A clogged filter makes the blower work harder, cuts airflow, and can cause the furnace to overheat and shut itself off. A good share of the “no heat” calls we get in the first cold week of the year end with a new filter.
Finding the filter and its size
Look where the return air duct meets the furnace, usually a slot on the side or bottom with a removable cover. Some homes have the filter behind a large grille in a hallway ceiling or wall instead.
Pull the filter out and read the frame. You will see a nominal size, such as 16 × 25 × 1. The actual size is slightly smaller, and that is normal. Buy by the nominal size.
What MERV means
MERV (Minimum Efficiency Reporting Value) rates how well a filter catches particles, on a scale of 1 to 16 for home use.
- MERV 1–4: flat fiberglass. Protects the furnace from large debris and little else.
- MERV 8: pleated. Catches dust, pollen and mold spores. A sensible default.
- MERV 11–13: catches finer particles, including much of the pet dander and smoke. Good for allergies, if your system can handle it.
Higher is not automatically better. A very dense filter restricts airflow, especially in a 1-inch slot on an older or smaller system. If you go to MERV 13, a 4-inch media cabinet is a better home for it.
How often to change it
- 1-inch pleated: every 1 to 3 months during heating and cooling season
- Pets, smokers, or ongoing renovation: closer to monthly
- 4-inch media filters: every 6 to 12 months
- Washable filters: rinse monthly and let dry completely
The best check is visual. Hold the filter up to a light. If you cannot see light through it, change it.
Changing it
Turn the system off
Set the thermostat to off so the blower does not run with the filter out.
Note the airflow arrow
Every filter has an arrow on the frame. It points toward the furnace, in the direction air flows.
Slide the new one in
Arrow toward the furnace. It should fit snugly with no gaps around the edges.
Close the cover and restart
Make sure the slot cover is fully closed; an open slot pulls in unfiltered air.
Common questions
Does the filter matter in summer?
Yes, if you have central air. The same blower and filter serve both heating and cooling.
Are expensive filters worth it?
A mid-range MERV 8 to 11 pleated filter changed on time beats a premium one left in for six months.
Our guides are general information for typical homes. Codes, permits and conditions vary, so check local rules, and leave gas, main electrical panel and structural work to a licensed professional.
