This is an issue that has been long debated in the heating and air conditioning business.

Particles in the air

The particles present in air vary greatly in their size and number. Here are a few facts about the particles that we typically find in air.

Particle size is typically expressed in microns. A micron is very, very small — 1 micron is 1/1000th millimeter or 0.00004 in. The eye of an average needle is 749 microns across.

Particles typically found in the air range from about 100 microns down to less than 0.01 micron. Some common allergens, like pollen and plant spores, are among the larger particles; smoke, bacteria, and viruses represent the smaller particles. Airborne particles larger than about 10 microns can be seen with the unaided eye.

While all particles in the air are small, we find that there are many more of the smallest particles than the larger ones. If a sample of air is taken, typically 99% of the particles counted will be 1 micron or less.

Not surprisingly, the size of the particle determines how long it will remain airborne before settling out. Particles 1 micron and smaller remain permanently suspended; they never settle out.

Measuring efficiency

There are three types of tests to indicate how efficient an air cleaner is at removing particles.

1. Arrestance: In this test, defined by ASHRAE Test Method 52.1, an air cleaner is mounted in a clean test duct, into which is fed a measured weight of synthetic dust. The efficiency of the filter under test is calculated by comparing the weight of the dust captured by the filter, compared to the total dust fed into the test duct.

This is a poor test for determining the efficiency of an air cleaner against small particles. Generally all filters score high in this test, because catching few larger particles (10 to 50 microns) can make up for thousands of small particles (less than 1 micron) that are missed.

2. Atmospheric dust spot: This test is also defined in ASHRAE Test Method 52.1. As the name suggests, this test uses atmospheric (outdoor) air pumped into a test duct. Upstream and downstream of the filter under test, samples of the air are captured in high-efficiency, laboratory-grade filters, leaving “dust spot” stains on the filters.

An efficiency number is applied to the filter by evaluating the darkness of the dust spot on the laboratory filter, downstream of the filter under test. This test is a better reflection of the actual efficiency of the filter.

3. Fractional efficiency: This test, defined in ASHRAE Test Method 52.2, uses modern laboratory equipment capable of actually counting the number of particles and classifying them by size. The filter under test is challenged with an aerosol containing potassium chloride particles in the size range of 0.3 to 10 microns. The number of particles are counted upstream and downstream of the filter under test, and efficiencies are calculated for each size range.

The data are usually expressed in the form of a graph, showing the efficiency of the filter at each size range. This test method is the best because it is possible to compute the actual efficiency of the filter under test over the entire range of particle sizes, resulting in a far better indication of the true filtration efficiency.

Comparing efficiency

As might be expected, the same air cleaner tested under the three different test methods will yield very different numerical results.

An air cleaner measuring 90% efficient using the arrestance test method will be less than 20% efficient when using the atmospheric dust spot test. Likewise, an air cleaner measuring 90% efficient using the atmospheric dust spot test method will be only 40% efficient against the 0.3-micron particles seen in the fractional efficiency test.

It’s no wonder that the average consumer isn’t able to compare products tested using different methods.

Figure 1 shows a range of efficiencies typically seen for various filtration technologies. While the efficiency of typical flat-panel, duststop filters range from 50% to 90% using the arrestance method, they do not yield measurable values using the other methods.

A typical pleated media filter measuring up to 30% on the atmospheric dust spot test, just begins to yield measurable values against 0.3-micron particles. Electronic air cleaners and extended-surface media filters begin to be very effective against submicron particles.

Finally, high-efficiency particulate arresting (HEPA) filters are, by definition, 99.97% efficient against 0.3-micron particles.

In fairness to the arrestance and atmospheric dust spot tests, these test methods were developed many years ago, before the availability of the particle counters required for the fractional efficiency test.

But now that this technology is available, as well as a better understanding of the harmful effects of small particles ingested into the respiratory system, it makes sense to evaluate air cleaners on their ability to filter particles 1 micron and smaller.

Measuring resistance

As difficult as it is to obtain meaningful efficiency measurements, measuring the resistance to airflow of an air cleaner is very easy.

Measuring the pressure drop across the air cleaner will give a direct indication of the amount of resistance to airflow the air cleaner is responsible for. The pressure drop is proportional to the velocity of the air — the higher the air velocity, the greater the pressure drop.

