Change a standard 1-inch filter every 1 to 3 months during heating season, or monthly if you have pets or allergies. Thicker 4- to 5-inch media filters last 6 to 12 months. The size is printed on the frame of your current filter, for example 16x25x1. Turn the furnace off, slide the old filter out, and put the new one in with the airflow arrow pointing toward the furnace.
Why does the furnace filter matter so much in winter?
Your furnace pulls every bit of air it heats through that filter. When the filter clogs, airflow drops, heat builds up inside the furnace, and a safety switch called the high-limit shuts the burners off to protect the equipment. The furnace cools down, restarts, overheats again, and shuts off again. That on-off cycling is one of the most common reasons we get called out for a furnace that "keeps shutting off", and the fix is often a $10 filter.
Left long enough, the strain does real damage: overheated heat exchangers, burned-out blower motors and higher gas bills. In the Capital Region, where a furnace can run for most of the day from December through February, a clogged filter does its damage fast.
How often should you change your furnace filter?
| Filter thickness | Typical household | Pets, allergies or renovation dust |
|---|---|---|
| 1 inch (e.g. 16x25x1) | Every 2 to 3 months | Every month |
| 2 inch (e.g. 16x25x2) | Every 3 months | Every 1 to 2 months |
| 4 inch media (e.g. 16x25x4) | Every 6 to 9 months | Every 4 to 6 months |
| 5 inch media (e.g. 20x25x5) | Every 9 to 12 months | Every 6 months |
Treat those as starting points. The real test is to pull the filter out once a month during heating season and hold it up to a light. If you cannot see light through it, change it, whatever the calendar says. A practical habit: check it when the gas bill arrives.
How do you find the right filter size?
Look at the cardboard frame of the filter that is in your furnace now. The size is printed on the edge, usually in the format width x height x thickness, for example 16x25x1 or 20x25x5. That is the nominal size, and it is the size you buy.
The filter itself is slightly smaller than the label. A 16x25x1 actually measures about 15.5 x 24.5 x 0.75 inches so it slides into the slot. If you measure the slot with a tape measure, round up to the nearest standard size rather than buying the exact measurement.
Can't find the filter? In Capital Region homes we usually find it in one of three places:
- A slot on the side of the furnace, between the return duct and the blower compartment, behind a thin metal cover
- Inside the blower door at the bottom of the furnace, common on older units
- A media cabinet, a box several inches thick between the return duct and the furnace, holding a 4- or 5-inch filter
- Some homes have the filter behind a large return-air grille in a wall or ceiling instead
Which MERV rating should you buy?
MERV measures how fine a filter catches particles, from 1 to 16 for home filters. Higher is not automatically better: a very dense filter can choke the airflow on a furnace that was not designed for it.
| MERV | Catches | Best for |
|---|---|---|
| MERV 8 | Dust, lint, pollen | Most homes. A good balance of filtration and airflow |
| MERV 11 | Plus pet dander, fine dust, smoke | Pets or mild allergies, if your system handles it |
| MERV 13 | Plus fine particles and some bacteria | Systems designed for it, usually with a 4- or 5-inch media cabinet |
If you want MERV 13 filtration, the better route is a 4- or 5-inch media cabinet rather than a thin 1-inch MERV 13 filter. The thicker filter has more surface area, so it filters finely and breathes easily. We can add one during a furnace tune-up or a new heating system installation.
How to replace a furnace filter in 5 steps
- Turn the furnace off at the thermostat, or use the service switch near the furnace.
- Slide the old filter out and note which way the arrow on its frame points.
- Check the size printed on the old frame matches the new filter.
- Insert the new filter with the arrow pointing toward the furnace, in the direction the air flows from the return duct into the blower.
- Close any cover, turn the furnace back on, and write the date on the new filter's frame with a marker.
Filter mistakes our technicians see every winter
- Arrow pointing the wrong way. The filter still fits, but it loads up faster and restricts airflow more.
- The wrong size, leaving a gap around the edges so dirty air goes straight around the filter and into the blower.
- Two filters stacked together, or a filter in both the return grille and the furnace. That doubles the restriction.
- A media filter forgotten for years. Homeowners change the grille filter but don't know the furnace has a 5-inch filter in a cabinet too.
- A washable filter put back damp, which grows mold and clogs faster.
When is it more than the filter?
A fresh filter fixes a lot, but if you still notice any of the following, the furnace needs a technician:
- The furnace still short-cycles or shuts off with a clean filter
- Weak airflow from every vent even after the change
- A burning smell that lasts more than the first few minutes of the season
- Repeated clicking without the burners lighting
These are jobs for furnace repair in Albany. And if you are heading into winter without a checkup, a filter change is a good reminder to book a full furnace tune-up. A furnace that runs reliably is also your best defense against frozen pipes in a cold snap.
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Frequently asked questions
Can a dirty furnace filter make the furnace shut off?
Yes. A clogged filter restricts airflow, the furnace overheats, and a safety switch shuts the burners off. The furnace then restarts and repeats the cycle. Replacing the filter often stops it; if it continues with a clean filter, call a technician.
Is a higher MERV filter always better?
No. A dense filter such as a 1-inch MERV 13 can restrict airflow on a furnace not designed for it. MERV 8 suits most homes. For finer filtration, a 4- or 5-inch media filter gives better filtering with less restriction.
Why does my furnace filter get dirty so fast?
Pets, recent renovation dust, gaps in the return ductwork that pull in basement or attic dust, or a filter that is too thin for the system. If a new filter is grey within a few weeks, have the return side of the ductwork checked.
Do I need to turn off the furnace to change the filter?
Yes. Turning it off at the thermostat or the service switch stops the blower from pulling dust and debris into the system while the filter slot is open.
What if the filter size isn't printed on the frame?
Measure the filter or the slot and round up to the nearest standard size, for example 15.5 x 24.5 becomes 16x25. The furnace manual or the model number on the furnace label will also list the filter size.
Are 4-inch and 5-inch media filters worth it?
For most homes, yes. They last 6 to 12 months, filter finer particles, and restrict airflow less than a thin filter of the same rating. They need a media cabinet, which can be added to most existing furnaces.
Written by the ALJ Heating & Cooling team
Licensed and insured HVAC technicians serving Albany, Latham, Troy, Schenectady, Clifton Park and the Capital Region. BBB Accredited, SBE and MBE certified, and rated 4.9 stars across 198+ Google reviews. This guide is based on what our technicians see on service calls in Capital Region homes.
Last updated November 9, 2026
