Go To Dust Problems Page


There is a whole lot of hype and confusion out there these days about Indoor Air Quality (IAQ) and filtration. Should you buy a filter to purify the air in your home? What should you expect from a High Efficiency Filter? Which filter does the best job for your home? If I buy a high efficiency filtration system to go with my air conditioner will it eliminate dust in my home? Do room model air purifiers really work? Read on for the answer to all these questions and more.

The first question is what is the main purpose of a filter in your air conditioning system in the first place? A filter was first installed in a central air conditioning system simply to keep the equipment clean and to reduce the maintenance on the system. As air flows through the system the dust and dirt that is in the air deposits on the blower and coil of the air handling unit. This dirt build up can cause the blower and coil to become restricted and not move the proper amount of air. On gas or electric heat systems the dirt and lint can build up on the heater and when it goes into the heat mode it can smoke or burn off. This is not safe. The answer is to filter the air before it goes into the system. Thus Air Conditioning filters were born. The first filters were basic low density filters called Furnace Filters because they were made for heat only systems. These filters I call “Rock Stoppers” because all they catch are the very large particles. With no cooling coil on a system they did not need a very efficient filter. These typically fiberglass filters are very loosely woven and are very inefficient.

With the increased number of central air conditioning systems being installed a better filter had to be used to keep the inside cooling coils from plugging up with dirt. The standard rock stopper fiberglass filter would not do the job. An upgrade to a polyester tighter woven filter was made. This catches the big stuff and some of the smaller stuff and will keep the equipment cleaner. These worked fine with the older style cooling coils because they had very wide fin spacing. The newer higher efficiency cooling coils get part of their extra efficiency because the manufacturers have made the fins on the coils thinner and much closer together. When these coils get saturated with water in the cooling process they become very efficient filters, much more efficient than a standard filter. They catch the smaller particles of dirt and dust that the filters do not. The problem with this is that the coils catch and hold this dirt and then become restricted and cause cooling problems. This lead to the increased use of higher efficiency filters. These filters have to have increased surface area because of their increase in density of the material. The way this is achieved is by pleating the material to give more surface area in the same filter space. Then they went to thicker filters so that they could increase the density of the material even more and catch smaller particles. The trick here is to not cause too much restriction in airflow which will cause cooling problems.

The standard fiberglass “Rock Stopper” filter.  Generally this comes in one or two inch thickness.  This filter is considered a Furnace Filter and is really not suitable for a system with air conditioning.  It is inexpensive and widely available, but a homeowner that uses this type of filter will need to have their cooling coil pulled and cleaned approximately every two or three years because of the inefficiency.  This can actually be one of the most costly filters you can use.  Not only do you have to service the system much more, but the additional inefficiency of the equipment costs you dearly on your electric bill.

Probably the least efficient filter you can use is the aluminum mesh washable filter.  This filter allows the dirt and dust particles to slide off the aluminum mesh surface and is even less efficient than a fiberglass filter.  If you use this type of filter you will need to have your cooling coil pulled and cleaned every year.  Some people use these filters because they are not disposable and they don’t have to buy new filters every month, but it is costing them much more in inefficiency in the operation of their cooling equipment.

The standard polyester filter.  Generally this comes in one or two inch thicknesses.  This filter is an upgrade from the fiberglass filter, but is still pretty inefficient.  With this type of filter even if you change it regularly you will usually need to have your coil pulled and cleaned about every five years.  These filters are only slightly more expensive than the fiberglass but are several times more efficient.

A polyester pleated type filter comes in one, two, or four inch thicknesses.  These filters are made of a much denser material which catches smaller particles.  The material used is the same for the different thickness filters it just gives added surface area so that it will hold more dirt and you have to change it less often.  The efficiency is slightly higher for the thicker ones because the increased surface area allows the air to pass across it slower which makes it a little more efficient.  These filters are more restrictive to airflow and you need to be careful that you have a large enough filter to allow for the amount of air that you need to move through it.  If you have a restricted return space with too small a filter you can cause freeze up problems or sweating problems in your air conditioning system.

The next step is the HEPA type high density pleated filters.  These filters are generally four or five inches thick and provide a much denser material than the standard pleated filter.  They increase the surface area by providing more pleats so that there is not too much restriction.  These filters are very good and generally are rated at about a MERV 10.  These are cartridge type filters and generally need to be replaced every six months to one year depending on the home.  Another advantage to these filters is that they get more efficient as they get dirty.  The dirt build up tightens up the pore spacing in the material and makes it catch more of the smaller particles.   These units generally have to be installed by an air conditioning professional and do not just slip into the normal filter slot.

The April-Aire high efficiency air cleaner type filter is a high density pleated filter that comes in a completed unit.  The media has a MERV rating of 10, and is available in a MERV 16 also, and because it has over 76 sq. ft. of surface area it can generally go for about a year between changes.  This is a very good filter with no moving parts and nothing to go wrong.  Like the cartridge type media filters above, this filter also gets more efficient as it gets dirtier and has to be professionally installed.

One of the biggest falsehoods in the filter business is the Electo-Static filter.  This is a washable one inch thick filter that is touted to be up to 80% efficient.  These filters are made of a series of mesh screen materials that supposedly create a static charge as the air moves across them which is supposed to attract the dirt particles.  This is the same principle as running a comb through your hair and then picking up small pieces of paper with it.  This static charge is killed if there is any humidity in the house.  Basically when you can walk across a carpet in your home and shoot sparks to the door knob this filter will build a static charge.  In Corpus Christi, that is about 3 days out of the year.  This filter is very restrictive to airflow and not very efficient.  I do not recommend this type of filter at all.

