IAQ ‘Hot Spots’
Proper specification, inspection and maintenance of air filters, coils and ducts can go a long way toward improving indoor air quality
Poor indoor air quality (IAQ) generally occurs when one of two things happens. Either bad air enters a building, or good air goes bad.
Most maintenance and engineering managers have little control over the first situation, aside from making sure air intakes are located away from loading docks or at street level. But managers have many options to ensure that air, once pulled into air intakes, is cleaned and stays clean.
The job of keeping indoor air clean centers on the proper maintenance of three critical HVAC components: filters, coils and ducts. Making sure each of these components functions properly will go a long way in helping facilities avoid IAQ problems.
Cleaning the Air
Any filter will trap particulate matter in the air stream, but trapping the right particulates and enough of them depends on both the type of filter and maintenance of the filtration system.
The media used for particulate filters include paper, cotton fibers, glass fibers and a variety of synthetic materials. Some filters using synthetic materials have an electric charge that attracts particles, in addition to filtering them out of the airstream. Filters can be loose blankets, they can be flat, pleated or bagged, and they can come in various widths. The type of filter medium and its density determines efficiency, while the width determines holding capacity.
Selecting the filter to use and determining its efficiency usually is driven by two criteria: manufacturer recommendations and activities in the conditioned space. For example, operating rooms require much higher efficiency than a commercial office.
“Our operating rooms require the highest-quality air possible,” says Chris Lawrence, HVAC lead for Yale-New Haven Hospital in New Haven, Conn. At the hospital, a 30-40 percent pleated fiberglass prefilter precedes a 99 percent efficient multifilter bag system. The prefilters are a cost saver because they increase the longevity of the more expensive bag filters, Lawrence says.
Air-handling units serving the hospital’s office areas and other public spaces usually have 30–40 percent, 2-inch pleated filters.
Jack Gookin, supervisor of mechanical systems at Grant Medical Center in Columbus, Ohio, says that while a prefilter is a good idea, a pre-prefilter is even better. Gookin says he has begun to include a blanket-type 30 percent filter over the prefilter, which is changed every two months. That strategy means that the prefilter now only needs to be changed every quarter, and the main filter needs to be changed every six to eight months.
The Right Fit
All of these measures are beneficial for IAQ, but managers need to be careful about adding filtration willy-nilly, says Bill Smith, program director of facilities for Okaloosa (Fla.) County School District. Providing good IAQ is a constant challenge for his maintenance department, but Smith says he adheres closely to manufacturer recommendations for filtration. To do otherwise risks seriously reducing airflow, freezing the coils and increasing energy costs.
“If a teacher asks for a filter grill on the return air, we will usually take out the coil filter,” Smith says. While filter grills aren’t necessarily the best filtering system, they are more obvious to the staff and, therefore, are sometimes changed more often than a rooftop filter, he says.
Efficiency is important to filtration. The higher the filter efficiency, the smaller the particles it can remove. But higher efficiency also means a greater pressure drop in the system, reducing airflow. So it is important for managers to determine the contaminants in the air and the level of filtration necessary to remove them, and adjust the system accordingly.
These factors also have an impact on how often filters are changed. Most technicians visually check filters regularly and frequently change them at that point. But, too often, technicians change filters before they need to because a filter that is just getting dirty is just getting good, says H.E. Barney Burroughs, president of Building Wellness Consultancy Inc. and chairman of ASHRAE’s Standard 52.2 committee.
To determine the need for changing a filter, it is better for a technician to rely on pressure change as a gauge. Changing a filter at the right pressure change is important not only to get maximum filter performance life but also to avoid blowout around a filter that is too dirty and clogged.
Managers and front-line technicians have long struggled with understanding how well a new filter will work and what they can expect it to remove from the airstream. But a new way to measure filter efficiency might help clear up some of the ambiguities.
Determining particulate filter efficiency has long been based on the atmospheric dust spot method. But this method never offered a good indication of a filter’s efficiency at certain known particle sizes.
