Due to new homes being built to be “tighter” standards against air heating and cooling losses, the need for cleaner indoor air and air filtration is rising rapidly.
We often find the broad phrase “air filters” used to describe both the equipment and the internal element, media or process inside the equipment that captures particles from the air. The removal of unwanted particles or gasses from the air is called air filtration. The filtering of air is one of the four main areas of indoor air quality. The other three consist of: correction of problem sources (example: carbon monoxide gas), ventilation, and humidity control.
To help you understand air filters and purifiers, we will first divide the equipment into two categories: (a) “whole house” air filter equipment that is built into the air handler or ductwork, usually in the attic, closet, or basement of a home or building, and (b) portable, freestanding room air filter units. Due to the technical nature of the work, whole house air filters should be installed by licensed heating and air conditioning contractors. Portable room air filters or purifiers are sold in many outlets, from specialty stores online to the big box discounters. Portable air filters usually do not require special installation, but do require filter element or media replacement or maintenance. The filtering capacity of these systems and costs vary significantly.
Although some of the same principles apply, this section of the website focuses on whole house air filter equipment and air filter elements for your central air conditioning and heating system.
The main benefits available from whole house or central air filters are (1) Correct air filtration and flow to protect your air conditioning and heating equipment and (2) Removal of the specific quantity, size or type of particles or gas you want out of your indoor air. Additional considerations are cost and convenience of changing or cleaning the air filter elements.
Why indoor air quality is important
Indoor air quality affects our lives in a big way. Why? We spend up to 90% of our daily hours inside our homes and places of work, and that indoor air can be many times more contaminated than the air outside.
People who may be exposed to indoor air pollution in one location are often those most susceptible to its effects. Examples of these include infants and toddlers, the elderly, and individuals suffering from chronic illnesses, especially respiratory aliments such as asthma or cardiovascular diseases
Sources of indoor air pollution are gasses such as those from combustion for heating or cooking (when the appliances are worn or out of adjustment), off-gassing of volatile chemical compounds from carpet; furniture; composite or pressed construction materials and furnishings; chemicals used in household cleaning; room deodorizers; dry cleaning chemicals in clothes; chemicals in hobbies such as glues and paints; pest extermination and more. Particulates that can be removed with air filters include dust, mites, pollen, mold spores, fungi, bacteria, smoke and grease from cooking, pet hair and dander, and tobacco smoke particulates.
The ways air filters work and basic requirements
Air filters work by using a fan to pull air through a framework that contains a filter element, media or process. The purifying ability of indoor air filtering equipment depends on two main factors: (1) how much air the equipment can pull through the filter element (in cubic feet per minute, or cfm), and (2) the ability of the filter to collect particles and gas pollutants from the air (described by % efficiency) These two attributes must be present in the proper balance to be effective. A third factor that affects overall air filter performance is proper maintenance.
The most basic mechanical function of an air filter in a HVAC system is to remove airborne particles that would otherwise clog or damage the coils and other parts of the air conditioning and heating equipment. While doing so, the filter must allow the required minimum volume of air to pass through to prevent the coils from freezing or the overheating of heat exchangers. Failing to get the correct flow for your HVAC system, either from the wrong type of air filter element or a dirty filter, will cost you in efficiency, your utility or repair bills, and eventually comfort.
To improve human health and comfort, the basic filtering function can be enhanced to capture and remove particles in a specific size range and some gasses of specific types and odors.
Understanding air filter elements and media construction and materials
We can first divide air filter equipment designs and features into two categories: (1) the paper or fibrous filter media variety and (2) the electrostatic or electronic types without a fibrous medium for the air to pass through.
Within the fibrous media category, we can divide further as follows:
- Disposable paper or fiberglass mesh type air filter elements or media
- and washable or reusable air filter elements or media
There is a wide array of disposable filter elements available including:
- HEPA (High Efficiency Particle Air) type air filters
- Paper, fiberglass, etc. filter elements that last 1 to 3 months.
- Thicker, deeper, filter elements that require changing only once per year or so.
- Including of additional materials or treatments, such as carbon, for specific purposes such as removing odors from pets, smoking, or cooking. Other treatments inhibit microbial growth, etc.