The electronic filter which is a filter generally about five inches thick is run with an electric current.  This filter electronically creates a charged ion field which the particles pass through.  In this field they are electrically charged and then they pass through to a collector cell which has the opposite electrical charge.  The particles are then attracted and collected on the plates of the collector cell.  This technology has been around for decades.  These filters can be very efficient because they capture even the smallest of particles.  The problem is that in normal operation in your home the collector cells need to be cleaned every three to four weeks in order for them to stay efficient.  As the dirt and dust build up on the collector cells the efficiency decreases drastically.   The other problem with this type of filter is that in order to create the proper ion field and collector field they have to produce between 6,000 to 9,000 volts.  Because of the high voltage and the sensitivity of the cells these units tend to be very fragile and need service quite often.  Plus if the unit is not working then the only filter it uses is a small ¼ inch thick aluminum mesh filter which is almost like having no filter at all.  Because of the maintenance and the mechanical problems with these units I do not recommend them.

The Trane CleanEffects filter is a combination of High Efficiency Media type filter and Electronic air cleaner built into one.  The media prefilter catches the larger particles and then the Electronic part catches the small particles.  The prefilter keeps the electronic part from loading up with the large dust and makes it to where it doesn’t have to be cleaned as often as the standard electronic air cleaners.  It still has to be maintained and serviced regularly and there are electronic components in it that can go out.  If you have bad allergies and need the extremely small particles to be filtered out of the air then this is a good system for you.

Another type of filter is the filter that utilizes Ultra Violet (UV) light technology.  There are several different designs for these filters with different uses and efficiencies.  The main purpose for the UV filter is to eliminate Volatile Organic Compounds (VOC’s).  These VOC’s can be chemicals, odors, or organic gasses such as the toxins given off by growing mold and fungi.  The so called Self Cleaning filters that only utilize UV lights to filter the air do not work.  They may eliminate the Organic compounds, but the inorganic compounds such as polyester fibers and other dust components are not treated.  The Lennox Pure Air system utilizes the UV technology along with a high density pleated media type filter.  The UV part of this filter is to eliminate Odors and very small particles, and the media part catches the bigger stuff.  The drawback to this type of filter is the replacement cost of the UV bulbs and activation cartridges.  They are very efficient but the initial cost plus the ongoing costs can be prohibitive.

What Should You Expect From a High Efficiency Filtration System

Generally a high efficiency filtration system will capture all the particles that go through it that the naked eye can see, plus a whole lot of the particles that can only be seen with a microscope.  The High Density Media type filters will generally catch most all of the particles bigger than 10 microns in size.  As the particles get smaller the efficiency gets lower.  At about 1.0 microns the efficiency goes to about 40%.  A micron is a very small particle (1 micron = 1/25,400 inch).  A human hair is about 100 microns in diameter.  House dust ranges from 0.01 microns to over 100 microns in size.  Pollens and spores range from 10 microns up.

The biggest problem with filtration systems is that they only catch the particles that go through them.  Most of the dust and particles that you can see would be considered very large, heavy particles.  These particles do not stay airborne for very long.  They have a tendency to fall out of the air stream quite rapidly and therefore deposit themselves on the flat surfaces in your home.  Most of these particles never make it to the intake of your air conditioner and therefore your filter never has the opportunity to catch them.  If your intake grille for your air conditioning unit is in the ceiling then it is even harder to get the large particles to stay airborne long enough to be drawn up into the system to be caught in the filter.  So don’t expect any filtration device to eliminate the dust in your home or completely purify the air that you breathe. The higher efficiency filters will keep the air conditioning equipment cleaner which will make it operate better.  I have seen systems that were installed with a high efficiency media type filter that have run for 15 years or more and the coils were still clean when they were changed out.

So What Do You Recommend In the Way of Filters

I have always liked the high density pleated filter such as the April Aire or the Cartridge type filters.  These filters are very efficient and catch any particle you can see and most of the ones you can’t see also.  The other good thing about these filters is that they have no moving parts to go out.  You replace the media in them every 6 months to a year and have very little maintenance.  They also do a great job of keeping your air conditioning equipment clean and operating efficiently.  The only draw back to these filters is the size and the fact that they have to be installed.  The best time to put one in is when you are changing your equipment.  They can usually be installed on existing horizontal systems in the attic, but upflow units in a closet normally don’t have enough space under them to install one.  On an upflow system you usually have to remove the entire inside system to install the filter so it is best to install it when your equipment is being changed out.

If you have bad allergies and need to filter out the smallest of particles then I would recommend utilizing either the Lennox Pure Aire system or the Trane CleanEffects air purifier.  Both of these systems catch the big stuff and the extremely small stuff in the air.  But remember that a lot of the biggest particles in the air fall out of the air before they ever reach the filter and the filter can only filter whatever is in the air that passes through it.

If you are using a one inch disposable filter at least upgrade to the polyester filter, don’t use the fiberglass ones.  If you can upgrade to the one inch pleated filter it is even better, just make sure it does not restrict the airflow.  If you have an Electrostatic filter change it out to a pleated filter as soon as you can.  If you are using an Electronic filter you need to make sure that you clean the cells at least once a month, preferably every 2 to 3 weeks, and make sure the system is working properly.  If the cells do not get dirty within the one month period then the system is probably not working with full voltage and efficiency.