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Air Filters for Heating and Air Conditioning Systems AirCondAPedia © Air conditioning / heating filter defects & repairs - a photo library of air filter mistakes Air conditioning / heating filter suggestions for optimum indoor air quality Air conditioning / heating system filter product sources and recommendations Air conditioning / heating system air handlers - Fan Coil Unit Inspection, Diagnosis, Repair, Replacement

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

Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

AIR CONDITIONING FILTERS - Air Conditioning Filter Location, Condition, Problems, Repairs

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 OPTIMUM INDOOR AIR FILTERS 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. I 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.

I 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.

• 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

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 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

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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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

AIR CONDITIONING & HEAT PUMP SYSTEMS
AGE of AIR CONDITIONERS & HEAT PUMPS
AIR CONDITIONER BTU CHART
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR CONDITIONER NOT WORKING
AIR FILTERS for HVAC SYSTEMS
  Air Filter Location
  Dirty Air Filter Problems
  Missing Air Filters
 OPTIMUM INDOOR AIR FILTERS
  Air Filter Efficiency
  Optimum Air Filter Design / Location
  How to Cascade Air Filters
 CONTINUOUS BLOWER FAN OPERATION
 AIR FILTER EFFECTIVENESS
  MERV, HEPA Definitions
  MERV Air Filter Ratings
  How Air Filters Work
  Particle Sizes & IAQ
  Gasketed air filters
  Debris in Return Air Plenum
  Washable air filters
 AIR FILTERING STRATEGIES
 FIBERGLASS & AIR FILTERS
  Air Filters Shed Fibers?
  Fiberglass & Test Lab Accuracy
  Variations in Fiber Size
 SOURCES FOR AIR FILTERS
  OTHER AIR CLEANERS
AIR HANDLER UNIT
  DIRTY A/C BLOWERS
    Leaks, Rodents In Air Handlers
    Mold Growth in Air Handlers
  DAMAGED COOLING COIL
  DIRTY COOLING COIL
  DIRTY COIL CLEANING PROCEDURES
  FROST BUILD-UP
  BLOWER LEAKS, RUST & MOLD
  ADDING A/C: RETROFIT SIZING
BACKUP HEAT for HEAT PUMPS
BLOWER DOORS & AIR INFILTRATION
BOOKSTORE - Air Conditioning "How To" Books
CLEANING & Legionella BACTERIA
COMPRESSOR CONDENSER
CONDENSATE HANDLING
CONTROLS & SWITCHES
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
COOLING COIL or EVAPORATOR COIL
DATA TAGS on AIR CONDITIONERS
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
DEFINITION of Heating & Cooling Terms
DEW POINT TABLE - CONDENSATION POINT GUIDE
DUCT SYSTEMS
DUCTS - Asbestos
DUCT SYSTEM DEFECTS
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCTS, Asbestos Transite Pipe
DUST FROM HVAC?
FAN AUTO ON Thermostat Switch
HEAT LOSS (or GAIN) in BUILDINGS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSPECTION LIMITATIONS
LOST COOLING CAPACITY
MOTOR OVERLOAD RESET SWITCH
OPERATING COST
OPERATING DEFECTS
OPERATING TEMPERATURES
REPAIR GUIDE for AIR CONDITIONERS
REPAIR & DIAGNOSTIC FAQs for A/C
REFRIGERANTS
SEER RATINGS & OTHER DEFINITIONS
SYSTEM OPERATION
THERMOSTATS
THERMOSTATIC EXPANSION VALVES
CRITICAL DEFECTS on A/C SYSTEMS
FURNACES WARM AIR HEATING SYSTEMS
INDOOR AIR QUALITY IMPROVEMENT GUIDE

• Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator.
• Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
• 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.).
• Wikipedia provided background information about the definition of HEPA and airborne particle interception

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

• Fiberglass in Indoor Air, HVAC Ducts, and Building Insulation
• Fiberglass Particle Identification in the Fiberglass Test Laboratory
• Goodman Gray Flex Duct Deterioration and Failures
• Mold in Fiberglass Insulation
• Optimum Indoor Air Filters
• Owens Corning Flex Duct Deterioration and Failures
• World Trade Center Dust Particle Identification

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

• Asbestos Identification in Buildings: How to find & identify asbestos-containing materials
• Asbestos in Good Condition
• Asbestos HVAC Ducts - Asbestos Materials On or In Heating and Cooling Duct Work
• Asbestos paper duct wrap
• Asbestos Transite Chimneys, Flues, & Pipes in Buildings
• Asbestos under the microscope
• Asbestos Vibration Dampers & Asbestos in the Air Handler
• Ceiling tiles: How to recognize ceiling tiles that may contain asbestos
• Examples of unusual uses of asbestos in buildings
  

How to diagnose and fix an air conditioning system that is not working

Since the failure of an air conditioner to turn on, loss of air conditioner cooling capacity, reduced air conditioning output temperatures, loss of cool air supply, or even loss of air flow entirely can be due to a variety of problems with one or more components of an air conditioner or air conditioning system, after reviewing the lost air conditioner cooling diagnosis procedures described in this article, be sure to also review the diagnostic procedures at each of the individual air conditioning diagnosis and repair major topics listed just below. To return to our air conditioning and refrigeration home page go to AIR CONDITIONING SYSTEMS.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

• 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 UNIT 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
  

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Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.

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