To correct this, ASHRAE released Standard 52.2, Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiencies by Particle Size. Standard 52.2 changes the way filters are tested and rated. Manufacturers now will begin providing a value for filters based on their minimum efficiency in removing a range of particle sizes, rather than its average efficiency. Now, managers and technicians will know with some certainty what filter systems are capable of removing from the airstream.
For instance, ASHRAE Standard 62 on IAQ requires a minimum efficiency reporting value (MERV) of 6 or greater for many applications. A MERV 6 corresponds roughly to a filter that is 20 percent or less efficient and removes 85-90 percent of particles that are 3.0-10.0 microns. Research indicates that particles of 1.0-5.0 microns are the most troublesome and can enter deep into the lungs.
Particulate matter is not the only thing that filters can remove. Gas absorption filters containing activated carbon or chemically treated clay particles remove compounds that are or behave like gases. No standards exist for rating gas absorption filters, and operating expenses for them can be quite high. But schools located near sewer or industrial plants that have to contend with disagreeable odors might benefit from gas absorption filters.
Clean Coils and Ducts
Coils play a critical role not only in conditioning air but also in keeping it clean — or at least in not making air dirtier. The exterior surface of coils and condensate pans can be breeding grounds for bacteria, molds and fungi that can enter the supply air. Eliminating or at least keeping these potential problems to manageable levels requires regular coil cleaning.
Typical cleaning procedures usually involve using a hose to spray a non-acid cleaning detergent on the coils and rinsing it off. ASHRAE recommends a pressure washer set at least 100 psi, with a sprayer head orifice of 1/16 inch.
Some managers have had success using a mild soap, such as dish soap, to clean the coils or a commercially recommending product. A final cleaning procedure involves cleaning out condensate pans and, if necessary, adding an antibacterial agent.
In really dire cases or where both sides of a coil are difficult to access, technicians can remove and clean the coil.
Gookin says the medical center has increased the frequency of coil cleaning to six months, adding that annual washings allowed the coils to collect more dirt than was good. He also points to a slight gain in energy efficiency from washing them more often.
Gookin says that although technicians already add biocide tablets to the condensate pans, they might start using an antimicrobial product when cleaning coils. Smith, however, isn’t sold on the antimicrobial products.
“They may retard growth, but they aren’t going to stop it,” he says. He prefers a non-bleach-based detergent. Many of the hospital’s coils are cleaned by hand.
Anti-microbial products might present other concerns, particularly if used on ductwork. The U.S. Environmental Protection Agency (EPA) says that because these products weren’t designed for use in ductwork, no data exists to show either effectiveness or safety. The EPA has no protocol that measures efficacy, chemical exposure limits or risk assessment of these products. So a company interested in stating that its products could be used for ducts would have to develop such a protocol and supply the EPA with data.
To prevent the growth of bacteria, molds and fungi in drain pans, as well as to avoid water splashing onto coils and into the airstream, managers can direct technicians to check coils and pans every time filters are checked or changed. Technicians also should make sure pans drain properly.
When it comes to major duct cleaning, most managers turn to contractors. But it often is cost-effective to have in-house staff periodically check and clean small sections of duct. These inspections usually involve short stretches of ductwork just downstream from air-handling units.
Duct cleaning is expensive and not often called for unless diffusers show a buildup of dirt or building occupants report dirt on surfaces, Lawrence says.
“When this happens, it tends to be our older systems,” he says.
Older facilities also could have interior-lined ducts, which cannot be cleaned. Smith, who battles high humidity most of the year on Florida’s panhandle, tries to dry these ducts out as much as possible if there are signs of mold.
“That’s the best we can do until we replace the ducts and put in only exterior insulated ducts,” he says.
When it comes to IAQ, a good preventive maintenance program is crucial. It has made the difference at Okaloosa County School District. Smith says that eight years ago, IAQ problems were the biggest maintenance concern facing the district. But since the district instituted a PM program, Smith says that deferred maintenance, not IAQ, has become his chief maintenance headache.