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AIR CONDITIONING & HEAT PUMP SYSTEMS
A/C - HEAT PUMP CONTROLS & SWITCHES
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR FILTER EFFICIENCY
AIR FILTERS, FIBERGLASS PARTICLES
AIR FLOW MEASUREMENT CFM
APPLIANCE DIAGNOSIS & REPAIR
APPLIANCE EFFICIENCY RATINGS
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - Air Conditioning "How To" Books
CAPACITORS for HARD STARTING MOTORS
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
CONDENSATION or SWEATING PIPES, TANKS
DEFINITION of HEATING & COOLING TERMS
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCT SYSTEM & DUCT DEFECTS
DUCT SYSTEM NOISES
DUCTS, Asbestos Transite Pipe
DUST, HVAC CONTAMINATION STUDY
ELECTRIC MOTOR OVERLOAD RESET SWITCH
EVAPORATIVE COOLING SYSTEMS
FAN LIMIT SWITCH
GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION INSTRUMENTS
HEAT LOSS (or GAIN) in buildings
HEAT LOSS (or GAIN) INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HEATING SMALL LOADS
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LEED GREEN BUILDING CERTIFICATION
LOST COOLING CAPACITY
LOW VOLTAGE TRANSFORMER TEST
MOTOR OVERLOAD RESET SWITCH
MOLD in AIR HANDLERS & DUCT WORK
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
REPAIR GUIDES A/C / HEAT PUMP
REPAIR & DIAGNOSTIC FAQs for A/C
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
Where air filters are installed on air conditioning or heating systems: this article explains in detail how to find the air filters on an air conditioning or hot air heating system. Knowing where air filters are typically located can help find dirty or clogged air filters that are increasing air conditioning or heating cost. Before assuming that your HVAC system does not have an air filter installed, or that it has only one air filter installed, check the locations we describe here.
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AIR FILTER LOCATION - How to Find the Air Conditioning Filter on Air Conditioners or Hot Air Heating Systems
If you can't find your air conditioner system filters (there may be more than one air filter) here are some places to look. After reviewing this list of air filter locations, if you still cannot find your heating or air conditioning system air filter read our detailed instructions on how to find air filters which follows just after this list.
If after reading this simple catalog of air filter locations you still can't find your air filter, below we provide a Detailed Guide of where and how to look for filters on an air conditioning or hot air heating system.
Central air conditioning filters (or heating system filters if hot air heat is used as well) are sometimes hard to find. Here are some tips on where to look to find the filters on your central air conditioning system:
How to Find Air Conditioning Filters Located in the Air Duct System
Central Air Return Registers and Filters on Air Conditioning or Heating Systems
Individual room air returns - look in each room where there is an air conditioning supply register to see if the room also contains a return air registers. Since some rooms have multiple air supply registers and ducts we need to know how to distinguish a supply air register from a return air register.
Once you've located the air conditioning or heating air supply registers, look in the same room to see if there are other registers that collect return air to take it back to the air conditioning air handler. Return registers are usually louvered as well, but will normally not have adjustable openings that can be opened or closed.
Main air filters are rarely installed on individual room return (or supply) air registers, but identifying the presence and the location of both supply and return air registers will help us understand whether or not the system uses central returns or individual room air returns, or a mix of both. That in turn will tell us whether or not to look for filters at central return registers.
Individual air register filters: Sometimes you may see some filter like material installed right on individual air supply or air return registers in rooms. These are usually "add-on" products that occupants have installed, perhaps in complaint that the duct system itself is dirty and that central filters have proven ineffective, or people may install individual register filters out of a general anxiety about building indoor air quality.
OPINION: individual air supply or return register air filters are either ineffective (not trapping much debris), or if they are effective, they are at risk of significantly reducing the air flow into the rooms where they are used, reducing the effectiveness of the cooling or heating system or increasing system operating costs. If the building duct system is so dirty that people are installing these filters it may be preferable to have the duct system professionally cleaned.
If a building uses both supply and return air registers in every room, that is, if it is not using central air return registers at just a few locations, then the air filters for the system are most likely going to be found on or very close to the individual air handlers or blower units themselves.
How to Find Air Filters Located At or Near Air Conditioning or Heating Air Handlers or Air Blower Units
When air filters are not found at return registers in a building the most likely place for an air filter to be found is at or even inside the air handler itself. Some buildings use multiple air handlers, providing air conditioning and/or heating to individual floors or to different building areas on the same floor, so be sure you locate all of the air handlers or blower units in the building.
