Air Filter Tutorial


Air filters, AKA furnace filters are installed in the central path of the airflow going to your furnace to be heated or cooled.  All the air going through the airflow on its way to the blower system in the furnace or air handler must pass through them.  The filter traps and retains particles that are contained in the air stream.


Filter Types


  1. Disposable filters
  2. Permanent washable media filters
  3. Electronic air cleaners
  4. Hybrid models.  These are electronic air cleaners with disposable filter media.


The vast majority of people have #1, disposable filters.  These filters come in many frame sizes and thicknesses.  But you can only use the size that will fit in the spot where your filter goes.  So if that spot is designed to accept a 16” x 25”, 1” thick filter, then that is the size you must buy.  If you wish to use a larger filter, you must have the ductwork modified to accept that filter size.


There are two major factors to consider when buying disposable filters.


  1. The MERV rating
  2. The amount of airflow restriction that filter causes


The MERV rating is a measure of how good that filter is at trapping and retaining small particles.  The higher the MERV rating is, the smaller the size of the particle it will trap and retain.  The problem here is that the higher the MERV rating is, the higher the resistance to airflow will typically be.


Restriction to airflow (“pressure drop”)


A disposable air filter is basically just a piece of woven fabric made of fiberglass or other synthetic materials.  The higher MERV filters have a more closely woven fabric than the lower MERV ones.  So there is less “free area” between the pieces of fabric for the air (containing the particles) to pass through.  So the particles that are larger than the openings become trapped.  Obviously, the higher MERV filter is going to cause more restriction to airflow, because it has less total free area for the air to pass through.  All disposable filters have one thing in common: The more dirt they accumulate, the smaller the particle size they will trap, because the air passages in the filter are becoming progressively smaller.  So when you buy a filter at the store, the MERV is at the lowest point it will ever be.  But so is the restriction to airflow, because there is more “free area” in the filter for the air to pass through.  The resistance to airflow of a brand new filter is called the “initial pressure drop”.  And the totally useless specification for how much restriction to airflow the filter will have when it’s full of dirt is called the “final pressure drop”.  That one is really dumb because it will kill any furnace or A/C out there before it ever reaches the “final pressure drop”.  The term “final” is a good one in that case, because it can mean that the next filter you buy will be serving a new furnace or A/C.


All filter media of a given MERV is not created equal.  Some filter media designs are more efficient than others at allowing more airflow to pass through the media with less resistance to airflow.


So when you’re going down the aisle at the hardware store looking through the maze of air filters, how do you know how much restriction to airflow a given filter has?  Unfortunately, with most of the less expensive brands, the pressure drop (restriction to airflow) specs are not listed on the package or the unpackaged filter.  Good luck finding out that information too.  Some of the filter manufacturers don’t publish those specs at all, for a very good reason, i.e. if you knew how restrictive they were, compared to the others, you wouldn’t buy them.  And let’s keep in mind that the manufacturer’s specs are for the “initial” pressure drop (the pressure drop of a CLEAN filter).  So they don’t show you what happens when they get a little bit of dust built up in them.


(For the techies)

The average blower system in a residential furnace is capable of overcoming 0.50 inches of water column of static pressure in the entire airflow system while still being able to deliver its rated CFM output.  The airflow system includes all the return and supply ductwork, air grilles, dampers, cooling coil, and last but not least, the filter.  The cooling coil alone uses up between 0.20 and 0.30 inches of water column of that 0.50.  So we only have 0.20 to 0.30 remaining for everything else.  The average duct system including the grilles uses up about 0.22.  So there is little or nothing left for the air filter.  The pressure drop ratings of air filters vary from 0.07 (for an unpleated disposable filter) to 0.40 for some of the pleated models.  As the velocity of the air increases, so does the pressure drop across the filter.  The increase in pressure drop per increase in FPM of the velocity is usually exponential in most filters.  Some of these exponential increases in pressure drop can exceed the power of 2.  So if the velocity doubles, the pressure drop quadruples.  This would mean that a filter with a seemingly low pressure drop of 0.07 would have a pressure drop of 0.28 if we were to double the air velocity at which the filter was rated at for the 0.07 pressure drop.  Yes, this happens all the time, because the typical rated velocity for the specs is done at a meager 300 FPM air velocity.  Velocity = CFM divided by area in square feet.  And when we halve the area, the velocity doubles.  So if our filter setup is only designed to accept a filter that is half the proper size for our CFM, the velocity is going to double.  And in a filter with an exponent of 2, this means that the pressure drop is going to quadruple.  So you’re really screwed if your filter setup is only half the proper size for the CFM moving through it.  If you’re going to use filters that are rated at 300 FPM velocity, you must use the formula: Filter size in square inches of cross sectional area = CFM divided by 2.08.  This will give you the filter size required for that CFM at a velocity of 300 FPM.


