Types of HVAC Filters
There are a wide variety of commercial heating and air filters that have special applications. For rooftop HVAC units (RTUs), there are three basic types of air filters:
Selecting the Right Filter
Fiberglass filters are the most common type of filter used in commercial buildings. They’re generally low-cost, and they provide minimally acceptable performance to protect HVAC equipment.
Most HVAC professionals agree that fiberglass filters are less than 10% efficient. Additionally, there is concern in many circles about the effect of fiberglass on health. In fact, the Wall Street Journal stated “Fiberglass fibers are similar to asbestos fibers and thus pose similar health threats.”
Polyester filters are gaining in popularity because they are only slightly higher in cost than fiberglass, yet they are more than 25% efficient. No health risks are currently being debated about polyester filters. They can also be tackified to improve efficiency. An added advantage of polyester filters is that two densities can be bonded together to provide a two-stage filter that will catch larger pieces of dirt in the first part. Finer particles of dirt, meanwhile, will be caught in second part that has smaller openings and is tackified.
Because it comes in rolls and can be cut with ordinary household scissors, a polyester filter can be set into a custom frame, thus eliminating by-pass (see below for an explanation of by-pass). For example, consider an air conditioning unit with three 20” x 25” filters. A frame could easily be made to accommodate a single 25” x 60” filter. This, in turn, would eliminate all of the gaps between filters. If spacers are used, then the frame could be oversized to include the additional area.
Poly cotton pleated filters are about three times more expensive than fiberglass filters. Yet they feature the highest efficiency of the three filters discussed. One of the problems with pleated filters is that it "surface loads" (dirt build-up).
A Word About Blow-By
Blow-by, or by-pass, describes air that moves around or between filters. It’s one of the biggest challenges in keeping the blower and evaporator coil clean. In most cases, simply taping the filter joints together with duct tape can dramatically reduce by-pass. Some RTUs utilize filter spacers; if your RTU does, then make sure these spacers have not been inadvertently removed. You can eliminate many of these problems by making a frame of the actual size of the total filters and using seamless polyester filter media.
RTUs with a fresh air or outside air intake can ingest rain into the filter section. If you are using air filters with cardboard frames, the frames will lose their rigidity when wet, and the filter can be drawn out of its tracks. This dramatically increases blow-by. If you use cardboard frames, then you should order “beverage quality” cardboard.
From a filtering standpoint, all three types of filters will protect your HVAC equipment – provided that the filtering system is maintained properly.
You get more filtering capacity from tackified polyester filters.
Pleated filters generally are the most efficient of the three filter types we identified earlier. One caution, though: The higher the efficiency, the more airflow restriction you place on your system. This reduces the capacity of the air conditioner. If you require more airflow and better efficiency, then you can retrofit a 1” filter track with a 2” track. 1” pleated filters are rated at 300 FPM (feet per minute), while 2” filters are rated at 500 FPM.
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