Within the washable, reusable types of air filters, some are described by their manufacturers as “lifetime” filters. These gets a bit complicated, because these can include electrostatic properties within the mesh filter media plus other features to increase filter life while avoiding unwanted growth of mold, mildew, etc. on or inside the media.
Manufacturers use the following units of measurement to describe air filter element function:
- Peak Arrestance, or filtering ability expressed as a percentage
- Initial Resistance, expresses in Inches W.G.
- Dust Holding Capacity, expressed in grams
- Smallest size of particle that can be captured, expessed in microns.
- MERV (Minimum Efficiency Reporting Value) Rating, which describes the ability of an air filter to remove particulates from the air, with a higher number being better. A clean room application would need a MERV up to 20, better residential and commercial air filter elements around MERV11, and the filter in a window A/C unit has a MERV around 1.
Electrostatic and electronic air filters
Electrostatic air filters are commonly found in both free standing room air filters or in air filters within a central system. Equipment options within the whole house category range from one-inch thick air filter units that replace disposable filters to more elaborate electrostatic precipitators. In electrostatic filtering, instead of being physically trapped in a filter as the air passes through, the particles receive a physical charge and get removed by being attracted to the opposite charge inside a contained system. The collected dust and particles must still be removed, and the designs include canisters that must be emptied and cleaned to collection pads to discard.
Some of the factors to consider when comparing air filters and elements
If you are buying a new air conditioning and heating system, you will likely be offered the choice of different types of air filter equipment and elements that match the new HVAC equipment. If you have been using portable room air filters and want to convert to a whole house system, getting the type of air filtration you need in conjunction with a new HVAC system installation makes a lot of sense.
Starting with the most basic requirements, you need to get a filter element or process that meets the flow and filtration requirements of the air conditioning, heating, or air filter equipment in which it will be used. Since basic filtering for the equipment comes with all HVAC systems, this should be already calculated during the matching of the equipment.
Next, get the filtration features that meet your indoor air quality goals for health or comfort reasons in your geographic location. Finally, you can compare the features you need with the dollar cost over the life of the filter. Whether you plan to change or clean your own air filters or have it done for you, don’t forget to figure in the value or cost of the time required for air filter element replacement or cleaning. Some useful questions or considerations:
- Do you need to remove any specific contaminate from your indoor air for your comfort or health? (example: airborne allergens from inside your home, inside or outside plants or trees, asthma control, etc.)
- Special considerations for your situation: pet hair or dander, smoking, pollen in your area, amount of dust inside, etc.
- What is the minimum filter maintenance specified for the equipment in your central heating and air conditioning system?
- Are you willing to change the air filter elements yourself on schedule?
- Disposable air filter elements vs. the washable type.
- Effectiveness and cost (initial and long term) of whole house systems vs. portable room units.
- Effectiveness and cost (initial and long term) of the types of air filter elements for your system.
- If you plan to change the filters yourself, is the size (width, height, thickness) and type of any disposable filter element required for an equipment purchase readily available, or will it be difficult to locate?
- Can you purchase the filter elements from many stores, or are you required to get them from a single source?
Changing or cleaning air filters on schedule
Changing the air filter element per the manufacturer’s specifications is one of the keys to proper functioning and longevity of the air conditioning and heating system. If you have the type of air filter that needs to be changed monthly, many filter elements are owner replaceable. If you have a lot of filters to change or prefer not do it for any reason, you can hire a service to do this. If you use a monthly service, it helps to save a list of the number, size, and location of the air filters in your home or building. Otherwise, unless the service sends the same person each visit, you are likely to spend more time than necessary showing the new personnel the location of your filters.
If it is time to replace your heating and air conditioning system, you also could consider getting the type of air filter system that holds the elements that last twelve months or more and have the change done during the annual service of your system.
For best results, talk to an air filtration professional in your area before making a decision
Due to the number of options available in air filter equipment as they relate to conditions in your home or building, your satisfaction with the air filter you choose will likely depend on getting expert technical advice.
A quick way to learn about your options for air filter systems to fit your situation is to talk with a professional who specializes in air filtration. Although whole house air filters can be retrofitted, the ideal time to install one is during an equipment change. Ideally, since you have your air conditioning and heating equipment checked annually, you could use the same company to help with your air filter decisions.