Furnace & A/C killers


An overly restrictive air filter can literally destroy your heating and cooling system.  Modern furnaces require much more airflow than their older counterparts.  This is because the higher D.O.E. mandated efficiencies are only attainable by using thinner metal in the furnace heat exchangers (firebox).  The thinner metal heats up more efficiently.  The drawback to this is that the metal can’t be allowed to reach anywhere near as high a temperature as the old, thick steel heat exchangers because the metal will crack.  So the newer furnaces have a much higher airflow requirement to prevent this from occurring.


If you run the A/C system with less than the required amount of airflow, the temperature of the indoor cooling coil can drop below 32-degrees and ice up.  When that occurs, water runs over the ice and pours down through the furnace, including the electrical components and the heat exchangers.  This can result in serious furnace damage.  This also forces the compressor in the outside unit to attempt to compress the “Freon” in a liquid, rather than a gas state, which is an impossible task to accomplish, because you can’t compress a liquid.  You can only compress a gas.  So the compressor can be destroyed.


If you are fortunate enough to escape these disasters, you still pay a hefty price in the form of extra energy costs for both the heating and cooling system, because the system must run much longer to maintain the desired temperature in your home, because the Btu/h output (heating or cooling power) per hour of the furnace and the A/C is reduced by poor airflow.  You may also notice that the rooms that are the farthest from the furnace will be less comfortable, because the airflow doesn’t have the force to reach those areas.


How often should I change my filter?


You’ve probably seen the “big lie” printed on the air filter package:

”Lasts 3 months”.  This is a perfect example of blatantly deceptive advertising.  I’ve seen those filters plug up solid with dirt in 5 days.  Each cubic foot of air in each environment contains x amount of dust.  And each air filter will only hold x amount of dust before it totally kills the airflow.  So you would have to know how many cubic feet of air containing x amount of dust have passed through that filter in 3 months before you could make the assumption that the filter would last 3 months.


(Update 7/25/10)  One manufacturer that had previously been putting the label “Lasts 3 months” on the package apparently read this page, as they decided to reword it as “Lasts up to 3 months” a while back.  Thanks! 


Obviously, if your system is running longer due to colder winter conditions or hotter summer conditions, the filter will have to be changed more often, because there are more total cubic feet of air going through the filter each month.  And obviously if your thermostat setting preferences are higher in the winter and lower in the summer, the filter will have to be changed more often for the same reason.  And in those periods where you are using neither heating or cooling, no dust will enter the filter.


A good, basic, accepted rule is that the filter should be checked monthly.  Obviously if the filter looks pretty badly plugged up in a month, you’ll need to change it more frequently.  Don’t wait until they get really dirty to change them or you could trash your furnace and/or A/C system, or at the very least cause it to have to run a lot longer and therefore use much more energy.


Pleated filters vs. non-pleated filters


Pleated filters are relatively new to the residential market.  They have the potential advantage of giving the filter more free area for the air to pass through, while still having closely woven fibers.  And they also have the potential advantage of being able to hold more dust.  If you take one of these filters apart and unfold the filter media, you’ll see that the media is much larger than the frame size.  While this is a very good concept, the effect isn’t always as good as you might expect when your filter setup will only accept a 1” thick filter.  But the effect is very good if you have a multi-inch thick filter setup, i.e. a 5” to 7” thick filter.  You can often have your cake and eat it too with these multi-inch thick filters.  Here’s why:  Let’s say that you currently use a 20” x 25” unpleated filter.  This equates to 500 square inches of media area, or 3.47 square feet.  A 20” x 25” multi-inch thick Aprilaire 2200 media filter has a staggering 76 square feet (11,000 square inches) of media area.  So the pressure drop is very low and the dust holding capacity is enormous.  You only have to change the filter media once per year under average conditions.  And it’s a 10 MERV filter. 


The big thing to watch out for when you’re shopping for a pleated filter is the number of pleats per foot.  The more pleats it has, the more media it has.  So with all other things being equal, it will have a higher dust holding capacity and a lower pressure drop than other filters of the same thickness having less pleats per foot.


Permanent washable media filters


These are the ones you see in the stores with the plastic or metal frame around a washable media.  I do not recommend these filter types, because they have a VERY high pressure drop, i.e. the highest I have ever seen in any air filters.  The problem I’ve seen with many of them is that they have 3 layers of media.  When you wash them out, it is practically impossible to dislodge the particles from the center layer.  Many of these tout the extra advantage of using electrostatic energy to snatch the particles from the airstream.  They supposedly are able to generate this electrostatic charge by causing the synthetic fibers to rub against each other as the air flows through them.  And this supposedly generates static cling, like what you see when you pull the clothes out of the dryer.  The problem is that the fibers generate little or no electrostatic field when the particles start to build up on them.


