Which Filter Is Recommended for HVAC Systems?

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As soon as you open a door to your home, sneeze, cook, or even sleep, particles in the air get stirred up. The matter found floating within the ductwork in your home, called particulates, needs to be removed for good indoor air quality. To keep the HVAC system running well and help protect air quality, good filtration is essential. A standard air filter is designed to capture large-sized particulates and is crucial to the performance of an HVAC system. By routinely replacing the air filter by following a manufacturer’s guidelines, you can prolong the life and efficiency of your system.

According to Energy.gov, “a clogged air filter can increase your air conditioner’s energy consumption by up to 15%.” Air filters are rated on a minimum efficiency reporting value (MERV). Residential furnaces and heat pumps commonly use 1 to 4 MERV filters and commerical buildings typically use filters rated from 5 to 8 MERV.

When the MERV rating number is high, a significant amount of pet allergens and a small portion of dust mite allergens will be trapped by the filter. Some HVAC systems may not be designed to accommodate high MERV-rated filters. Discuss the options with a licensed HVAC technician about your HVAC manufacturer’s recommended filter type and which MERV rating your system can handle.

Consider the whole house, not just individual rooms when deciding on indoor filtration. When you replace a filter with a better, high-efficiency filter than what was previously installed, you are almost always going to increase the static pressure of the system. Your contractor should measure the static pressure of the system to make sure you can still move enough air with the new filter in place. The duct system should also be evaluated for leakage and to ensure the home has pressure-balanced rooms. This can determine if the house has excessive dust due to being under negative pressure, or because of a return leak that is drawing in an attic, basement, or crawlspace dust and dirt.

Filtration Overview

What is filtration? Defined as the mechanical or physical process that is used for the separation of solids from liquids (air being a liquid), by interposing a medium through which only the liquid (air) can pass, removing any solids.

Filter History

Air filters were originally installed in heating equipment to keep the dust from entering the system, lessening the chance of mechanical failure. Filters have evolved over time, with increased customer expectations. Homeowners want filters to not only protect the equipment but to also keep the home clean.

Installed in the return side of the heating system, the sole job of the filter was to keep the dust from entering the system. By stopping dust before it entered, the blower section had less chance of mechanical failure and efficiency loss due to improper airflow. Then, because air conditioning was added to the heating equipment, the original wire mesh filter was improved to trap finer dust that would clog the new evaporator coil. However, the primary purpose of the filter was still to offer protection for the mechanical system. Another emerging use for filtration began to show up when electronic air filters were introduced, primarily limited to a few homeowners. During the 60s and early 70s, a huge influx of electrostatic filters hit the market, with many making claims of greater than 90% efficiency – and the filter industry was officially born.

While history may show that this was the beginning of filters being used as a means for improving air quality, there were some problems associated with these new electrostatic filters – restriction of airflow, most notably. In addition to the mechanical issues, the rating system of using either dust spot efficiency was confusing to consumers and left the door open for somewhat misleading claims by manufacturers.

Filter Types

Fiberglass 

Spun fiberglass is usually the least expensive, least restrictive, and least effective filter type. Typically constructed in 1” and 2” thicknesses, these filters can be found in most homes and can be purchased at most grocery or home improvement stores for very little cost. Since HVAC systems are different, finding the right size for your grille air returns can be a challenge. Some homes have multiple sizes, so stocking these can be a hassle. These are most effective if changed monthly. The main benefits of fiberglass filters include:

  • Inexpensive
  • Disposable
  • Good for capturing large particles like lint and dust
Electrostatic

Most electrostatic filters are made of tightly woven mesh and plastics, with some containing layers of foam or charcoal filtration material. Electrostatic filters vary as far as air resistance. Many HVAC professionals agree that these filters may be a problem when it comes to restricting airflow through the system. This is an attractive filter type because they are marketed as “lifetime” filters and only require regular washing with the water hose for maintenance. However, many individuals do not do this routine maintenance and the filters become even more of a problem for airflow restriction. The main benefits of electrostatic filters include:

  • Can be cleaned and reused
  • Require replacement only every few years
  • Protect furnace motor and trap larger particle
Pleated Media 

Pleated media filters have pleats (accordion-style folds) in them. This design creates more filter surface area, which allows for better filtering capability. The idea behind the pleated filter design is that if it is constructed with a tighter weave (to stop more particles) there must be a larger surface area to allow the air to pass through the filter with minimal pressure drop. However, these filters are being installed inappropriately in the 1” filter rack the manufacturer designed for equipment protection. The result? In many cases, the pleated filters are too restrictive. The concept of more surface area is a great idea, but when we make a highly effective pleated filter and place it in the incorrect 1” size slot, the airflow restriction is significant. The main benefits of pleated media filters include:

  • Disposable
  • Larger surface area captures more particles
HEPA

The American Lung Association defines HEPA as a High-Efficiency Particulate Arrestance filter. HEPA filters were originally designed for the U.S. military during World War II to prevent the discharge of radioactive particles from nuclear reactor facility exhausts. This filter type quickly spread into medical, industrial, and consumer applications. A HEPA filter must be able to remove 99.97% of all particles down to .3 microns.

A filter as effective as the HEPA filter is extremely restrictive to airflow. Too restrictive, in fact, for a residential blower to operate against. Therefore, a HEPA bypass filter has become the product of choice, so it can be used in residential applications. The HEPA bypass filter is installed outside the air stream of the system, connected with a section of ductwork, and has its own blower that draws the air through the filter and circulates it back to the air stream. The concept is that a HEPA filter blower can bring the air into the HEPA bypass filter around the air handling unit blower. This allows the filter to do its job without “choking” the air-handling unit by severely restricting airflow.

Filter Thickness Makes a Difference

In addition to knowing the standard height and width dimensions, air filters also vary by thickness. Filters can measure under an inch thick or up to six inches. Sizing the fit is imperative for an air filter to work properly. The wrong size may cause the filter to stop working. To ensure you get the right fit, remove your current air filter and check the size printed on the frame. If you can’t find these at a local hardware store, check out online options, or your HVAC contractor can order it for you.

Routine Maintenance Is Key

Just as you have your oil and filter changed out in your car routinely, you can keep your system running smoothly by changing out your filters regularly. It is time to do so when you glance at your air returns and see dust or dirt on them. Indoor pollutants including dust, pet dander, and pollen can be trapped – simply changing out the filters can help keep your air clean.

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