At the blower unit observe that large ducts will be connected to bring return air from building return ducts into the blower unit and to send conditioned air (cooled or warmed) out of the blower unit into the building supply ducts.
Look first outside the blower unit where return air is entering the blower cabinet.
Air handlers are installed either horizontally (in an attic or crawl space) or vertically (in a basement, high attic, or other building area). Photos of horizontal and vertical air handlers are shown in this article and at our website.
Other vertical air handlers may be a "down-flow" unit which has return air entering the top of the air handler or blower and conditioned air passing out into supply ducts connected at the bottom of the unit.
Which side or end or top or bottom of the air handler blower has incoming return air and which has outgoing supply air?
Follow the duct work: If you were able to spot central or room air registers inside the living space, at the air handler unit you should be able to spot where those ducts enter the air handler itself. That will tell you which end of the air handler has entering return air.
Feel the outside temperature of duct work at the air handler: If you cannot determine where return air ducts are entering the air conditioning or heating air handler, if the system has been running in air conditioning mode the supply or outlet air end of the ducts connected to the air handler will be cool or even cold, and the return air end of the ducts connected to the air handler will be warmer to the touch.
If the system has been running in heating mode, the supply or outlet air ends of the ducts connected to the air handler will be warm to the touch and the inlet or return air ducts will be cooler.
SAFETY WARNING - injury risk: when opening a blower cabinet, older units may lack a safety interlock switch that turns off power to the blower fan. Reaching inside of a blower cabinet when the fan is running is very dangerous as you can lose fingers in the spinning air conditioner blower assembly fan or its drive motor or belt. To be safe, turn off power to the unit before opening the blower compartment to inspect for and change a filter.
Opening the blower compartment door on these systems will expose one or several filters which are simply set in place on a frame. Normally the filters are placed inside of this box in a location which will filter air before it reaches the blower fan itself.
SAFETY WARNING - filter direction: be sure to place the new air filter properly into its slot (at the air handler) or grille (for ceiling or wall mounted central air return filters). The edge of most air filters has an arrow showing the intended direction of airflow through the filter. The filter is installed so that the arrow (and air flow) show air moving through the filter INTO the duct system (at a central air return grille) or INTO the air handler (for filters installed on the blower unit). This is important because many filters are reinforced to prevent the airflow from collapsing the filter material and drawing it right into the blower fan itself. A collapsed filter that is drawn into a blower fan can cause fan overheating and even a building fire.
MAINTENANCE TIP: when inspecting the blower compartment interior of a central air conditioning or central heating air system look at the blower fan itself. Usually these are a round squirrel cage fan driven either directly by an electric motor or driven by a fan belt which is in turn connected to a pulley on a nearby electric motor. Use a good light to examine the blades of the blower fan itself.
If the air conditioner or heating blower fan blades are clogged with dirt (or mold (as in this photo)) you should have the air handler unit thoroughly cleaned by a professional, including the fan blades themselves. We've seen a 100% improvement in air conditioning or heating airflow when a very dirty blower fan was cleaned. Of course a very dirty air filter will also slow air movement and increase operating cost in an air conditioning or heating system.
Change the air conditioning or heating system 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.
After you have located the air filter(s) document the placement of the HVAC system air filters for future building owners, occupants, or repairmen. Knowing where the air filters are located, and that you've found all of them, eases air filter inspection and regular changing during the cooling or heating season. Each month during the heating or cooling season, inspect the air conditioning filter type and condition.
What about air filters that are missing completely or are very dirty?
What about other air filtration methods like electrostatic air cleaners, HEPA or high efficiency air filters, or UV light disinfection systems?
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 other sections of this article - see links below.
Frequently Asked Questions (FAQs) about the location of air filters on air conditioning & heating systems
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Question: Should I remove the air filter installed at the air handler and just filter at the return air inlets?
My house has a central air unit and 2 return supply grills in each hallway. I always had a big filter in the main unit (16x20x4) and then one in each of the return grills (16x25x1).
Last year my unit was not cooling very well and the tech said I should not have filters in all 3 spots, and suggested I not put one in the main unit, just the hallways.
Now the house cools great, but the dust is over whelming. Is there some adjustment I should make by adding a central filter again, or just buying super efficient hallway filters? BTW, we have lots of pets.
Reply: Suggestions for filtering HVAC air at the return air inlet grilles
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