A clean filter helps your heating and cooling system run the way it was designed to. It’s one of the easiest maintenance jobs in the house.
The essentials
- ENERGY STAR advises checking your filter every month, changing it if it looks dirty, and changing it at least every 3 months.
- The filter’s size is printed on its frame, and the airflow arrow should point toward the furnace or air handler.
- EPA suggests a filter rated at least MERV 13, or the highest rating your system’s fan and filter slot can accommodate.
- An annual pre-season check-up by a contractor catches problems a filter change can’t.
The air filter in a forced-air heating and cooling system catches dust, lint and other particles before they reach the blower and coils. When it clogs, the system has to work harder to move air. Changing it takes a couple of minutes and costs little, which makes it one of the best-value maintenance jobs in a home.
Find your filter and its size
Look for the filter in a slot next to the furnace or air handler, behind a return-air grille, or inside the furnace cabinet. Some homes have more than one return grille with a filter in each.
The size is printed on the frame, for example 16x25x1: width, height and thickness in inches. These are usually rounded “nominal” sizes, and the actual filter is often slightly smaller. Buy the size printed on your old filter rather than measuring the slot.
Which way does it go?
Every filter has an arrow on the frame showing the direction of airflow. Point it toward the furnace or air handler, and away from the return duct the air comes from. A filter installed backward doesn’t work as well.
What MERV ratings mean
MERV stands for Minimum Efficiency Reporting Value. According to EPA, the rating describes a filter’s ability to capture particles between 0.3 and 10 microns, and higher ratings trap more of them. The common scale runs from MERV 1 to MERV 16. HEPA filters are rated separately.
For homes, EPA suggests choosing a filter rated at least MERV 13, or as high as your system fan and filter slot can accommodate. Some systems, especially older ones with thin filter slots, may struggle with very dense filters. EPA recommends consulting an HVAC technician to find the best option for your system.
How often to change it
ENERGY STAR’s guidance is straightforward:
- Check the filter every month, especially in heavy-use months in winter and summer.
- Change it if it looks dirty after a month.
- At a minimum, change it every 3 months.
Homes with pets, smokers, nearby construction or a lot of dust usually need more frequent changes.
Pro tip
Write the date on the edge of the new filter before you slide it in, and keep a spare on hand. Next month, a glance tells you how long it has been in.
How to change it
- Turn the system off at the thermostat.
- Slide out the old filter and note the size and arrow direction.
- Check the slot for dust or debris and wipe it out if needed.
- Insert the new filter with the arrow pointing toward the furnace or air handler.
- Close any cover and turn the system back on.
Beyond the filter
ENERGY STAR’s maintenance checklist recommends annual pre-season check-ups by a contractor: cooling in spring and heating in fall. During a tune-up, a technician typically:
- Checks thermostat settings and the system’s controls
- Tightens electrical connections and checks motors
- Lubricates moving parts
- Inspects the condensate drain
- Cleans the coils and checks the refrigerant level on air conditioners
- Checks gas or oil connections, gas pressure, burner combustion and the heat exchanger on furnaces
Between visits, keep supply and return vents unblocked by furniture and rugs, and keep plants and debris clear of an outdoor unit.
Safety first
A furnace that burns gas or oil can produce carbon monoxide if something goes wrong. Make sure you have working CO alarms. Our guide to smoke and CO alarms covers where to put them and how to test them.
Make it a habit
Filter checks are easiest to remember when they’re part of a routine. They’re on our fall home maintenance checklist, along with other jobs that keep your heating system ready for winter.
Common questions
What happens if I don’t change the filter?
ENERGY STAR warns that a dirty filter can increase energy costs and damage your equipment. Restricted airflow can also leave rooms less comfortable.
Is a higher MERV rating always better?
Higher-rated filters trap more small particles, but not every system is designed for the extra resistance they can add. EPA suggests the highest rating your system can accommodate and recommends checking with an HVAC technician.
Can I wash and reuse my filter?
Only if it’s sold as washable or reusable. Most pleated filters are disposable. For reusable ones, follow the manufacturer’s cleaning instructions and let the filter dry completely before putting it back.
Where is my filter?
Common spots are a slot beside the furnace or air handler, behind a return-air grille in a wall or ceiling, or inside the furnace cabinet. Check your system’s manual if you can’t find it.
This guide is general information, not professional advice. Follow the manufacturer’s instructions and your local building codes, and hire a licensed professional for electrical, gas, structural or roofing work.