Particle Tech Furnace Filter Study

Using Furnace Filters as Whole House Room Air Cleaners

Ker-Ching (K.C.) Hsieh, Ph.D., Particle Tech, Inc. St. Paul, MN

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 particles(2) inside the house.

Furnace Filter Field Test 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. 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.

TABLE 1. Furnace Filters Used in Field Tests

Description Media Type Propery
MERV11 Split Fiber (Fat Fibers) Electret
MERV12 Melt-Blown Electret
MERV8 Cotton and PET Mixed Non-Charged
MERV10 Tribo-Charged (Fat Fibers) Electret
MERV12 Tribo-Charged Composite Electret
Glass Fiber Fiberglass Throwaway-Type Non-Charged
CFP* MERV8 Composite Filter Pads* Non-Charged

Furnace Filter 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.

The time that was required to remove 50%, 75% and 87.5% of the initial p article concentration for each tested filter is shown in Table 2.

TABLE 2. Time Required for Particle Reduction Recorded by CNC No. 1 (0.02-1 µm)

Particle Reduction Percentage (SF) (SF) MERV11 (MB) MERV12 (C+PET) MERV8 (TC) MERV10 (TCC) MERV12 Loose Glass Fiber (CFP) MERV8
50% 2070 1710 3270 1995 2250 4740 3405
75% 4395 3735 7620 4140 4455 11130 8605
87.5% 7440 5880   6555 7080   22155

Among the electret filters, both "fat fiber" filters (split fiber and tribo-charged) showed faster removal rate for respirable particles than the other two electret filters. Lower pressure drops of the "fat fiber" filters that result in higher air exchange rate might have contributed to the differences seen here. This proves that both pressure drop and efficiency of a filter are important for this application and not just efficiency of the filter.

Electret filters showed the quickest drop in particle concentration, followed by the cotton & PET mixed media filter. The fiberglass furnace panel filter and CFP showed slower particle concentration drop than the rest of the pleated type furnace filters. It should be noted that the range covered by OPC (0.5-5 mm) coincides with so-called respirable particle size range. The particle deposition percentage in human respiratory airway is usually higher for respirable particles. In this test, the MERV11 and MERV10 filter removed respirable particles better.

It should be mentioned that all the results were the initial performance and not averaged lifetime performance. There are different characteristics between electrets and non-charged media. The initial performance of an electret furnace filter might be the best over a period of usage time. The performance can drop as the filter ages due to shielded electrostatic field by deposited particles. On the other hand, the initial performance of a non-charged media is usually the lowest and the performance can gradually improve as the filter loads with the deposited materials. It is not the intention of this study to compare the averaged lifetime performance among various furnace filters.

Furnace Filter Test Summary

The initial performance of pleated type furnace filters made of electrets shows much faster particle removal rate when compared with furnace filters made of non-charged media. All electret pleated type furnace filters are able to reduce 90-97% of fine and ultra-fine particles (0.02-1 mm). Even the non-charged media pleated type furnace filters can remove more than 80% of fine and ultra-fine particles in a longer period of time.

For respirable particles (0.5-5 mm), electret pleated type furnace filters can be very effective for particle reduction of 90- 94%. However, this represents a tougher challenge for some of the new non-charged media pleated type furnace filters. Lower pressure drops of the "fat fiber" filters result in higher air exchange rates. This proves that both pressure drop and efficiency of a filter are important for this application and not just efficiency of the filter alone.

1" electret pleated type furnace filters, as shown in current study, not only protect the residential HVAC system, they can be useful tools for improving the overall indoor air quality in your home or workplace.

Ker-Ching (K.C.) Hsieh is president of Particle Tech, Inc. located in St. Paul Minnesota. He is a member of ASHRAE and active on Technical Committee (TC) 2.4. K.C. is also involved in testing standards committees of SAE, IEST and ASTM.

References