by David Kozlowski,
Failing to pay attention to the role of air filters is one way to undercut the performance of HVAC systems
Given all the cost, design and technology that goes into sophisticated HVAC systems, it is a bit unnerving that a system can be very easily compromised by a cheap, throw-away filter. But it happens all the time.
As important as filters are, they often are given the least amount of attention of all components in an HVAC system. Aside from hospitals, which fall under a strict code covering air filtration, too few maintenance and engineering managers could say exactly what kinds of particulates their filters can remove from the air.
Recent changes to American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) standard for indoor air quality could turn this around. The standards arm managers with the knowledge of their filters' capabilities. This knowledge, along with a closer look at filter design and maintenance, can help managers improve the air quality of their facilities and save money.
Move to MERV
For instance, ASHRAE Standard 62 on indoor air quality recently was amended to require a minimum efficiency reporting value (MERV) of 6 or greater for many applications, says Andrew Persily, chairman of Standard 62 committee. This minimum requirement is designed to keep dirt and moisture from mixing on the cooling coils, Persily says.
A MERV 6 rating corresponds roughly to a filter that is 20 percent efficient or less and removes 85-90 percent of particles measuring 3-10 microns. These particles include mold spores, hair spray and cement dust. A human hair is about 100 microns thick.
"Right now, I have MERV 6-7 circled as a recommendation, but I am not sure that that is going to be enough," says Al Abend, chief of school facilities for Maryland's Department of Education. "There is still a lot of educating we need to do before we can really start talking about what is best for our schools."
Research indicates that particles from about I micron to 5 microns are the most troublesome and can enter deep into the lungs.
But MERV 6 might not be enough filtration, says H.E. Barney Burroughs, president of Building Wellness Consultancy Inc. and chairman of 52.2 committee. Burroughs is conducting a 50-site, five-city filtration project to determine a sufficient level of filtration by measuring downstream particles.
Embracing the MERV rating system is important, however, because it means managers will have a better handle not only on the quality of the air their systems provide but on the cost of providing it. Managers understand the energy and labor costs, but they don't really know how the good the air is, he says.
Understanding the relation between air quality and cost means understanding the operating costs of filters, says Norm Nelson, senior project manager and forensic engineer for CH2M Hill.
The cost of air quality
"Labor and energy decide the cost of filtration more than the cost of filters do," Nelson says.
One premise of life-cycle cost is that a larger filter media surface area means less pressure drop and more dirtholding capacity, resulting in cleaner air, fewer filter changes and greater energy efficiency.
"The first thing to know is that filters that look dirty are probably just beginning to do their job," Burroughs says. The dirt that builds up in a filter is actually increasing the filter's efficiency, but only up to a point, he says.
To improve life-cycle costs, managers should try to improve a filter's dirt-holding capacity. Typical tactics to achieve this goal is using pleated filters or increasing the depth of the filters.
The labor savings that result from using filters with increased dirt-holding capacity are obvious. A standard 2-inch, throw- away filter in bulk quantities might cost $1.50. A good-quality, 2-inch pleated filter with the same efficiency might cost $4-6. But the pleated filter will last four to six times longer than the standard filter, thus requiring fewer changes.
Burroughs is conducting a direct comparison at an Atlanta-area hospital using several different types of filters. So far, he has found that a 1 -inch, blanket-type filter, though it offered a lower first cost, needed to be changed after just one month. In contrast, the more expensive pleated MERV 5 and MERV 7 filters have remained in place and are performing well seven months later.
The amount of filter surface area not only is critical to efficient filtration and dirt-holding capacity. It also can have an effect on energy.
If everything else is equal between a 2-inch and 4-inch filter, the 4-inch filter has an average lower pressure drop, which means less energy used.
Taking into account labor and energy savings, if there is room to increase the size of a filter rack, it could be cost effective to do so, Nelson says.
Managers could do the same thing by lowering the air velocity, but that is a more expensive option, he says.
The Maryland State Department of Education has done its own energy analysis. The department compared the performance of a disposable, 2-inch coarse-fiber filter for unit ventilators to that of a 2-inch, extended pleated filter.
The results show significant energy savings when using the latter filter. The former cost $3.80 per 1,000 cubic feet per minute, and the latter cost $3. The savings resulted from cleaner, more efficient coils.
A 4-inch pleated filter still saved more than the disposable filter but less than the 2-inch filter because of the increased cost of the filter. At 6 inches, Abend says, the pleated filter costs significantly more.
"As you improve your filtering and increa sethe sizes, you save more, but only up until a certain point," Abend says. "After that, you have to make a judgment," he says. "That's when it becomes a question of how much more do I want to pay for better air quality."
Burroughs encourages managers to start thinking of filters as an investment.
Says Burroughs, "If managers buy the right filter and maintain it properly, then the payback is in clean coils, clean ducts, less housekeeping and less energy."
Filter Options
HEPA filters. High-efficiency particulate arrestance (HEPA) filters can remove more than 95 percent of most particulate matter, including particles as small as 0.100.20 microns.
Gaseous phase filters. These filters use a carbon or charcoal pad to remove gases and are most effective in removing odors. But recharging the carbon can be messy.
Electric static filters. These filters can be added to I -inch unit ventilator filters to improve filter efficiency. The electric pad uses activated charcoal and draws low voltage when wired to the fan.
Ultraviolet light filters. These filters can destroy many biological and chemical contaminates, but effectiveness drops off quickly. While it is 100 percent effective next to the bulb, it is only 20 percent efficient 2 inches away.
What MERV Means
The following are MERV ratings relative to dust-spot efficiency percentages:
This article appeared previously in the November 2001 issue of Maintenance Solutions. ASHRAE Standard 52.2 was used as a source for this article.
Earlier this year, ASHRAE released Standard 52.2, Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiencies by Particle Size. The standard changes the way filters are tested and rated. Gradually, manufacturers will begin labeling air filters with a value based on the filter's minimum efficiency in removing a range of particles, rather than its average efficiency. This means managers now will know with some certainty what their filter systems are capable of removing from the air stream.
Not many managers consider life-cycle cost when it comes to filters. And what they might learn when they do so is that better filters cost less.
Managers have a range of options in using filters to help meet facilities' indoor air quality needs. Among the filter types are these:
ASHRAE Standard 62 on indoor air quality recently was amended to require a minimum efficiency reporting value (MERV) of 6 or greater for many air filter applications. Here is an explanation of MERV:
First published December 2001 Month Year
