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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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:
Central air returns - Air conditioning filters located on central air return registers: if the system uses one or just a few central return registers very often a filter is placed behind the inlet grille at the central air return and is easily spotted in installations such as this Arizona central air conditioning air return inlet in the ceiling of a new home - the large rectangle is a return inlet where the air filter is quite visible.
In this photo the small opening is a poorly-installed supply register - air coming out of this register will simply be drawn right back into the return air inlet and won't do much good for the rest of the building.
Look for central air returns on building ceilings and walls in central areas such as in the ceiling near the top of a stairwell to an upper floor, or on lower floors look on hallway ceilings. Don't forget to look for central air return registers on floors too, such as shown in this photo of an air return in a front hallway floor.
Sometimes there will be a central air return in a ceiling or wall but without any filter (as shown in this photo of a wall-mounted return air inlet). In that case we'd expect to find an air filter at the air handler unit. It would have been better to put the filter at the wall or ceiling return register grille since that way the return air ducts are protected from debris accumulation.
Central return registers are normally located in one or two common areas of the building where they will not be accidentally "shut off" by someone closing a door.
For example, a bedroom won't normally have a central return register since closing the bedroom door would shut off other building areas from the return air path.
This photo shows what you'll see when you open a ceiling return air register and remove the filter that (should have been) installed there. The duct surfaces in this ceiling look rather clean - the filter has been doing its job.
You may see house dust deposits around the edges of the opening into which the air filter is mounted - showing where air bypass leaks are occurring.
Some air filters include a gasket to seal these locations against air and dust leakage. Sometimes these leaky openings at air filters also cause noises when the air handler is running.
Watch out: If a building air conditioning or heating system air handler gets its return air right at the air handler such as in this photo of a basement air handler the system has several operating problems: it is unsafe if it can draw combustion gases or mold into the duct system, the system lacks adequate return air and so will not deliver good air flow into the living area.
And the system will have high operating costs because the air flow is "one-way" drawing constantly "new" air from the basement into the air handler to be warmed or cooled and sent "one way" into the living area. Return ducts should bring air from the living are back to the air handler for cooling or warming.
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.
Air conditioning supply registers usually have an adjustable louver shown in this photo of a ceiling supply register.
The louver (notice the little lever at the right end of the register cover?) that permits the air supply register to be opened or closed.
If the heating or cooling system is operating, you can expect to feel air blowing out of a supply register and you may feel air moving into a return register.
But don't be confused. Air moving into a return register also may just be felt as an airflow across your hand and you may not be sure which way the air is moving.
Holding a simple piece of tissue paper next to an air register will by its movement show you for sure which way the air is moving.
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.
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.
Horizontal air handlers such as the attic mounted horizontal air handler in this photo will have return air entering one end of the air handler unit and supply air exiting at the other end.
Vertical air handlers may be an "up-flow unit" which has return air entering at the bottom of the air handler or blower and conditioned air (cooled or warmed) exiting at the top of the unit.
The photograph shows a vertical or up-flow heating and air conditioner unit whose cooling section was mounted as an add-on atop a hot air furnace.
In this case the overhanging size of the top mounted cooling section suggests poor design - the sizes of the air moving sections are not matched.
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.
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.
Air handler filters may be located between the return air plenum and the rest of the air handler which contains a cooling coil for air conditioning or a heat exchanger for heating systems, and also a blower fan that causes air to move across these components and out into the supply ducts.
This photo shows an air filter located between a return air plenum (at photo right side) and the bottom of the air handler unit.
Locating the air filter slot at the air handler:: Now that you've identified the return air end of the air handler look closely in that area for a slot which is perhaps 1 1/2" wide and runs the entire width or height of the metal of the return air plenum (metalwork box connected to the inlet end of the air handler).
The slot may have a removable cover, but inside here should be located a central return air filter. Open the slot, remove the filter, and install a new filter. If an electrostatic air cleaner (photo) is installed at the air handler an air filter is usually installed in the air path just before or just after the electrostatic unit itself.
In this photo of a slot intended to receive an air conditioning filter at the blower unit, the filter had simply been left out and the slot left open - drawing dirt, debris, moisture, and mold from nearby basement areas. At the right side of the upper portion of the slot you can see our little piece of clear adhesive tape bending inwards, showing that air is flowing into the air handler at this filter slot.
Many air conditioning system or warm air heating air handlers, both horizontal and vertical units, have one or more air filters that are installed inside of the blower assembly itself. These systems move air from a return plenum (connected to the return duct end of the system) across a cooling coil (for air conditioning) or a heat exchanger (for heating Systems).
On nearly all central air cooling or heating systems, the blower is located at the return air end of the system.
For many of these systems a filter or a set of air filters are located inside of the cabinet that contains the blower fan itself. Such systems can be recognized by a handle that is turned or pulled to open the blower cabinet.
This photo shows a blue fiber type filter located inside the blower compartment of the air handler. The photo at the top of this page shows another conventional air filter located inside of a vertical air handler blower compartment - in that case the filter was very dirty and blocked with debris.
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.
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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.
In my OPINION what your HVAC tech said was reasonable.
I would NOT want to operate an HVAC system with no air filters - doing so leads to debris collection in the duct system (expensive to clean or correct later on) and debris clogging at the cooling coil of an air conditioner and debris clogging on the blower fan - reducing airflow further and creating system operating problems.
The problem with filtering ONLY at the air handler is that debris collects in the ductwork, especially the return ducts - that can be costly or even impossible to clean, depending on duct routing and the materials used.
So it's best practice to filter return air right at the return air inlet grilles just as your tech suggested.
You can try using higher efficiency filters at the return inlets (the inlet mounting grille may need to be changed to accommodate a thicker air filter) but choose your filters carefully. A thicker filter that is deeply pleated, by providing greater total surface area than a more flat air filter, can improve air filtration significantly without reducing the air flow rate through the device.
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