Around the house · First steps · about 10 minutes

Change a Furnace Air Filter

Most homes with central heating or air conditioning have an air filter that catches dust before air runs through the system. A clogged filter blocks airflow, wastes energy, and can let dirt reach parts that are hard to clean. Changing it is one of the easiest home jobs there is.

When to teach it: Watch from about 6, help from about 8, do it alone from about 11.

The filter sits where the return duct meets the furnace. Its size is printed on the frame.

You'll need

A new filter in the exact size printed on the old one
Marker
Flashlight
Screwdriver, if the filter cover has screws
Step stool or ladder, if the filter is up high or in the attic

Before you start

Turn the heating and cooling system off at the thermostat.

Step by step

Tick each step as you go if you like; ticks stay on this device.

Find the filterGo to the furnace or air handler, which may be in a utility room, basement, closet, attic, crawl space, or garage. The filter usually slides into a slot between the return duct and the unit. In some homes it sits behind a return-air grille in a wall or ceiling instead.
Turn the system offSwitch the system off at the thermostat before you open anything.
Take out the old filterOpen the cover (it may have screws, clips, or a latch) and slide the filter out. Handle it carefully; the wire on some filters can be sharp.
Read the sizeThe length, width, and thickness are printed on the cardboard frame. Buy that exact size. It should cover the opening and fit snugly without bending.
Find the arrowLook for the airflow arrow on the new filter's frame. Point it into the duct, toward the furnace, the way the air moves.
Write the dateTake off the plastic wrap (not the cardboard frame) and write today's date where you'll see it next time.
Slide it in and close upSlide the filter into the slot, arrow toward the furnace. Put the cover back on. Don't tape it shut, since you'll open it again.
Turn it back onSet the thermostat back to normal. Put a reminder on the calendar for the next check.

Physics

How it works: How a filter catches dust

A fan in the system pulls air from your rooms through the return duct, pushes it through the filter, heats or cools it, and sends it back out. The filter is a mat of fibers. Air slips through the gaps, while many particles, like dust and pollen, get caught.

MERV stands for Minimum Efficiency Reporting Value. It's a number for comparing filters: the higher the MERV, the better the filter is at trapping small particles. But a filter that catches more can also clog faster and slow the airflow, so your system has to be able to handle it.

As a filter fills up, air has a harder time getting through. That lowers the system's efficiency, and some dirty air can sneak around the filter onto the cooling coil. Energy.gov says replacing a dirty, clogged filter can cut an air conditioner's energy use by 5% to 15%. One more fact: a filter only cleans the air while the system is running.

energy saved ≈ cooling energy cost × 0.05 to 0.15 (5% to 15%)

Try it: Hold the new filter and the old one up to a lamp. Which lets more light through? Then do the math: if cooling costs $100 a month, a clean filter could save about 5% to 15%, or $5 to $15.

Keep learning: Science

Teaching it to your kids

Let your child read the size off the old filter and find the match at the store.
Make the arrow rule a chant: 'Arrow points to the furnace.'
Hold the old and new filters side by side so they can see what the filter caught.
Let them write the date on the new filter.

Tips

How often depends on your filter and system, so follow the maker's directions. EPA says many makers suggest every 60 to 90 days. Energy.gov suggests every month or two during cooling season.
Check more often if you have pets, a dusty home, or the system runs constantly.
If the old filter looks very dirty when you change it, change it more often.
If your slot fits more than one thickness, the thicker filter has more surface area for catching dust.
Buy a few filters at a time so you always have a spare.

Common mistakes

Mistake	Instead
Putting the filter in backward.	Point the arrow into the duct, toward the furnace.
Buying a filter that's 'close enough' in size.	Match all three numbers on the frame. It should cover the opening and fit snugly without bending.
Forgetting when you last changed it.	Write the date on the frame and set a calendar reminder.
Jumping to the highest MERV without checking.	Ask an HVAC pro whether your system can handle it.

Safe habits

Turn the system off at the thermostat before you pull the filter.
Handle filters carefully. The wire on some can be sharp.
Use a ladder if the filter is in the attic.

When to call a pro

Before switching to a MERV 13 or higher filter, ask an HVAC technician or your service company whether your system can handle it.

Sources

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