Is the filter in your furnace or air handler there to protect the mechanical parts of the system, or is its purpose to provide clean air for your family?
Air Filter Purpose - Clean Air or a Clean-running HVAC System?
The answer is: BOTH. Because your heating and cooling system may eventually suffer a breakdown with a long-term buildup of dust and debris, it was designed to have a filter to trap these particles before they reach the inner workings. And a good air filter will trap indoor pollutants such as dust, pollen and pet dander, helping to improve the air quality in your home.
Having clean, breathable air circulating through your home is probably just as important as protecting your HVAC equipment from damage. According to the U. S. Environmental Protection Agency (EPA), most Americans spend an average of 90% of their time inside their homes. Of course, it varies by family and time of the year. However, the fact remains – we’re indoors a lot (too much?) In your home, believe it or not, the concentration of air pollutants are often 2 to 5 times higher than normally found outdoors. Unfortunately, the air in your home contains the pollutants found outside plus the pollutants created indoors – excessive carbon dioxide from breathing, cooking fumes/odors and the use of cleaning agents all raise the pollution level inside your home.
A visit to your local home improvement store or a survey of online shopping sites could leave you very confused as to what kind of filter you should buy for your home’s central heating and air conditioning system. Here is a rundown of the options available to help you make that decision. But before we get into the details, it will be useful to understand common terminology used when describing air filters.
Let’s Come to Terms
Before we begin, here are definitions for a few terms that we use when discussing furnaces, HVAC systems and filters. They are placed here for quick reference should you need to look back.
1. MERV stands for Minimum Efficiency Reporting Value. OK. What does MERV mean? It is a measure of the ability of a filter to trap particles from the air. MERV values vary from 1 to 20. The higher the MERV value, the more efficient it is in trapping smaller particles. Perhaps the most often recommended MERV rating for a home air filter is 8, though people with respiratory ailments might do better with a MERV 11 to 13. Just keep in mind that using a filter with a MERV rating that’s too high for your equipment might cause it to malfunction or break down. Sad, but true.
2. Air Flow, aka airflow, simply refers to the air that travels through the ductwork of your HVAC system, including the filter. The amount of airflow in your HVAC system is determined by the size and power of the blower motor.
3. Pressure Drop occurs within your system when anything interferes with air flow. In this article, it is primarily the filter. Some filter types and densities cause more resistance to air flow than others when new. When a filter is dirty, it will cause more pressure drop than when it is clean. Examples of this are easy to come by – Let’s say your vacuum gets clogged – the motor is still working as hard as ever, but the air flow into the head or hose is greatly reduced by the clog. The filter in a central heating and air conditioning system (HVAC system) by design slows the airflow because it traps pollutants.
4. Media is the material inside the filter that traps the dust and other air pollutants. Artists use the term “media” for the material they draw on, paint on or otherwise work with. The HVAC industry uses the term in a related way.
Types of Furnace Filters
Here are your options. We’ll describe them and then make suggestions for their use.
These filters are made of spun fiberglass fibers that allow maximum air flow, but also have the lowest MERV rating (1 to 4), meaning they will trap the least number of small particles. This type of filter was primarily designed to trap dirt and dust particles that could cause damage to your furnace blower motor. Though it will filter out the largest kinds of air pollutants, it may not be the right choice for significantly improving the indoor air quality.
These filters are cheap – Under $10 per filter and often less than $5.
Pros: The main attraction of fiberglass filters is their price — usually the lowest priced on the market. A six pack can often be found for less than the price of one pleated filter.
Cons: With a smaller surface area, they are quite ineffective in filtering smaller dust particles. This makes them a poor choice for people who have asthma and other breathing issues.
Pleated Furnace Filters
These air filters offer filtration from moderate to superior. We recommend pleated filters for your home’s HVAC system.
Pleated filters are the most commonly used filters today. They are available in many standard sizes and with MERV ratings from 5 all the way to 16. Their filter material can be polyester, microfiber, nanofiber or a cotton/paper blend. They are called “pleated” because these filters have an accordion-shaped face. This results in a surface area that is 2 to 3 times more than the perimeter of the filter would normally afford.
Pleated filters usually allow less air flow than fiberglass filters because their media is denser. This will vary, however, depending on the type and thickness of the media. The frame is normally made of cardboard, since pleated filters are designed to be thrown away when dirty and replaced with a new one.
Upgraded filters: Some pleated filters have enhanced their filter material with activated charcoal. You will find them advertised as Carbon or Charcoal Filters or Odor Eliminating Filters. They usually have a MERV rating of 8 to 11 and are supposed to remove many common odors from cooking, cleaning or pets as well as filter dust and other airborne particles from the air.
Pros: Pleated filters are very good at catching most airborne debris thanks to their larger surface area. They can be found in almost every size to fit dozens of HVAC brands and models. Pleated filters also last longer than fiberglass filters, meaning fewer replacements per year.
Cons: Depending on brand and MERV rating, pleated filters can be quite costly. Expect to pay $10 to $35 or a bit more (price changes are hard to keep up with currently). Caution should be exercised to not place a pleated filter in your system that will cause excessive pressure drop. The result could be low airflow through your air vents – and warm/cold rooms as a result. Consult your user’s manual or an HVAC technician to determine the right MERV rated filter for your system.
