Photograph of a filthy air conditioning air handler filter

Air Filters for Heating and Air Conditioning Systems
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Air Conditioner, Heat Pump, or Furnace Air Filter Location, Condition, Problems, Repairs
Air conditioner filter location: filters should be readily accessible
Dirty Air Conditioner Filters Cause Multiple Problems for an Air Conditioning System
Improperly-Fit or Wrong Sized Air Filters Cause Bypass Leakage
How to Install or Replace an Air Filter in Heating or Air Conditioning Duct Systems
How to Construct or Obtain Large or Special-dimension HVAC System Air Filters
The Effects of & Problems Caused by Missing Air Conditioner Filters
Questions about air filters: buying, installing, using, and problems with HVAC air filters

Air filters on HVAC systems: this article series answers almost any question you might ask about air filters for central heating or air conditioning systems. We explain how an air conditioning service technician will diagnose certain common air conditioning system failures or defects. In these articles we are referring to filters installed on central air conditioning or central heating systems that move air through air handlers and duct systems. Standalone or portable "air cleaners" are generally ineffective in buildings where a problem particle reservoir is present, and they are not capable of removing an indoor air quality problem.

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Air Conditioner, Heat Pump, or Furnace Air Filter Location, Condition, Problems, Repairs

Readers should also see Air Filter Effectiveness and Real-World Effectiveness of Air Cleaners in our INDOOR AIR QUALITY IMPROVEMENT GUIDE article series.

First locate and document the placement of the HVAC system air filters - for examination and regular changing during the cooling season.

In our photo at left you can see a blue and white electrostatic air cleaner on the air handler.

But notice at the upper left of the photo just below the brown metal of the air handler body: see that silver sheet metal handle? Removing the two screws on either end of the pull-out will permit you to expose another air filter that is in this location - the handle is a tip-off that the air handler is meant to be opened at this location in order to remove/replace an air filter.

Are these the only two air filters on this system? Nope. Our discussion of cascaded air filters found at AIR FILTERS, OPTIMUM INDOOR includes photos of a front-end air filter found at the return air register. In sum, finding a filter on a duct system or air handler is no promise that it's the only air filter installed. Inspect the system thoroughly. If more than one air filter is provided, document the location of all of the filters installed.

Next inspect the air conditioning filter type and condition. What about filters that are missing completely or are very dirty? What problems can a dirty or blocked air conditioner filter cause for the air conditioning system and how do we fix these snafus? That's what we'll cover in this article.

Air conditioner filter location: filters should be readily accessible

Filter accessibility: Air filters which are hard to access are rarely changed as often as necessary. We frequently see HVAC systems designed by someone who obviously has never had to service them.

Placement of filters and air handler access doors in very hard-to-access locations such as at the far end of a minuscule attic behind a forest of trusses means that the system is very unlikely to receive the periodic inspection and maintenance it needs.

We prefer to see A/C and heating filters placed at the building side of the air return register or grille, so as to protect the return duct from debris accumulation. The more common filter placement on many systems is right at or in the air handler.

After reading the text just below, if you still cannot find your heating or air conditioning system air filter read our detailed instructions on how to find air filters in our article: Air Filter Location. Just below we summarize some common locations where air filters are found.

At the central air return register, grille located in a wall or ceiling if your system uses centralized air returns instead of individual room-air return ducts. There may be several central return points, depending on the design of your system. If there are more than two, chances are the filter was placed at the air handler instead of at these grilles. Unfortunately that means that the return ducts themselves become more soiled with dust and debris from the building.
At an attic air handler look for a slot which has a removable cover. The slot may be just an inch or so wide if 1" thick filters are used, or it could be several inches wide if a wide high-capacity pleated or similar filter was used. The return air plenum on an attic or basement air conditioner blower unit will usually be a large metal enclosure about the same dimensions in width and height as the air conditioner blower unit itself. Look for a filter slot right where the return plenum contacts the blower fan assembly.
At a basement air handler we also look for a filter at the return air plenum which is often next to the bottom of the air handler if the system is an "up-flow" unit (or vice versa for the less common case of return air entering at the top of the air handler and exiting at its bottom).
Next to an electrostatic air cleaner: if your air conditioning air handler has an electrostatic air cleaner installed, look for the filter, if there is one, next to the electrostatic air cleaner. In addition, the electrostatic air cleaner, which is a type of particle incinerating filter itself, needs to be removed and cleaned periodically. (Check with your unit's manufacturer for cleaning interval and procedures. Often the electrostatic unit can be cleaned inside a dishwasher). Often there is also a thin metal washable air filter installed along with the electrostatic air cleaner.

