The old adage “out of sight, out of mind” is especially appropriate for the air filter in an HVAC system. Hidden filter locations are more likely to be forgotten about until the filter clogs and the lack of air flow causes a system shutdown or other problem.  

    If you don’t see a panel on or near the air handler (indoor unit) that has “FILTER” stamped on it, then try checking these locations:

1. Sometimes the air filter is behind the bottom front panel of the air handler. There might be two removable front panels, and you may have to lift the top one off first. The filter will be in the bottom or side of the lower compartment, often under a tangle of wires, like in the photo below.
2. The air filter may be hiding in the ceiling, wall, or floor, behind a return air register (vent). These registers do not have a handle to adjust the angle of the louvers, which is one way to tell them apart from a supply register. Look for a filter behind the louvers. A flashlight will help if it is located on a tall ceiling or wall. The louver panel is hinged and will fall open when the securing points are released. Hopefully the filter will not look like the one in the photo below when you find it.    There is often more than one filter location when they are at the return air register, because larger homes will have one large register at the main living area and an additional smaller one in the master bedroom, or even one in each bedroom.
3. You may have the never-ending curse of a filter in the attic if your furnace or air handler is up there. The location is usually well marked, but you may need a flashlight to get around. There is supposed to be solid flooring from the attic hatch opening to the air handler, unless it is accessible directly from the attic hatch opening.
4. Even if you find a filter at one of these locations, check to see if there is also an additional filter all the other possible spots if you have recently bought a home that was owned by someone who was obsessive about air filtration. You may find filters at both the return air register(s) and behind the “FILTER” slot at the furnace/air handler. Double filtration puts an unnecessary load the blower in your system and the second filtration layer you didn’t know about will eventually clog if you don’t find it.

    One more note: be sure to check the arrow on the side of the filter frame and install the filter so that it points in the direction of the air flow. The additional stiffness built into the downstream side of the filter prevents premature buckling.

      If you are uncertain about which filtration rating is right for your replacement filter, go to our blog post What is the right MERV number for my air conditioning filter? Also see Frequently Asked Questions (FAQ) about Air Conditioning Troubleshooting.

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I find out the age of my air conditioner or furnace?

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning?

• How can I find out the size of my air conditioner? 

• How can I find out the age of my air conditioner or furnace? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.