HVAC Air Filter Location & Renewal

How to Find he Furnace Air Filter or Air Conditioner Filter

Air Filter Locations at the Air Handler / Blower Unit

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 that the air handler that you are examining is the one supplying heated or cooled air to your building or building area. Here are the locations to check:

• Find the air filter at a central air return register
  
  Grille located in a ceiling or wall 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.
• Find the Air Filter at but not inside of the Air Handler
  
  Look first outside the blower unit where return air is entering the blower cabinet. 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.
  
  

  Locating the air filter slot at the air handler: Once 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 (or it could be several inches wide if a wide high-capacity pleated or similar filter was used).

  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.
  
  
  
  The above photo shows an air filter located between a return air plenum (at photo right side) and the bottom of the air handler unit.

  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.

  
  

  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.

  [Click to enlarge any image]

  If an electrostatic unit 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.
  
  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.

• Find the air filter at an attic air handler or horizontal or "low boy" unit
  
  Look for a filter slot right where the return plenum contacts the blower fan assembly - marked as AIR FILTER SLOT and yellow arrow in my photo. If you click-on and enlarge the image you may see a black bolt or screw at the top of the air filter slot cover - remove this to lift the filter slot cover up and out.
  
  The air filter itself will slide directly out - or rather it would if some dope hadn't taped the electrostatic air cleaner power cord to the condensate drain so tightly that it will block air filter removal. The homeowner changing the filter will need to untape and loosen the blocking cord.

• Find the air filter at a basement air handler front removable panel or door
  
  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).
  
  
  
  Above: this basement oil-fired heating furnace is an up-flow unit: return air from the building (blue arrows) moves from the building's occupied space through return ducts (blue arrows) into the bottom of the heater.
  
  Heated air moves out of the unit into supply ducts (red arrows) and back into the building.
  
  [Click to enlarge any image] to see more detail.
  
  Watch out: We marked in yellow an unsafe return air inlet grille placed right in the basement on the return air plenum itself.
  
  Someone probably added this air inlet (yellow arrows) to improve the total air flow through the system, but drawing return air from close to the oil or gas burner on a furnace can
  • cause improper burner operation (interfering with combustion air supply)
  • draw oil fumes or gas odors into the building supply air
  • can kill building occupants if these conditions cause carbon monoxide production and spillage at a gas burner

Also see UNSAFE DUCT OPENINGS for more about this hazard.

• Find the Air Filter that is Located Inside of an Air Handler Blower Compartment
  
  

  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.

  Watch out: 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.

  Watch out: 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.

• Find the air filter at a vertical air handler or blower unit at a rear removable door or panel

  Below: this photo shows a very common air filter location: behind a service door on the back of an upright warm air furnace or furnace/air conditioner combination unit.

  

• Find the air filter at a slot in the return duct where it connects to the air handler side or back

Below: a closer look at an air filter slot at a typical upright air handler on a basement heating or heating+air conditioning system.

• Find the air filter next to an electrostatic air cleaner:
  
  If your air conditioning air handler has an electrostatic air cleaner installed ELECTROSTATIC AIR CLEANER [PHOTO], 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.
  
  In our photo below, the silver handle above the white electrostatic air cleaner is a tip-off that a removable filter is installed there.
  
  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.

Find the Air Filters in the Ductwork of an Air Conditioning or 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 Return Registers and Filters on Air Conditioning or Heating Systems:

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 [photo] 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. Below is a central air return on the wall of a New York home.

This wall return air inlet sports a pleated air filter right behind the hinged grille opening. To minimize the airborne dust levels in this home, we used

a CASCADED AIR FILTER DESIGNS for which the wall filter shown here was the easily accessed first-stage of dust filtration.

Don't forget to look for central air return registers on floors too, such as shown in this photo of a return air inlet in the front hallway of an older New York home.

We noted that a bit of reduction in air flow and a bit of increase in home heating cost may be the result of leaving stuff on top of the return air inlet.

Sometimes there will be a central air return in a ceiling but without any filter as shown in our photo at left and in this additional example of

a WALL MOUNTED RETURN AIR INLET [photo] .

In that case we'd expect to find an air filter at the air handler unit.

It is usually 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.
• 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 area back to the air handler for cooling or warming.

See BACKDRAFTING HEATING EQUIPMENT and

also COMBUSTION AIR for TIGHT BUILDINGS for additional details.

Individual Room Air Supply & Returns for Air Conditioning or Heating Systems

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.

Which way is the air blowing, in or out of a heating or cooling register?

If you see an air register in a wall, floor or ceiling and you're not sure if it's an air supply register (conditioned air is being sent into the room at this opening) or if instead it's an air return register (room air is being returned to the air handler for cooling or heating), here we explain how to figure that out.

• Air supply registers blow air into the room or area being heated or cooled
• Air return registers take air out of the room or area and return it to the air handler for treatment

If the heating or cooling system is operating, you can expect to feel air blowing OUT of a SUPPLY REGISTER [photo] 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.

Another tip: air filters are almost always installed in the return air system, not the supply air duct system. So if you see an opening sporting an air filter, that may indeed be a return air inlet.

Depending on building design there will often be multiple air supply outlets, often one or more in each room, and sometimes we also have multiple air return inlets, either located centrally or located in individual rooms.

Identify individual room SUPPLY air registers

The supply registers are usually located in a CEILING [photo] or in a building WALL [photo] but may be in a floor, too (as shown here), usually near an exterior wall.

Air conditioning supply registers usually have an ADJUSTABLE LOUVER [photo] at a ceiling or wall supply register.

The louver (notice the little lever at the right end of the register cover?) permits the air supply register to be opened or closed.

Identify individual room RETURN air registers

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 air inlet registers

are usually louvered as well, but will normally not have adjustable openings that can be opened or closed.

Operable control louvers to adjust air flow are normally found only on supply air outlets into the occupied space, while return air registers or grilles normally have no lever controls.

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.

Photo: I'm holding a bit of tissue paper in front of an air register while the furnace (or air conditioner) is operating.

Even a weak air flow at a register will move the tissue showing the direction of air flow. This is a supply register: the tissue is blown away from the register's opening.

We would have guessed that this is a heat or cool air supply register by the presence of louvers inside of the register grille and by the control lever that can open or close them.

How to Identify Horizontal, Vertical, Up-Fow or Down-Flow Air Handlers or Blower Units

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 the photo above will have return air enterig 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.

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.

General maintenance tips for your air filter

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.

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.

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 cover in other sections of this article series - see links in the Recommended Articles list below.