Air Conditioning-Furnace Filters: The Right Choice Makes Your Unit More Efficient, Saves You Money

Have you ever been reading the paper or watching TV when you are suddenly disturbed by a humming sound in the background and realize it’s your air conditioning or furnace unit pumping cool or hot air into your home (depending on the season) in order to make you feel more comfortable? Naturally you want to make the unit as efficient as possible and even if it is in good mechanical condition there are certain areas that you should check out to keep it in tip-top performance. A poor or clogged intake air filter in your unit could reduce the cooling or heating efficiency in your home by 30% to 50% and even more. With today’s energy costs that could mount up to a large sum of money, so making a little effort to select the right air filter for your A/C furnace unit is certainly worthwhile.

I had been replacing old air filters over the years in my A/C furnace when I suddenly realized that my favorite type has been slowly disappearing from the store shelves and then I found out that it was almost impossible to get the size that I needed. This prompted me to investigate what type filter I could substitute that would come close to what I had been using.

Upon my investigation of the filters being used in today’s market I was surprised that there had been no real radical changes in the design or improvement from the days when I worked on them many years ago. I might mention at this point that I had been in the air filter business and worked in a laboratory where we tested and designed air filters that were used in a forced air and heating systems that are very similar to those being used in today’s housing market.

So with some effort I started looking at all the different types of filters being used in today’s housing market that I could substitute for the type filter that I had been using in my household.

I am what you call semi-retired and got all fired-up when the need for air filters for my unit could not be found, so I am relying on my early experiences in this field. I thought that I would like to share these experiences with others who might benefit by them. I got into the air filter business purely by accident. Many years ago when I landed my first job as a project engineer for a fiberglass company located in New York. The company made a fiberglass mat that was used in lead acid automobile batteries to reinforce the lead oxide on the positive plate. The glass mat ranged in thickness from 15 to 20 mil. I found out later that they used the same mat material for making air filters. The mat was exactly pleated in the same manner as they use in today filters and then placed in a one or two inch cardboard frame. My job was to test these air filters and improve on their performance.

Not only did I test these filters but I also tested many of our competitor’s air filters to see whose filters were most efficient and had the highest dust holding capacity. The unit used for testing these air filters was designed in accordance to (ASHRAE) test method number (blank). The test numbers I do not recall. The test unit consisted of a 12” by 12.” upright air flow tunnel equipped with water gauges that measured resistance of the air flow before and after the filter being tested. Part of the testing procedure was to place artificial dust on to a vibrating trough that fed it into the intake top orifice of the tunnel at a slow given rate and the air flow velocity tunnel rate had been adjusted to the feet per minute (100 fpm to 300 fpm) that is normally used in supplying the average household A/C furnace units. The test method was intended to determine what each filter design was capable of in terms of. 1. Efficiency 2. Dust holding capacity 3. Air flow resistance before reaching.50wg” (inches of water gauge). At this reading (.50″wg) a filter is loaded to capacity and will not be able to allow a sufficient amount of air flow to pass through a filter to be effective in a heating and cooling system.

What is the function of an air filter and how does it work in your A/C furnace unit? Most air forced units in houses today are equipped with a blower that is designed to move large volumes of air through air ducts that are strategically located throughout the house. If the house blower was completely blocked either by placing a heavy polyethylene sheet or solid plywood board on the air intake side of the blower the air flow on the exhaust side would be nil (no air movement at all). Therefore it is important that the air intake of the house blower should not totally be restricted, but controlled. The air filter is placed directly on the intake side of the blower which will cause some resistance to the air flow however as the filter begins capturing the dust particles, the blockage of the air flow starts to diminishes thus restricting the air flow into a room.

Now that we know the basic function on how the air filter works in a household unit we now have to select the right filter for your unit. The only way to know is to test them. When turning on the testing (ASHARE) unit an air flow stream develops velocities that can range around 100 to 300 feet per minute depending on the material thickness and is measured by using a water gauges. When you insert the air filter test material you then have interrupted (blockage) the air flow and that is measured by the water gauge that is located on the output side of the air flow.this reading can is usually around.05″ to.08″ of w.g. and is called initial resistance to the air flow.

As the test proceeds the artificial dust that is being slowly fed into the dust chamber is gradually caught by the filter will increase the resistance of the air flow until it reaches.50″w.g. At this reading the filter has reached it full capacity.