When comparing pressure drop specifications of various filters, it is important to make sure that the data from all filters is measured at the same air velocity. Because pressure drop is proportional to the square of the velocity, relatively small differences in velocity can result in large differences in pressure drop.

Results can also be misleading if the air cleaner is installed in an application where the free area is substantially different from the total face area of the filter. For example, if a 24- by 24-in. filter was installed on an air handler with a 20- by 25-in. blower deck opening, for a constant airflow rate, the face velocity would increase by 15% and the resulting pressure drop would increase by more than 30%.

Differences in pressure drop

The differences in pressure drop between air cleaners can be as different as the efficiencies. In fact, the trade-off between efficiency and pressure drop is one of the great challenges in filter design.

Although there are some exceptions, in general, increasing the efficiency of the filter also increases the resistance to airflow and the pressure drop.

Media air cleaners also have the characteristic of suffering an increase in pressure drop as the filter becomes loaded with collected particles. The filter becomes more efficient as it loads, but at the expense of an increased resistance to airflow. As we will see later, high-pressure drops from loaded media filters can have a disastrous effect on the heating and cooling equipment.

Initial pressure drop, the pressure drop of a new filter, is probably the most important indicator of a filter’s resistance to airflow. As the filter loads, the pressure drop will increase, so a low initial pressure drop is very important.

As you can see in Table 1, there is a wide range of initial pressure drops that will ultimately affect the life of these types of filters.

When to change filters

Most manufacturers of furnaces and air conditioners recommend that the pressure drop across the air cleaner not exceed 0.5 in. wc. Residential heating and air conditioning systems are generally not designed to operate with pressure drops exceeding 0.5 in. wc.

The life of the filter, or how long the filter can be in service before it needs to be replaced, becomes a function of its initial pressure drop, its efficiency of particle collection, and its surface area available to spread out the collected particles.

In general, pleated media filters have a greater surface area than flat-panel filters, and will have longer life. Similarly, filters with greater pleat depth also have greater surface area. In contrast, within the time span of a typical media filter, the pressure drop across electronic air cleaners generally does not increase.

In a laboratory environment, a common test is to feed an air cleaner large quantities of a test dust to accelerate loading of the filter. The pressure drop and efficiency at intermediate steps of loading are measured, and trends of pressure drop vs. the amount of dust collected are drawn.

An important measure is how much dust can a filter collect until the pressure drop reaches 0.5 in. wc. This will correlate directly with the length of elapsed time before a filter needs to be changed.

Once again, there is a wide range of performance. Even within the media air cleaner product category, there can be up to a 10 times difference between the amount of dust that a filter can hold until it reaches the pressure drop threshold.

Also note that, as mentioned above, in comparison to a typical media filter, the pressure drop across electronic air cleaners generally does not increase.

Effect of pressure drop

In a forced-air system, the movement of conditioned air through the heating and/or cooling equipment to the living space is how comfort is delivered. Obviously, one of the most important aspects of the conditioned air that is delivered is the airflow in the system.

If the actual airflow in the system is significantly different from the design value, not only comfort, but also the efficiency of the heating and cooling equipment may be compromised.

We have seen the differences in efficiency and pressure drop of various air cleaners. Not surprisingly, the selection of the air cleaner has a large impact on the system airflow.

Testing was recently conducted at four similar homes in the Dallas area. Various types of air cleaners were installed, and the pressure drop across the air cleaner and system airflow were measured.

A total of 25 different filter-system combinations were tested. The data showed, as expected, that system airflow can be greatly restricted with some types of air filters.

Although all filters tested were new, based on the airflow data and our knowledge of how filters load in use, projections can be made as to the system performance with dirty filters.

The data in Table 1 shows that system airflow will be significantly reduced as the pressure drop increases, up to 50% in extreme cases. This will have the effect of reduced comfort in the house, as less air will reach the rooms at the end of the duct runs.

The heating and cooling equipment will operate less efficiently and, over time, this condition may reduce the life of the equipment.

In comparing the overall effectiveness of various filter types, a clear trend emerges — you get what you pay for. The low-cost options come with the greatest compromise in either low filter efficiency or high pressure drop.

Within the higher-cost product category of deeper pleated media, acceptable performance is available for both efficiency and pressure drop.

Finally, an electronic air cleaner will provide superior efficiency and pressure drop characteristics. While the initial installed cost is high, there is no recurring expense for replacement media.