Furnace Filter: Size, Rating and When to Change It
Buy the right filter once, write the size down, and set a date to swap it.
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Most homes need a new furnace filter about every 3 months, and every month or two in a house with pets or heavy use. Heating season is the time to check. A clogged filter makes the furnace work harder, and the wrong one can do the same.
Key takeaways
- Read the size printed on the edge of your current filter. Take a photo of it.
- Pick a rating your furnace can handle. Higher is not always better.
- Check it monthly and set a repeat reminder. Swap it at least every 3 months.
How do I buy the right furnace filter?
- Find the size. It is printed on the cardboard frame, such as 20x25x1. That is the nominal size. The real filter is often a little smaller, so buy the number on the old one, not a tape measure reading.
- Check the thickness. A 1 inch filter is the common kind. Thicker filters, often 4 or 5 inches, fit a special cabinet and usually last longer between changes. You cannot swap one for the other.
- Pick a rating. Filters carry a MERV number. Higher numbers catch smaller particles. For most homes, MERV 8 to 13 is the range commonly recommended. Check your furnace manual first. A very dense filter can restrict airflow on a system not built for it.
- Match the filter to your home. Pets, allergies, or a dusty area are reasons to look at the upper end of what your furnace allows. A quiet two person home may be fine in the middle.
- Compare the unit price. A multipack is often cheaper per filter. Divide the price by the number of filters, like the unit price shelf tag does for groceries. Check current price before you buy.
- Skip the claims you cannot check. Words like "ultra" or "premium" are not a rating. Look for the MERV number on the box.
How often should I change it?
A common rule is to check it every month and replace it at least every 3 months. Your filter may need it sooner:
- Pets that shed: look at it every month and expect to swap it earlier.
- Heavy use in winter: the furnace runs more, so the filter fills faster.
- Thick filters: often last 6 months or more. Follow the box.
- Construction or a dusty season: swap it after the work is done.
A simple test: hold the filter up to a light. If you cannot see light through it, replace it.
Mistakes that cost money
- Installing it backwards. Look for the arrow on the frame. It points toward the furnace, the way air flows.
- Washing a disposable filter. Only filters marked washable can be cleaned.
- Buying a higher rating than the furnace handles. Read the manual before you move up.
- Forgetting it. The most common mistake. Write the date on the frame in marker.
- Leaving the gap open. A filter that does not fit snugly lets dust go around it.
Quick action checklist
- [ ] Pull out your current filter and photograph the size.
- [ ] Look up the MERV range in your furnace manual.
- [ ] Buy a multipack in that size and rating.
- [ ] Write today's date on the new filter's frame.
- [ ] Add a repeat reminder to Nusafa, and test your alarms the same day. See space heater safety checks.
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Product guidance changes, so follow your furnace manual and the filter box. Check current price before you buy. See our disclosure policy for details.