Electrostatic filters have several layers of material, usually synthetic materials including aluminum or polypropylene. Friction as the air passes through the first layer creates static electricity – like rubbing a balloon on your hair. The unwanted particles in the air become positively charged, and then as they pass through the remaining layers of media they cling to the material and don’t enter your duct system. These filters are designed to trap larger particles mechanically (Stop, you’re too big to get through here), and smaller particles electrostatically (by electrical attraction).
Electrostatic filters made from synthetic materials are intended to be cleaned and reused rather than replaced by a new one. Their frame is made of plastic or metal to withstand repeated washings. The few brands that advertise a MERV rating are usually less than 8. Most are designed to be washed by gently spraying with a garden hose or rinsed in the shower every 30 to 90 days.
Don’t wash these! A few electrostatic filters are made from organic, pleated material and are disposable rather than washable.
One manufacturer, Air Threads, offers a MERV 8 Polyester 3D mesh material that is a “natural electrostatic material”. It is best described as a mesh bag that slides over a plastic frame. To clean, the bag is simply removed from the frame and tossed into the washing machine, either alone or with regular laundry. It is supposed to last as long as 36 single-use filters, or 3 years.
Pros: Considered a cost-effective option to “throw away” filters. No need to keep buying replacements. Helps reduce waste.
Cons: While the static charge can trap smaller particles, the larger mesh may let larger ones – such as dust and mold spores - through. This may not be the best choice for people who have respiratory issues. Higher initial cost than replaceable filters. Care must be taken to allow the filter to dry completely before reinserting. A wet filter can introduce mildew and mold into the ductwork.
Electronic Air Filters
Not to be confused with an electronic air cleaner, these are high-tech 1” thick filters. Most consist of three metal grids and two fiberglass pads. They must be plugged into a standard electrical receptacle with a plug that looks like a phone charger. The top of the filter is a powerhead that places a high-voltage (low amperage) current on the metal grids, which charge the dust particles coming into the filter. At the other end of the unit, the oppositely charged grid traps the dust particles like a magnet. The fiberglass pads need to be replaced every 90 days.
Pros: Effectively captures dust and pollen – approximately MERV 8. Uses only about as much electricity as a night light. Replaceable pads available online.
Cons: Much higher initial cost of $80 or more. And the filter requires access to an electrical receptacle near the furnace.
Media Filters - The Most Popular Furnace Filters
Media filters are considered an “upgrade” from the standard 1” spun fiberglass filters that most HVAC systems are designed for. The media filter is made of a deeply pleated and compressed paper-like material that fits inside a special cabinet, not the normal filter slot. These filters start at 1” thick but most are 4”-5” thick. Because of their increased filtering area, they have the ability to have a high MERV rating without producing a significant pressure drop on the other side of the filter. And they have a long life – up to two years, depending on the environment.
Pros: Low maintenance, since they need to be changed so seldomly. High MERV ratings and low pressure drop due to their increased surface area.
Cons: Will not fit most systems and will likely need to be professionally installed. To have your air handler/plenum/ductwork modified from accommodating a standard 1” (one-inch) filter to a 4” or 5” filter (four-inch / five-inch), expect estimates in the range of $300 to $600 for materials and labor.
A High-Efficiency Particulate Air (HEPA) filter would be an upgrade to the top MERV filter, but they aren’t made for residential HVAC systems.
But just for information purposes, micro-glass fiber filtering paper is used in HEPA filters, which is able to trap most known airborne allergens and pollutants – up to 99.97%. MERV rating only goes to 16. HEPA filters have the equivalent of a MERV rating of 17 to 20. They can remove many of the smallest airborne particles common in most homes including tobacco smoke and bacteria. Normally used in hospitals and other places that require a high degree of air purification, an upgrade to HEPA may be the answer for people who live with allergies or other respiratory issues.
Pros: Very effective at collecting very small pollutants. May be a great benefit to those who suffer from allergies.
Cons: Because HEPA filters for HVAC use are thicker than standard 1” (one inch) filters, most will not fit HVAC systems without modification to the ductwork. In fact, HEPA filters slow airflow too much for residential HVAC systems, so are not made for this purpose. Plus, the cost is very high, and few homes would not benefit from filtration levels this extreme. Your vacuum might have a HEPA filter, or your air purifier, but furnace and air handler blowers are not made to push/pull air through them.
Final Thoughts to Keep in Mind
1. All filters need to be changed (or cleaned if washable) at appropriate times. Many of them will specify how often this should occur on their label. Unless you have a super memory, figure out a way to be reminded when it’s time to clean or change the filter.
2. What’s running - and for how long? If you have both central air conditioning and heating, filters often collect more dust in the summer than in winter. This is because there is more dust in the air when things are growing outdoors. It’s also true that your AC fan will often run faster when the system is in COOL mode than when in HEAT mode. That’s a great topic for another article.
Money-saving Tip: The long-story-short tip is this – Check your filter more often during heavy use. In Minnesota, the furnace runs more in winter than the AC runs in summer. In Arizona, the opposite is true – the furnace or heat pump will run very little during warmer months while the AC runs from spring well into fall – or longer. Therefore, to keep your airflow running as designed, check your filter regularly, and change it as needed.
3. Thicker is usually better. Most residential furnaces and HVAC systems are designed for a 1” thick filter. Filters are also available in many sizes from 2” to 5” thick, which almost always outperform their thinner cousins. In most cases, it would require some changes in the ductwork to be able to use one, but it might be worthwhile to check into that possibility, especially if anyone in the household has breathing issues.