By Darwin Zhang Li '09 from St. Ignatius College Preparatory, San Francisco
One day last winter, I went home thinking I could escape the cold. Something was wrong. I felt no gradation in temperature when I stepped in, so I asked my dad if the heater needed repair. In an unhappy tone, he told me that the heater worked, but the new filter he installed impeded the process of heating. He then added, “It’s because of the greedy of capitalist system, in order to gain more profit by high end product, Home Depot no longer carries large selections of the old style and inexpensive fiberglass filters, that we used to use. The ones they provide now are expensive ones that block so much airflow that our house can’t get warm fast enough.” Seeing the dilemma in not having warmth, I decided to lend my capabilities in a father-son quest. When we rallied at Home Depot, the sight, horrific like a warzone, displayed pleated, synthetic fibers but the most heart wrenching sight was the price tag. They cost around eight to twenty dollars; a painful hit on the wallet compared to the old ones that cost merely one dollar and ninety-nine cents. We tried Lowe’s, The Orchid Hardware, and even A’s and True Value stores. The competitors were obviously competing incorrectly. Greatly disappointed, my dad blamed greed.
Are these new style filters any better than the old ones? Are they worth the money? I picked up the burden from my dad and began research. The descriptions for new style filters, including their properties and ratings, confused me. I came face to face with terms such as MERV, Pressure Drop, Arrestance, Dust Spot Efficiency and Dust Holding Capacity. Along with using the flashy image of the products as advertisements, these terms also boost the manufacturers’ claim that new style filters can clean very small dusts, pollen, and even micro allergy off heated air before they travel into the rooms.
First things first, the definitions I derived from the 2004 ASHRAE Handbook are as follows. Pressure Drop (sometimes also referred to as Initial Resistance) refers to the effect that the filter has on airflow. The higher the pressure drop, the lower the operating efficiency of hot or cold air from the source to the living space. Arrestance is the measurement of a filter’s ability to capture dust particles under test conditions. It is worth noting that during testing, only large sized artificial particles are used for the measurement. Dust Holding Capacity, as its name suggests, is the amount of dust a filter can hold at a given pressure drop. MERV stands for Minimum Efficiency Reporting Value. Engineers determine MERV ratings (1 – 12) by adding particles of varying sized into a controlled testing environment. The particles are added upstream to the test filter and a laser particle counter samples the air before they enter and after they leave the filter.
Generally speaking, these terms serve to grade the filters’ ability to capture small particles. Depending on the pore size of the filter, which is inversely correlated with the pressure drop, the higher the MERV rating, the smaller the pore size, and the better it is to capture the small dust. For example, a nominal size 10X10X1, Micro Allergy Pleated Filter with the efficiency rating MERV 11 will have an initial resistance of 0.24 a.u. (a.u stands for arbitrary unit. According to the manufacturer’s own description, a same sized old style Fiberglass Disposable Filter, graded at MERV 4, will have an initial resistance at 0.07. A more than three fold increase in airflow resistance for new ones!
While there is no doubt that the new style filters can perform better to clean off the small dust from airflow coming into the home than the old ones, these filters also come with a high price; it takes much longer to either heat up or cool down the living space.
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