How to change a furnace filter (and find the right size)

Of all the jobs that keep a house running, this may be the best deal going. It takes five minutes, costs a few dollars, and needs no tools. A dirty filter slows the air moving through your system, and ENERGY STAR says that makes it work harder and waste energy.

Where to find the filter

The filter is somewhere along the path air takes on its way back into your furnace or air handler. Look in these places, in this order:

• At the furnace. A slot on the side of the unit, usually where the big return duct meets the cabinet, often with a thin removable cover.
• At a return grille. A large vent in a wall or ceiling with no airflow blowing out of it. Some swing open on a latch.
• In the air handler. In an attic or closet, if that's where your system lives.

Some homes have more than one. If you can't find it, look at the manual, or ask at your next service visit and write down the answer.

How to find the right size

The size is printed on the edge of the old filter, something like 16 × 25 × 1. Buy that number.

If the actual measurements look a little different, that's normal. Filters are sold by a rounded "nominal" size, and the real dimensions run slightly smaller. So a filter labeled 16 × 25 × 1 can measure closer to 15¾ × 24¾ × ¾. Go by the label.

No label? Trane suggests measuring the slot for height, width and depth and rounding up to the nearest inch.

Take a photo of the label while you're there. The next time you need filters, you'll be standing in the store, and the furnace won't be.

How to change it

1. Turn the system off at the thermostat. You don't want it pulling air in while the slot is open.
2. Slide out the old filter. Before you pull it all the way, find the arrow on its frame and note which way it points.
3. Bag it. It's full of dust. A trash bag keeps it from shedding on the way out.
4. Slide in the new filter with the arrow pointing toward the furnace. The arrow shows the direction of airflow. Filtrete says it should point at the blower motor. On a horizontal furnace, it follows the air: if air moves right to left, the arrow points left.
5. Replace any cover and turn the system back on.
6. Write the date on the filter's frame. It takes a second and ends the "when did I last do this?" question.

How often to change it

ENERGY STAR's advice is to check it monthly, especially in the winter and summer, and to change it at least every three months. If it looks dirty after a month, change it then. A quick way to judge: hold it up to a light. If you can't see much light through it, it's done.

Those are the rules for a standard one-inch filter. You'll want to look more often if you have:

• Pets
• Allergies or asthma in the house
• A remodel going on, or smoke from outside
• A system running hard during a very hot or cold stretch

Thicker media filters, the 4- or 5-inch kind in their own cabinet, often last longer. The manufacturer's directions will tell you how long.

Which rating to buy

Filters are rated by MERV. Higher numbers catch smaller particles. The EPA suggests a filter rated at least MERV 13, or as high as your system will accommodate. It says most systems can handle MERV 13 if you replace it often, and that a technician can confirm what will work with yours.

That last part matters, because a denser filter can also make it harder for air to get through. Your furnace's manual may name a maximum. If it doesn't, ask at your next tune-up, and until then stick with the rating you've been using.

If something seems off

Change the filter first. It's a common cause of weak airflow, and it's cheap to rule out. If the problem doesn't go away, the next stop is a qualified technician, and ENERGY STAR recommends a yearly tune-up in any case.

If you ever smell gas or a carbon monoxide alarm goes off, this isn't a filter problem. Leave the house and call your gas company or 911 from outside.