The other quite obvious downside to these filters is that it’s just a lot better idea to dispose of something that may contain allergens.  The bottom line is that you should just say no to these washable filters.


Electronic Air Cleaners


These have been around for a long time, and they claim to have some very high particle trapping rates, i.e. so high that they’re off the MERV scale.  But those of us who’ve also been around a long time know full well that they usually don’t work any better than a MERV 2, unpleated fiberglass filter.  We can tell at a glance how poorly they work in real world conditions, rather than those found in the testing laboratories.  These systems work by sending a few thousand volts DC into electrically charged cells with opposing positive and negative plates.  This places an opposite charge on the particles, and they are attracted to the oppositely charged plates.  The problem is that in most systems, the dust particles are moving so rapidly that it has little effect on them.  So they just move right through the cells.  The other problem is that as soon as a very minute layer of particles builds up on the cells, it acts as an insulator that reduces the intensity of the electro-magnetic field.  So there isn’t enough electromagnetic force to grab the particles as effectively.  This is the exact opposite of the way disposable filters work, because the dirtier a disposable filter gets, the more efficient it becomes at trapping the particles.  The normal aging process also takes its toll on the cell as well.  So the only reason they look so good in the specs is because they are using a very low air velocity with brand new squeaky-clean cells to test them in the laboratory.


Other disadvantages:

The cells are also very fragile and easily damaged when they’re removed to be cleaned and are placed back in the unit.

If a piece of hair or a large dust particle becomes trapped on the face of the cells, you’ll hear this popping sound.

If the electronics in the unit malfunction, you have basically no air filter at all.  So the entire airflow system gets trashed.

They’re expensive, and they use electricity.

If you put the cells back in before they’re completely dry, you can fry the electronics in the unit.  Many people are aware of this and wait for a day or so to turn the air cleaner back on.  But if they forget to turn the air cleaner back on, they have no filtration and the airflow system gets trashed.

They emit ozone, which is a ground level pollutant that can cause lung damage.  Ozone is a serious trigger for asthmatics as well.

It’s no fun cleaning the cells and it should be done at least on a monthly basis.  The best way to clean them is to take them outside and wash them out with a hose equipped with a pressure nozzle.  But this is impractical or impossible to do in the winter.


Hybrid air cleaners


This is actually a cross between a multi-inch thick disposable air filter and an electronic air filter.  These have a multi-inch thick disposable air filter media in them that has electric circuits built into the disposable media.  These have extremely high (off the charts) MERV ratings, just like the electronic air cleaners do.  They are very expensive to have installed, and the media is a bit more expensive to replace as well, because it has the electrical circuitry contained in it.  The very good thing about them is that the filter media is a MERV 10 with the electronics turned off or not functioning.  So you’re guaranteed a minimum of 10 MERV no matter what happens.  The downsides (other than the high cost) are that they are subject to a few of the same problems as an electronic air cleaner, i.e. the electronics can and do fail.  And they do generate ozone as well.


Allergies and air cleaners


I often run into people with severe allergies and asthma.  Some of them have very expensive filter systems costing up to $1200 per system.  But many of them claim that it isn’t helping with their ailment.  And many of them claim that the house is just as dusty as it was before they had their high dollar filter systems installed.  Unfortunately, these people have been the victims of advertising that leaves out one simple fact.  Any furnace filter will only trap the particles that are actually reaching the filter.  The simple truth of the matter is that there is no blower system capable of moving enough air through the cold air return grilles and ductwork to the filter.  This has been proven in numerous tests.  So while a given filter might be capable of removing 95% of the particles that reach it, only a tiny fraction of all the particles in the house are actually reaching it.  If you don’t believe this, then I urge you to perform a very simple test.  Turn on your blower system, then stand in the middle of a room and throw a handful of dust in the air and see how much of it gets sucked into the return grille.  And once the particles settle on an object, like the floor, there’s no chance in the world of there being enough air pressure to move them toward the grilles. 


So if you have allergies, the best plan is to place a good, portable air cleaner in the room you spend the most time in, like the bedroom.  That will at least put a big dent in the amount of allergens that reach your respiratory system.


And one of the best investments you can make is a vacuum sweeper equipped with an HEPA filter system.  If you use a regular vacuum, you’re picking up all of those particles and throwing them back into the air.


Where does all that dust come from?


