Installation & Setup
Furnace Filter Guide: Sizes, MERV Ratings and When to Change Them
Use the size printed on your current filter, choose MERV 8 for basic protection or MERV 11–13 for better air quality if your system can handle the extra resistance, and change 1-inch filters every 1–3 months. Thicker 4–5 inch media filters last 6–12 months and restrict airflow less than a thin high-MERV filter.
What a furnace filter actually does
The filter's first job is protecting the equipment: it keeps dust off the blower wheel, the AC evaporator coil and the heat exchanger. Its second job is cleaning the air you breathe. Every filter also adds resistance to airflow, and that trade-off between capture and resistance drives every decision on this page.
Filter sizes: nominal vs actual
Filters are labelled with a rounded nominal size, such as 16 × 25 × 1, while the actual size is usually about 1/4–1/2 in smaller so it slides into the slot. Buy by the nominal size printed on the frame of your old filter. If there is no old filter, measure the slot and round up to the nearest whole inch.
| Common nominal size | Typical actual size |
|---|---|
| 16 × 20 × 1 | 15.5 × 19.5 × 0.75 in |
| 16 × 25 × 1 | 15.5 × 24.5 × 0.75 in |
| 20 × 20 × 1 | 19.5 × 19.5 × 0.75 in |
| 20 × 25 × 1 | 19.5 × 24.5 × 0.75 in |
| 16 × 25 × 4 | 15.5 × 24.5 × 3.75 in (varies by brand) |
| 20 × 25 × 5 | 19.75 × 24.75 × 4.4 in (varies by cabinet maker) |
Media cabinets for 4–5 in filters vary by manufacturer, so match the replacement to the cabinet brand.
Choosing a MERV rating
MERV (Minimum Efficiency Reporting Value, from ASHRAE Standard 52.2) runs from 1 to 16 for residential and light-commercial filters. Higher numbers capture more fine particles. Our MERV rating chart covers every level in detail.
| MERV | Captures well | Good for |
|---|---|---|
| 1–4 | Lint, large dust | Equipment protection only |
| 5–8 | Dust, pollen, mould spores | Most homes; the usual default |
| 9–12 | Adds finer dust and some smoke and pet dander | Allergies, pets |
| 13–16 | Most fine particles, including many bacteria-sized ones | Asthma, wildfire smoke; best with a 4–5 in media cabinet |
HEPA filtration is a different, stricter standard that most residential furnaces cannot push air through; see the HEPA filter guide.
The airflow and static pressure trade-off
Your blower is designed to move a set amount of air, usually about 350–450 CFM per ton of cooling, against a limited total external static pressure (often 0.5 in w.c. for many residential systems). A clean 1-inch MERV 8 filter might use around 0.1 in w.c. of that budget; a thin MERV 13 filter can use two or three times as much, and any filter's resistance climbs as it loads with dust. Typical figures vary by brand, so check the filter maker's pressure-drop data.
Too much restriction has real consequences:
- Furnaces overheat and trip their high-limit switch, which can leave you with a furnace blowing cold air and, over many years, contributes to heat exchanger cracking.
- Air conditioners get a colder coil, which can freeze, and lose capacity.
- Variable-speed (ECM) blowers ramp up to compensate, using more electricity and making more noise.
The way to get high MERV without the penalty is more filter area. A 4–5 inch pleated media filter has several times the surface of a 1-inch filter, so a MERV 13 media filter often restricts less than a 1-inch MERV 8. Adding a media cabinet is a common upgrade.
A technician can measure static pressure before and after the filter with a manometer to confirm your choice is suitable.
How often to change it
| Filter type | Typical interval | Change sooner if |
|---|---|---|
| 1-in fiberglass (MERV 1–4) | 30 days | Visible dust buildup |
| 1-in pleated (MERV 8–13) | 60–90 days | Pets, construction, smoke, allergies |
| 2-in pleated | 3–4 months | Heavy run time |
| 4–5-in media | 6–12 months | Pets or very dusty homes |
| Washable/electrostatic | Clean monthly | They lose effectiveness if not dried fully |
Check monthly during peak heating or cooling season. Hold the filter up to a light; if you can barely see through it, replace it.
Step-by-step: changing a filter
- Turn the thermostat off (or the furnace service switch) so the blower does not pull dust in while the slot is open.
- Find the filter: in a slot beside the blower compartment, in a media cabinet, or behind a return air grille.
- Note the size and the airflow arrow on the old filter.
- Slide the new filter in with the arrow pointing toward the furnace (the direction of airflow).
- Close the slot cover or grille so no air bypasses the filter.
- Write the date on the frame and turn the system back on.
Common mistakes
- Installing the filter backwards.
- Using a filter that is too small, leaving gaps where air bypasses it.
- Choosing a thin MERV 13+ filter for a system with already high static pressure.
- Forgetting a second filter at a return grille when there is one at the furnace too, doubling the restriction.
- Running the system with no filter while waiting for a replacement.
Frequently asked questions
Is a higher MERV filter always better?
No. Higher MERV captures more, but a thin high-MERV filter can restrict airflow too much for some systems. MERV 8–11 suits most 1-inch slots; go higher with a thicker media filter.
Which way does the furnace filter arrow point?
Toward the furnace and blower, in the direction the air flows. On a return grille, that means pointing into the wall or ceiling.
Can a dirty filter raise my energy bill?
Yes. Restricted airflow makes the system run longer and, with variable-speed blowers, use more fan power. It can also cause icing on the AC coil and overheating in the furnace.
Last reviewed 2026-10-28