How to Change Your Furnace Filter

Why this matters

Every bit of air your furnace or air conditioner moves passes through one filter on its way in. When that filter clogs, the system has to pull harder for less air, and the parts that depend on airflow suffer for it: in summer the indoor coil can freeze into a block of ice, in winter the furnace can overheat and shut itself off on its safety limit, and in both seasons the bill goes up for less comfort. Changing the filter is the single maintenance job most squarely in a homeowner's hands. It takes five minutes, and the three mistakes people make (wrong direction, wrong size, wrong filter) are all easy to avoid once you know what you are looking at.

Where the filter lives

There are two common places, and some houses have both.

 A. AT A RETURN GRILLE (wall or ceiling)

   room air -->  [grille][FILTER] --> duct --> furnace
                  arrow on the frame points into
                  the wall or ceiling, away from
                  the room

 B. IN A SLOT AT THE FURNACE (upflow furnace shown)

                     to rooms (supply duct)
                            ^
                     +-------------+
                     | burners or  |
                     | cooling coil|
                     +-------------+
   return duct       |   BLOWER    |
   from rooms  ==>[F]==>  (fan)    |
                ^    +-------------+
        slot with a cover; arrow on F
        points right, toward the blower

A return grille is the larger grille, often in a hallway ceiling or low on a central wall, with no air blowing out of it when the system runs. If it has a hinged face with latches, the filter is behind it. At the furnace, look for a slot with a removable cover in the duct beside or below the blower, or, on some furnaces, a filter rack inside the blower compartment behind the lower door.

The one rule both share: the arrow on the filter's frame points the way air flows, which is always toward the furnace and its blower and away from the rooms.

Step 1: Turn the system off at the thermostat

Set the thermostat to off, and the fan to auto rather than on.

Why: with the blower running, an open slot pulls dust and debris straight into the blower and the coil with no filter in the way, and the suction can drag a new filter crooked as you slide it in.

If your filter is inside the blower compartment: also turn off the furnace's service switch, which looks like an ordinary light switch on or near the furnace, before opening the lower door, and follow the filter instructions in your owner's manual. Open only the door the manual names; the burner compartment is not yours to open. When you are done, close that door fully: on many furnaces a switch behind it keeps the furnace from running until it is shut.

Step 2: Read the old filter before you throw it out

Pull the old filter out and look at its frame. Three things are printed there:

• The size, as three numbers: width by height by depth in inches, such as 20 x 25 x 1. That is the nominal size, the one you shop by. Many frames also print a slightly smaller actual size, such as 19.5 x 24.5 x 0.75, which is what fits the slot. Buy by the nominal size and the actual size takes care of itself.
• The arrow, showing which way it was installed. If the arrow on the old filter points toward the room, it was in backwards.
• The rating, usually MERV followed by a number, or a store brand's own scale. Write it down.

Why read it rather than measure the slot: the frame is the one place the right answer is written down for you. A tape measure gives you the actual size, and shoppers who round that number down end up with a filter half an inch too small.

What breaks if you skip it: a filter with a gap around its edges lets air go around it instead of through it, and that unfiltered dust builds up on the coil, which a filter cannot clean off.

Step 3: Slide the new one in, arrow toward the equipment

Check the arrow before it goes in, then slide the new filter in with the arrow pointing toward the furnace: into the wall or ceiling at a grille, toward the blower at a furnace slot. It should sit flat and snug, with no gap you can see around the edges and no bowing. Close the grille or put the slot cover back.

Why direction matters: a pleated filter is built to catch dust on its upstream face and is often supported on its downstream face by a wire mesh or a stiff backing. Backwards, it catches less and can bow or collapse into the duct as it loads.

Step 4: Write the date on the frame

With a marker, write today's date on the edge of the frame you can see when the grille is open or the filter is in the slot.

Why: "I think I changed it in the spring" is how a filter stays in for a year. A date you can read without pulling the filter answers the question every time you walk past, and it answers it for a tech on a service visit too.

