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Using Furnace Filters as Whole House Room Air Cleaners

June 27, 2011

There have been significant improvements on 1″ furnace type filters over the last ten years. The fiberglass furnace filter was originally designed to keep the house ventilation system clean from large particles and debris. Since most of the house furnace blowers are not designed to operate in high static pressure environment, the fiberglass furnace filter can share only a limited amount of pressure drop. The 1″ fiberglass furnace filter with low-pressure drop and low cost naturally became the most common choice.

Two things have changed. First, is the better design of the filters by adopting pleated structure to reduce the media velocity. Lower media velocity typically leads to higher filter efficiency and lower pressure drop. Secondly, the variety of specialty media (e.g., tribo-charged media, split fibers, large effective fiber diameter (EFD) melt-blown electret…), which have significant higher initial efficiencies with relatively low pressure drops have been introduced for this application.

The enhanced performance of some of the pleated type furnace filters has elevated the traditional role of 1″ fiberglass furnace filters from protecting the residential ventilation system to improving the indoor air quality (IAQ) in a residential environment1. Pleated type furnace filters, when selected and used properly, can potentially reduce the indoor air pollutants significantly and the advantages are multifold. For example, for allergy sufferers, houses installed with specialty pleated type media furnace filters can potentially alleviate the symptoms of sneezing, watering eyes, itching throat, postnasal drip, coughing etc. during the pollen season. The reduction of indoor air particle concentration by pleated type furnace filters can also slow down the settling of dust and respirable particles2 inside the house.

Field Tests Setup

Seven different new furnaces were evaluated in this study. Identification of the seven filters can be seen in Table 1. Tests were performed in an actual residential home located in Florida with total square footage of 2900 ft2. The test house is a high ceiling with two stories in half of it. A heat pump with a slot for a 20″x20″x1″ furnace filter is the ventilation system used in the house. Air filters used were acquired from local retail stores.

Two TSI PortaCount Plus used as condensate nuclei counters (CNC) and one TSI 3755 (two channels: 0.5-5 and >5 micron) optical particle counter (OPC) were used as the primary monitoring instruments in this study. One CNC (#1) was placed on a ground floor dining table close to one corner of the house. One CNC (#2) was placed in a second floor bedroom whose location was in the exact opposite end of a diagonal of the house relative to the first CNC. The two-channel optical particle counter was placed in the first floor master bedroom. It forms a triangle (with the other two CNCs) that covers three corners of the house. Each monitoring instrument was connected to a computer for data acquisition.

CNC recorded the particle concentration from 0.02-1 mm and the sampling time was set at 15 seconds for each sampling period. The optical particle counter recorded the results of particle concentration in the range of 0.5-5 mm (respirable particle size range) and the sampling time was set at 20 seconds for each sampling period. Combination of those two types of instrument covers the particle size range from 0.02 to 5 mm.

Since the main objective of current study is to see how effective is each furnace filter working as a whole-house room air cleaner. The decay of particle concentration vs. time inside the house is the primary focus of each test.

Field Test Procedures

Several sliding doors and windows were open to let outside ambient particles enter the house through natural ventilation. Particle concentrations were monitored throughout the house to make sure the particle concentrations were stable before each experiment started. All the sliding doors and windows were then closed once the particle concentration was stable inside the house. All three counters were then reset to start to record the data of each test. The blower of the ventilation system was not turned on for another 10 minutes to establish the initial base line (particle concentration). Each test lasted 3-8 hours depending on the performance of each furnace type filter. No activities or human movements occurred during each test.

A background test was also performed. All the doors and windows were closed. The blower was off and there was no activity or any movement in the house. CNC#1 and OPC recorded the particle concentration over a period of time.

Results and Discussion
The data collected in the first 10 minutes of each test were averaged and used as the initial concentration. The numbers collected by each instrument after the blower was turned on were divided by the initial concentration and represented as the percentage of the original concentration.

Fig. 1 illustrates the decay curves of the CNC #2 placed in the second floor. The CNC #2 was not available when the test was performed for the WEB furnace filter. The results are very close to those collected by CNC#1. The time that requires removing 50%, 75% and 87.5% of the initial particle concentration for each tested filter is shown in Table 3.

Improved Test Methods for Electronic Air Cleaners; Fall 2002 issue of Air Media
Author(s): JT Hanley; et. al.