The sources of the particles come from both inside and outside the house.  On the inside, there is a constant breakdown of construction materials, paints and fabrics over time.  And there is dead skin sloughing off of the inhabitants (including the pets).  On the outside of the house, the sky’s the limit.  Older homes are often very loose.  So when the wind blows, the air pressure outside the house is greater than it is inside the house.  So the air and the particles in it enter the house.


Many new homes are built so tightly that they are actually placed into a negative pressure when the furnace, water heater, clothes dryer, bathroom fans, etc. are in use.  This creates the same issue, i.e. the pressure outside the house is higher than the pressure inside the house, so the air and particles in it are forced inside the house.


The cure for these problems is to have a special system installed in the house that places the house into a slightly positive pressure.  This prevents outside air from entering the house, because the pressure in the house is higher than the pressure outside the house.  The positive pressure in the house is created by a blower system that draws in a controlled amount of airflow through a duct entering the house from outside.  This controlled airflow is run through a filter system as it’s brought into the house.  So the only air entering the house from the outside is filtered as it’s brought in.


Is it possible for a high MERV rated filter system to be ineffective at trapping and retaining particles?


Absolutely.  I see this problem in the vast majority of the homes I go to on the first visit.  It’s one of my favorite rants.  Simply put, all of the air reaching the blower system in the furnace should first have to pass through the filter media in the furnace filter before arriving at the blower system.  But this is typically not the case in most systems.


There are 2 main causes for this:


  1. There are gaps around the outside edges of the filter frame in the ductwork.  This allows the dirt to go around the filter rather than through it.
  2. There are openings or gaps in the ductwork past the filter.  This allows the dirt from the area around the furnace to be sucked in after the filter.


Let’s start with number 1.  There are a few different scenarios where this is going to occur.  It’s a no-brainer that if you turn the furnace off and see these gaps around a brand new disposable filter, you’re not going to filter that air that is going through those gaps.  But many of the problems I see involve overly restrictive disposable pleated filters that warp and bend when the air hits them.  This is a very common problem when people buy overly restrictive filters where the cardboard frame isn’t sturdy enough for the filter to maintain its shape at the air velocity going through the filter.  So the filter actually starts to become like the sail on a boat.  This reduces the outside dimensions of the filter and creates a gap around the frame.  There are a few aggravating problems that make this problem worse, like if the air volume is too high, or the filter is too restrictive to airflow, or it just doesn’t get changed often enough.  If the problem is bad enough, the filter will get sucked right into the blower system and burn out the motor.


In the case of number 2, where the dirty air from the area surrounding the furnace is sucked into the system, this can happen with any filter system, including the multi-inch thick media filters or the electronic air cleaners or hybrids.  The filter system might be doing an admirable job of trapping or retaining the particles and not allowing anything to get around it, but the dirt from the basement air is being sucked into the system after the filter system.  So it doesn’t have to go through or around the filter to enter the airflow system. 


In either case number 1 or case number 2, the problem is further aggravated if the filter system is overly restrictive, because the dirt through the unwanted paths will be moving at a higher velocity due to the increased negative pressure caused by the overly restrictive filter system.


Could I have problems even if I use unpleated filters?


Absolutely, especially if the filter setup is undersized for the volume of airflow going through the system.  But the problems won’t be nearly as bad as they would be with a more restrictive filter of the same frame size.




Be careful which filter you buy.  Before making any decisions consult a professional with airflow testing equipment like the instruments I use.  They can tell you very quickly how much resistance to airflow your system configuration can overcome and still put out enough airflow to keep your system from being damaged.  If you’re in doubt about how much restriction your system can overcome, then you should purchase the unpleated, disposable fiberglass filters because they are the least restrictive.  They might not trap as much, but at least you won’t kill your system or restrict the airflow to the point where the higher negative pressures from a more restrictive high MERV filter will cause the filter system to become ineffective.


It’s impossible to have a disposable air filter system that is too big.  The larger the frame size it will accept, the less velocity there is to force the particles through or around the filter, and the less restriction to air flow there will be.  And the higher the MERV rating of a filter you can use without killing your airflow.


Areas Served

Greater Kansas City including:

Johnson County, Kansas     Kansas City, Kansas       Kansas City, Missouri

Fairway, KS

Lake Quivira, KS

Leawood, KS

Lenexa, KS

Merriam, KS

Mission, KS

Mission Hills, KS

Mission Woods, KS

Olathe, KS

Overland Park, KS

Prairie Village, KS

Roeland Park, KS

Shawnee, KS

Spring Hill, KS

Stanley, KS

Stilwell, KS

Westwood, KS

Westwood Hills, KS

Copyright 2011 Leonard Arenson Heating & A/C


Updated 12/16/15


Back to Tutorial Menu


Back to Main Page