Then turn the system back on at the thermostat, and the service switch if you turned it off. Stand by for a minute: you should hear the blower start and feel air at a supply vent, with no new rattle or whistle at the grille or slot.

Why a tighter filter is not always better

A higher MERV rating catches finer particles, which sounds like an upgrade every time. The catch is that a filter that catches more also usually resists airflow more, especially in the common 1-inch thickness, and many systems were designed with return ducts that leave little room for extra resistance. Put a high-rated 1-inch filter in a system like that and you can get the symptoms of a dirty filter on day one: reduced airflow, a coil that freezes in summer, a furnace that trips its overheat limit in winter, and more noise at the grille.

Thicker filters, 4 or 5 inches deep in a cabinet built for them, solve that by spreading the same filtering over far more surface, which is why they can carry a high rating and still let the air through. Whether your system can take a higher-rated filter is a question for your heating and cooling tech, who can measure the resistance your system is already fighting. The ratings themselves are explained in the tech-facing Filter MERV Rating Guide.

How often

The filter's packaging gives a starting interval, and the common ones are 1 to 3 months for a 1-inch filter and 6 to 12 months for a 4 or 5-inch media filter. Treat those as a starting point and let your house set the real number. Pets, a dusty road, a remodel or a system that runs constantly in a hot summer all shorten it. For the first year, check the filter monthly: when it has gone uniformly gray and you cannot see light through it when you hold it up, that is your interval.

Worked example: one house, two filters, one question

The house: a hallway ceiling return grille with latches, and a furnace in the basement. The homeowner has always changed the grille filter.

October 12. He sets the thermostat to off and the fan to auto, opens the grille, and pulls the filter. The frame reads 20 x 25 x 1 nominal, 19.5 x 24.5 x 0.75 actual, MERV 8, with "7/10" written on it in marker. The arrow points up into the ceiling: correct. Its face is evenly gray and he cannot see light through it when he holds it up to the hall light. July 10 to October 12 is a little over three months, through a summer the air conditioner ran most days, so he decides his interval for summer is two months, not three.

He slides in a new 20 x 25 x 1, MERV 8, arrow up, writes "10/12" on the edge, and closes the grille.

While in the basement for something else, he notices a cover on the duct beside the furnace he had never opened. Behind it is a second 1-inch filter, almost black, with no date on it. The house has been filtering twice, in a row, and nobody knew about the second one.

What he does not do: throw in a fresh filter there too and assume twice the filtering is twice as good. Two filters in a row add their resistance together, and a filter loaded with years of dust is likely to be doing most of the restricting. He pulls it, reads 16 x 25 x 1 on the frame, writes down what he found, takes a photo, and leaves the slot empty with the cover back on until his heating and cooling tech's fall visit two weeks later.

What the tech settles: which of the two locations should hold the filter, since one filter in the right spot is the usual design. She also answers the question his allergist raised, whether to move up to a higher MERV: she measures how much resistance the system has with one MERV 8 in place and tells him whether a higher rating fits in 1 inch, or whether it needs a deeper filter cabinet.

What would have changed it:

• If the basement filter had been wet, or the slot had water or rust in it, that points to a drain or coil problem in a system with an air conditioner. See related: How to Clear Your Air Conditioner Condensate Line.
• If the house had only the basement slot, and the homeowner had never known it existed, the same black filter would have been the whole story, and changing it would have been the fix.

Stop and call your heating and cooling tech when

• The filter is wet, or there is water, ice or rust in the slot or around the furnace.
• You smell burning or hot dust that does not clear within the first few minutes of the season's first heating run.
• Airflow stays weak at the vents with a clean filter in.
• The filter keeps getting sucked out of shape, or the slot has no cover and leaves a gap.
• You are not sure which of two filter locations should be used.

See related: When to Stop and Call a Licensed Pro Around the House.

References

• See related: Filter MERV Rating Guide
• See related: Reading the Labels on Your Furnace and Air Conditioner
• See related: Seasonal House Care Calendar for Homeowners
• See related: Your House Records File and What Goes In It
• Your furnace's owner's manual, for filter location, size and any filter inside the blower compartment