The short version
- A thin one inch filter needs changing every one to three months. A thick media filter in its own cabinet usually runs six to twelve months.
- Buy by the size printed on the frame of the old filter. That number is deliberately about half an inch bigger than the filter itself.
- MERV 8 to 11 suits most furnaces. Higher grades catch more but make the blower work harder, so check before you jump to MERV 13.
- The arrow on the frame points toward the furnace, in the direction the air is moving.
How often to change a furnace filter
It depends on how thick the filter is. Thickness is the third number on the frame, and it changes the answer more than anything else.
- One inch. The thin one in a slot on the side of the furnace. Every one to three months. Furnace makers put the range at 30 days to three months.
- Four or five inch. A pleated media filter that sits in its own cabinet beside the furnace. Usually six to twelve months, and the maker's label will say.
- Washable. Rinse and dry it completely on the same schedule as a one inch filter. A damp filter put back in is worse than no filter.
Then adjust for the house. Shorten the interval if you have a dog or a cat, if anyone in the house has allergies or asthma, if you run the fan all the time, or if there is renovation work going on. Drywall dust will finish a filter in a fortnight.
Winter and summer are the heavy months because the system runs hardest then. A filter that lasted all spring can be grey by the end of January. Check it in December, not just in September.
The calendar is a reminder, not a rule
Pull the filter out and hold it up to a light. If you cannot see light through the pleats, change it, whatever the date says. Take a photo of the size printed on the frame while it is in your hand.
What size furnace filter to buy
The size is printed along the edge of the cardboard frame, as three numbers such as 16x25x1. Width, height, thickness. Buy that.
Here is the part that confuses people. Those three numbers are the nominal size, which is the name of the slot, not the tape measure size of the filter. A 16x25x1 filter actually measures closer to 15.5 by 24.5 inches. That is on purpose, so it slides in instead of binding. Every brand follows the same convention, so you order by the printed name and it fits.
If the old filter is gone and there is nothing printed anywhere, measure the slot itself and round down to the nearest common size. Do not force a filter that is slightly too big, and do not settle for one that is loose. Air takes the easy route, and a gap around the frame means dust goes straight past the filter into the blower.
Which MERV rating your furnace can handle
MERV stands for minimum efficiency reporting value. The scale runs from 1 to 16 and it tells you how small a particle the filter catches. Higher is not automatically better, because a finer filter is also harder to pull air through.
| Grade | Catches | Suits |
|---|---|---|
| MERV 8 | Dust, lint, pollen, mould spores | A house with no pets and nobody with allergies |
| MERV 11 | The above plus pet dander, fine dust, some bacteria | Most homes. Pets, or someone who reacts to dust |
| MERV 13 | The above plus smoke and the finest particles | Allergy and asthma households, on a system that can take it |
The US Environmental Protection Agency suggests MERV 13, or as high as your fan and filter slot can accommodate. That last part is the whole question. A one inch MERV 13 in an older furnace with tight ductwork can starve the blower, which shows up as weak airflow, a furnace that overheats and shuts off, or a frozen coil in summer.
So MERV 8 to 11 is the safe range for most one inch slots. If you want finer filtration than that, the right way is more filter area rather than a denser one inch filter: a four or five inch media cabinet, or a HEPA filter in its own housing. Both need the ductwork opened up, which is why they get installed rather than bought off a shelf.
Which way does a furnace filter go in
There is an arrow printed on the side of the frame. It points toward the furnace, in the same direction the air travels. Air comes back from the house through the return duct, passes through the filter, then reaches the blower.
So if the filter slot is below the furnace, the arrow points up. If the slot is on the side, the arrow points in toward the cabinet. The easy check: the arrow always points away from the return duct and at the furnace.
Getting it backwards will not break anything overnight, but the filter collapses toward the blower and traps less. If you cannot find an arrow, note which way the old one faced before you pull it out, or look for the stiffer wire mesh side, which faces the furnace.
What a clogged filter does, and when the filter is not the problem
A blocked filter starves the system of air. In winter the heat has nowhere to go, so the furnace overheats, a safety switch shuts the burners off, and it runs in short bursts without warming the house. In summer the same shortage of air can freeze the indoor coil solid.
- Rooms furthest from the furnace never quite get there.
- The system runs in short cycles, on and off, all evening.
- Dust settles again a day after you clean.
- A whistling noise at the return grille.
Change the filter first, because it is the cheapest thing to rule out and it is often the answer. Give it a day.
If nothing changes, the filter was not the fault. Ice on the coil, a failing blower motor, a dirty burner or a duct problem all look similar from the hallway. That is the point to book a furnace tune-up or, if there is no heat at all, furnace repair. The diagnostic price is agreed before anyone starts.