactively involved in conveying the engineer's design intent to the building manager and staff.
Improved IAQ is critically important as it relates to the health of building occupants. To combat mold-related problems and sick building syndrome, commissioning ensures that a building is pressurized and has the correct fresh air changes for IAQ. For example, if a building is required to utilize 20% outside air for code IAQ requirements, less outside air may cause occupants to become sick and the building to become negatively pressurized. A negatively pressurized building will be conducive for mold growth and subject to using excess energy. If outside air is more than 20%, energy costs can needlessly increase. Commissioning can ensure a building is both efficient and safe by verifying outside air usage and controls.
THREE STEPS FOR SUCCESSFUL MAINTENANCETo improve air filter maintenance, managers should understand the design intent, review purchasing criteria, and choose a supplier.
First, determine the design intent of the HVAC filtration system. The design intent should establish filter types - particulate filters, gas-phase filters, order control, etc. - filter efficiencies, MERV ratings, clean and dirty filter-pressure drops, minimum media square footage, filter depths, filter housings, and fan static-pressure calculations.
At the end of a construction project, a contractor, filter vendor and engineer should all be involved in training the building manager and maintenance staff. They should disseminate the filtration system design intent to the manager and staff.
Managers in older facilities who do not have easy access to the original specifying engineer will need to establish the filtration design intent. Consult with an engineer or a filter manufacturer to learn the current issues, and evaluate the current filtration replacement program based on design intent.
Changes or modifications to the original systems' configurations or usage may warrant revisions to the filtration system. For example, new filter housings might be beneficial in reducing leakage around and between filters and frames. More efficient filters might require adjusting fan speeds or even replacing motors.
Managers should establish purchasing criteria for new filter media that conforms to the design intent. Ensure that all filter vendors have the same information regarding required pressure drops and efficiencies before submitting their proposals.
Require each vendor to visit the facility to avoid blind quotes. Vendors need to understand systems and customize their proposals to meet the facilities' exact needs.
While an upgraded filtration system may appear to cost more, the overall cost of a more-efficient filtration system saves equipment and enhances the quality of the indoor environment. Purchases in the filter aftermarket based solely on price will always lead to false savings.
Managers should also consider each vendor's proposals for filter life, pressure drop, energy costs, and filter media. Often, managers call filter vendors and place an order for specific media. In such cases, a vendor is unlikely to know if the requested media is commensurate with the originally specified media.
Select a vendor that is knowledgeable about current codes and standards. A vendor unfamiliar with ASHRAE 62 will be unaware of the many IAQ issues facing today's buildings. Managers should make sure to purchase expertise, not just filter media.
The importance of air filtration design and maintenance cannot be over-emphasized. The goal is to create an acceptable indoor air environment to protect the HVAC systems and reduce energy consumption. It is essential to allocate the money and resources needed to maintain, upgrade, and/or enhance the building's filtration system to accomplish these objectives.
Filtration maintenance is more than just protecting equipment. It is an integral component in creating and maintaining an acceptable indoor environment. The costs of establishing and following the filtration design intent are worth it.