Change your air filters every month when the air conditioning system is in operation. Make sure you find all of the filters as some systems have multiple filters and even multiple types of filters installed, such as a fiberglass or pleated paper filter, a washable filter, and an electrostatic air cleaner. These last two are cleaned, not replaced, when they're dirty.

Dirty Air Conditioner Filters Cause Multiple Problems for an Air Conditioning System

See our detailed article at Dirty Air Filter Problems. Excerpts are below.

Photograph of a dirty air conditioning filter

Dirty Air Filters: are a source of increased operating costs and poor cooling system operation. Dirty air filters can:

reduce air flow in the building
cause dirt to accumulate on the fan blades, wasting your energy dollars
cause excessive dirt build-up inside the duct system, leading to mold or allergen problems in a building and to the need for more costly duct cleaning or replacement
block the cooling coil itself with dirt, reducing system effectiveness and possibly leading to costly repairs
lead to frost build-up on the cooling coil and reduced or totally blocked air flow in the system
eventually permit dirt to bypass the filter where it soils and blocks the blower fan itself, leading to more costly repairs.

The filters on an air conditioning or hot air heating system should be changed monthly when the system is in use. Discuss with your heating/cooling service professional the possible need to clean the blower fan and duct work.

Improperly-Fit or Wrong Sized Air Filters Cause Bypass Leakage

Bending over the end of an air conditioner or heating air handler filter such as shown in the photo at left above is a bad idea. If the filter does not fit there will be bypass leakage past the filter, soiling the blower fan, slowing air flow, and leading to more costly cleaning and service later.

Furthermore when you bend the filter as this owner did, you interrupt the structural integrity of the filter's frame, risking filter collapse. A collapsed air filter can be drawn right into the blower fan, causing damage to the fan motor or even leading to a fire!

The photo at right shows how a college HVAC maintenance crew kept the A/C system running when the school did not have the proper filter size in stock. This filter installation also will have severe bypass leakage around the filter where the pleated section contacts the edges of the filter slot.

Installing a filter that is the wrong size for the heating or air conditioning air handler defeats the purpose of air filters because of leakage and it may be unsafe. Install a properly-sized filter in locations like this as soon as possible and watch out for unsafe filter collapsing.

How to Install or Replace an Air Filter in Heating or Air Conditioning Duct Systems

Find all of the air filters or locations where air filters are supposed to be installed in your heating or cooling system.

Watch out: there may be multiple air filters to find, inspect, and replace. See Air Filter Location for help in finding where your air filters may be located.
Inspect the air filter that is already installed: is it dirty, bent, damaged, leaky? If so it should be replaced. See Dirty Air Filter Problems and Missing or Leaky Air Filters
Write down the air filter measurements: thickness (usually not critical or usually standard anyway), and the dimensions (length and width). Providing that the filter already installed fit properly, you will want to buy an air filter that is the same size so that it will fit properly.
Buy your replacement air filters. There are usually lots of choices in filter quality, fit, gaskets, etc. An air filter that has gaskets will be less leaky and the HVAC system will operate more efficiently with one installed. An air filter that has increased filtering capacity may help reduce the level of airborne dust, pollen, mold in your building.

Watch out: if you install an air filter that is so dense that it significantly reduces air flow through the system, the delivery of conditioned air (heated or cooled) may be noticeably reduced.
Install your new air filter, making sure it is seated properly, not bent or damaged, and that it is facing the right way: arrows on the filter edges show the direction of air flow through the filter.

Question: How do I know which way my air filter should face when it is installed?

I am in the process of changing the filters in my new home for the first time - there are three located in various areas, easily accessible. I have purchased the correct sizes. Although marked by "air flow" arrows, I'm still confused as to which way they should be installed - does the "webbed grill" side face in or out? I know this may be a "no brainer" but it is a first for me and I want to make sure I place them correctly. - Beverly Upperman

Reply:

Think about the problem we need to avoid: if an air filter gets sucked into a duct or worse, into a blower fan assembly, it can jam the blower fan, cause a motor overheat, and even cause a fire. So we want the air filter to be placed so that its reinforced side will prevent it from being drawn into the duct or blower system.

Air filters have an arrow on their edges to give us a clue about how they should be placed. Insert the air filter so that the arrow is facing in the same direction as the direction of air flow through the filter. So if you are placing a filter on an HVAC system central air return grille, air is "returning" or "entering" the grille from the building, so air is flowing from the building into the duct system at that location. So the arrow would point the same way - into the duct system.

