How to Change a Furnace Filter
Heating season is starting, and a clogged filter makes your furnace work harder, cost more and blow dustier air. Swapping it takes five minutes once you know where it is, what size to buy and which way it faces.
Check it now, then every month
ENERGY STAR says filters should be checked every month and changed at least every three months, or whenever the filter looks dirty. Pull it out and hold it up to a light: if it's gray and you can't see light through it, it's time.
Before you buy, read the size printed on the edge of the old filter, like 16x25x1. Take a photo of it so you have it next time.
HOWReplace the filter
Do them in order.
- At the furnace or air handler: look for a slot with a removable cover where the big return duct joins the unit, usually on the side or bottom near the blower.
- Behind a return grille: a large vent in a wall, ceiling or floor (often in a hallway) with clips or thumbscrews. The filter sits right behind it.
- Some homes have more than one filter, for example one at each return grille. Your furnace manual shows where yours go.
Set the thermostat to Off so the blower doesn't start and pull dust in while the slot is open. If the filter is inside the furnace cabinet, also switch off the furnace's power switch (it often looks like a light switch on or near the unit).
- Slide the old filter out slowly so the dust stays on it. Have a trash bag ready.
- Read the size on the frame: width × height × thickness in inches, like
20x25x1. This is the nominal size; the real filter measures a bit smaller, and that's normal. - Notice which way the arrow on the frame pointed. The new one goes the same way.
No label? Measure the old filter or the slot and use the size decoder below.
- Size first: buy the exact nominal size. A filter that's too small lets dusty air bypass it.
- Then MERV: MERV (1 to 16) rates how well a filter catches small particles. Higher catches more. Cheap fiberglass filters are usually low, MERV 1 to 4; pleated filters are often MERV 8 to 13.
- The EPA suggests at least MERV 13, or as high as your system's fan and filter slot can handle. Very dense filters can restrict airflow in some systems, so check your furnace manual or ask an HVAC technician if you're unsure.
| MERV | Typical filter | Good for |
|---|---|---|
| 1–4 | Flat fiberglass | Protecting the equipment from big dust |
| 5–8 | Basic pleated | Everyday dust and lint |
| 9–12 | Better pleated | Pets, more fine dust |
| 13–16 | High-efficiency pleated | Allergies, smoke, finer particles |
- Find the airflow arrow printed on the frame.
- Point it the way the air travels: away from the return duct and toward the furnace blower. At a wall or ceiling grille, that means the arrow points into the wall or ceiling, away from the room.
- Slide it in fully so it sits flat with no gaps, then close the cover or grille.
- Turn the furnace switch and thermostat back on.
- Write today's date on the filter frame with a marker, and set a phone reminder using the calculator below.
- Buy the next filter now, or a multi-pack, so you're not hunting for the size later.
If the filter won't fit, check you have the right nominal size and thickness. If the furnace won't start after, make sure the power switch and the panel cover are back on (many furnaces won't run with the door off). Still nothing? Call an HVAC technician.
TRYFilter size decoder
Type the size printed on your old filter, or what you measured with a tape measure, and see what to buy.
TRYReplacement reminder
How often you need a new filter depends on its thickness and what's in your air. This gives a starting point; your monthly look at the filter is the final word.
FAQCommon questions
Why is my 16x25x1 filter actually 15½ × 24½?
The printed size is a rounded “nominal” size. Filters are made slightly smaller so they slide into the slot. Always buy by the nominal size printed on the frame.
Is a higher MERV always better?
It catches more fine particles, and the EPA recommends MERV 13 where the system can handle it. But a very dense filter can restrict airflow in some older or smaller systems. If you're unsure, check the furnace manual or ask an HVAC technician.
Which way does the arrow go?
In the direction the air flows: toward the furnace and away from the return duct or grille.
Can I vacuum and reuse a disposable filter?
No. Vacuuming doesn't get dust out of the fibers. Only filters sold as washable or reusable are meant to be cleaned.
How often should I change it?
ENERGY STAR: check monthly, change at least every three months or whenever it looks dirty. Thick 4- to 5-inch media filters can last longer, often 6 to 12 months; follow the maker's guidance. Pets, allergies and dust mean changing sooner.