When testing filters I found that the filter material we made in forming our basic glass mat (when pleated) showed fair to poor results, clogged up quickly and also did not hold much dust before failing. I was not too disappointed as mostly all of the competitor filters were in the same class. However there was one filter that was consistently outstanding, made by Owens Corning. The test results would repeat themselves time after time and would be almost identical in dust holding capacity and efficiency. It was superior to any filter on the market at that time. It was made using coarse fiberglass strands that were criss-crossed and placed clockwise side on the intake side of the filter and with finer fiberglass strands on the discharge side. When this filter was examined after completed its test cycle (5.0″water gauge) it was cut open and we noted that ever glass fiber was coated with dust particles. Unfortunately this filter had been discontinued for reasons unknown; however it was a great model to follow.

I had been testing filters on and off for about two or three years. Most of the filters I tested were single sheet air filter material and were either pleated or waffled and they were also subject to surface loading (clogging). What is meant by surface loading is that when the dust particles traveling in the air flow stream and are caught by the small openings (pores) located on the surface of the filter material. They will slowly block the incoming air flow. With this condition, the air flow (resistance) will increase rapidly thus cutting way back on the amount air flow coming into a room. The resulting reduction of air flow into your home means your A/C furnace will have to work twice as long in order to keep up. Surface loading means that there is no actual storage space for dust to pile up except for on the surface of the air filter material.

Rating the efficiency in an A/C furnace unit comes in two parts. First comes the efficiency of the air filter and then and then the efficiency of the air flow output. Note: if you did not put a filter or any objects in front of the blower unit you would then have 100% air flow (100fpm – 300 fpm). Unfortunately the air flow is reduced when a filter is installed thus cutting back on the air flow efficiency. Air flow efficiency and filter efficiency must be carefully balanced to get the most out of your household A/C furnace unit.

When we first introduced the all fiberglass filter we had considerable problems that were eventually overcome with time. In testing the all fiberglass filter we noted that the dust particles did not surface load and that the duct particles were gradually being caught by the glass fiber below the surface of the filter (storage area). Testing showed that the all fiberglass filters had dust holding capacities two to three times greater than that of single sheet filter material and still allow the air flow velocities to be maintained until it reached(.50w.g. ) on the testing machine..

But all fiber glass filters are not created equal. In choosing an all fiberglass filter you have to look at five things.

1. The intake side should have an open weave to allow the dust particles to enter and the exhaust side should be a close weave to prevent them from leaving.

2. Touch the filter face lightly with your finger and then note if any fire retardant oil has been transferred to your finger. Note: some all fiberglass manufacturers do not add a slight mist of fire retardant oil(approved by underwriters) to the filter material, however we found that this greatly improved the filters efficiency and dust holding capacity so we adopted into our production.

3. The back of the filter frame should be strong enough to support at the air flow velocities that are being used by your unit. Note: as a filter loads up with dust the resistance increases and can be easily sucked into your furnace.

4. Hold the filter up to the light. You should see a faint shadow of light. Then scan the filter from side to side observing the density to see that it is uniform.

5. Test for rigidity. Press down on the face of the fiberglass mat and then let go. It should bounce right back to its original height. A one inch all fiberglass filter is a “true” one-inch filter as it very seldom surface loads and allows dust particles to store up in the one inch thickness area (holding from two to three more dust) than the pleated filter with slight affect on the air flow velocity, which in turn allows the filter life to be greatly extended. Pleated filters have a problem in the dust holding capacity as the filter surface is the only place that the dust particles can be stored thus making the filter life considerably shorter.

When selecting a non-fiberglass filter in today’s market you must keep in mind the conditions in your own household. It may be necessary to obtain a high efficiency filter due to the allergies that run in your family. Most high efficiency filters will note on the filter package of what the filter is capable of collecting in terms of dust, spores and etc. Keep in mind that when using high efficiency filters the life span is much shorter than normal filters and have to be changed more frequently.

As for the normal household in choosing right filter I must admit that I have not tested all of the new filters in today’s market. However here are some of the basic rules that I would follow when I was testing newly designed filters.

1. Uniform density–test by scanning filter from side to side.
2. Air flow resistance. Hold filter up to the light to see if you can see through it. If not this filter may be too high in air flow resistance for your unit.
3. Dust loading capacity. Examine the surface of the filter material to determine if there are small pockets where the dust can settle for storage.
4. Strong filter frame backing. The efficiency of a filter has to be adjusted to the to the air flow output or the heating or cooling effect will be lost.

Keep in mind the area conditions surrounding your home. If you should live in a high dust location with a lot of construction or southern region where the air conditioning (part of your furnace) is going almost eight months out of the year then your choices will have to be made accordingly, keeping in mind the basic rules in selecting the right air filter.