How to Construct or Obtain Large or Special-dimension HVAC System Air Filters

The same college HVAC maintenance staff who was struggling with improperly-fit air filters we discussed earlier was also faced with the task of coming up with a much larger air filter for the air conditioner air handler over their computer center. The neatly-taped "built-up" air filter shown in this photo was nicely constructed but we don't recommend this practice:

The filter may come apart and send fragments into the blower assembly, damaging the blower or leading to overheating and a fire.
The filter is not delivering the total cross-section of filtering area that was anticipated by the HVAC design engineer who specified the dimensions of the return air plenum at which this filter was installed - we're seeing less total CFM of airflow.

Air filter suppliers and manufacturers have no trouble providing air filters of special dimensions. Furthermore if the filter is built by a manufacturer it's more likely that they'll understand the structural and strength requirements of the filter as well as the required airflow characteristics and filtering ability. We list some suppliers of air filters at SOURCES FOR AIR FILTERS

Incidentally, except unusual cases with special requirements, wouldn't it have made sense for the HVAC or duct system designer to have specified a filter that is one of the many standard sizes?

The Effects of & Problems Caused by Missing Air Conditioner Filters

See our detailed article at Missing or Leaky Air Filters. Excerpts are below.

Photograph of a missing air filter opening.

Look closely at this photograph. On the right we can see a tan "Air Filter Cover" plate which marks the intended location of the HVAC air filter. But there is an open slot to the left of the air filter cover, possibly where another filter was previously being installed. When the new air filter slot was constructed and nicely covered (so as not to leak) the old slot was simply left open.

You can see my piece of adhesive tape bending into the opening, demonstrating (not too scientifically) that there was airflow into the unit from this location.

This is a great way to draw attic insulation fiberglass into the air handler and to blow it into the living area. And of course any other unwanted attic dust and debris is also being invited into the air handling system and blown into the occupied space.

Failure to properly filter dust from the return air supply will load the fan and cooling coil, dirty the duct system, and lead to the problems listed above. As the ductwork debris level increases you increase the risk of forming an allergen or mold reservoir, especially if there are water or condensate leaks into the duct system interior.

If Your Air Conditioner or Warm Air Heater Doesn't Have Air Filters, Install One or More Filter

If an air filter is not present on your air conditioner or warm air heating furnace system, have one installed. Installing an air filter is normally a minor expense involving placement of a rack to hold the air filter at each return air register, and installation of the air filter itself.

The alternative to installing and maintaining good air filtration on any air conditioner or warm air heating system is dirty ductwork and a dirty air handler. Duct cleaning or duct replacement can be a significant expense. Cleaning up a moldy HVAC system, where mold may have been caused by coil icing which was caused by a dirty coil or filter is still more costly.

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AIR FILTERS for HVAC SYSTEMS - see detailed links at page left

Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
Air Conditioning Inspection, Diagnosis, Repair, Efficiency all the basics for home owners, inspectors, new repairmen
Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
Asbestos HVAC Ducts and Flues field identification photos and guide
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
Wikipedia provided background information about the definition of HEPA and airborne particle interception
Thanks to reader Beverly Upperman for discussing air filter installatin deatils - August 2010

A/C - HEAT PUMP CONTROLS & SWITCHES describes air conditioner controls and switches - begin here if your A/C won't start
OPERATING DEFECTS major air conditioning problem symptoms and how to get the air conditioning system working again,e.g. compressor or fan noises, failure to start, and inadequate cool air volume
LOST COOLING CAPACITY describes what to do when not enough cool air comes out of the system
COMPRESSOR CONDENSER which describes problems with air conditioner compressor/condenser units
AIR HANDLER / BLOWER UNITS which describes problems with the air handler, air filters, and the cooling coil itself
DUCT SYSTEM DEFECTS which describes problems with the duct system
A/C REFRIGERANT LEAK DETECTION describing how to use a TIF5000 to detect air conditioning refrigerant gas leak
A/C DIAGNOSTIC FAQs air conditioning system diagnostic FAQs: Q&A about air conditioner repair

For more information about fiberglass as an indoor air quality concern see:

For more information about asbestos as an indoor air quality concern with focus on easy ways to identify asbestos materials in buildings, see:

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Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

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Air Conditioner, Heat Pump, or Furnace Air Filter Location, Condition, Problems, Repairs

Air conditioner filter location: filters should be readily accessible

Dirty Air Conditioner Filters Cause Multiple Problems for an Air Conditioning System

Improperly-Fit or Wrong Sized Air Filters Cause Bypass Leakage

How to Install or Replace an Air Filter in Heating or Air Conditioning Duct Systems

Question: How do I know which way my air filter should face when it is installed?

Reply:

How to Construct or Obtain Large or Special-dimension HVAC System Air Filters

The Effects of & Problems Caused by Missing Air Conditioner Filters

If Your Air Conditioner or Warm Air Heater Doesn't Have Air Filters, Install One or More Filter

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair