A filter is the cheapest part of your system and the one that causes the most service calls when it is wrong. Wrong size, wrong rating, or simply left in too long. Here is how to get all three right.
Finding your filter size
The size is printed on the edge of the filter you already have, in inches, as width by height by depth, for example 16x25x1 or 20x25x4. That printed size is the nominal size. The filter itself is usually slightly smaller, for example a nominal 16x25x1 may actually measure about 15.5 by 24.5 by 0.75 inches, so it slides into the slot. Buy by the nominal size printed on the old filter.
If there is no filter, or the label is unreadable, measure the slot or the return grille opening and buy the nominal size closest to it. A filter that is too small lets air bypass it, which lets dust straight onto the coil.
Where the filter lives
- Behind a return grille in a hallway or ceiling, common in Sacramento tract homes
- In a slot at the furnace or air handler, in a closet, garage or attic
- In a media cabinet beside the furnace, which takes a thicker 4 or 5 inch filter
Some homes have more than one return, each with its own filter. Replace all of them.
What MERV means
MERV, the Minimum Efficiency Reporting Value, rates how well a filter captures particles of different sizes, on a scale up to 16 for residential and light commercial filters. Higher numbers catch smaller particles.
- MERV 1 to 4: basic fiberglass. Protects the equipment from large debris, does little for air quality.
- MERV 5 to 8: pleated filters that catch dust, pollen and pet dander. A sensible default for many homes.
- MERV 9 to 12: better capture of finer dust and some smoke particles.
- MERV 13 and up: captures much finer particles, including a large share of wildfire smoke. The EPA suggests MERV 13 or the highest your system can accommodate during smoke events.
Higher is not always better
A higher MERV filter is denser and can restrict airflow. On a system with undersized returns, which is common in older Sacramento homes, a thick MERV 13 one inch filter can raise static pressure, reduce airflow, freeze the coil in summer or trip limits in winter. Deeper 4 inch media filters offer higher MERV with less restriction because they have far more surface area.
If your system gets louder, cools worse, or ices after you switch to a higher rated filter, that is the filter restricting airflow. See duct and return sizing.
How often to change it
- 1 inch pleated: every 1 to 3 months, more often in summer, with pets, or near fields and construction
- 4 to 5 inch media: commonly every 6 to 12 months, depending on the manufacturer
- During wildfire smoke: check weekly, it loads much faster
Write the date on the edge of the filter when you install it. It removes the guessing.
Signs your filter is the problem
- You cannot see light through it
- A whistle at the return grille
- Weak airflow from the vents
- Ice on the refrigerant lines in summer. See why an AC freezes.
- A furnace shutting down on a limit fault. See furnace light codes.
What we do
We check filter fit and airflow, measure static pressure with the filter you want to use, and recommend a media cabinet when a higher rating makes sense. See our indoor air quality services, filters for wildfire smoke, and the summer checklist.
Frequently asked questions
How do I know what size air filter I need?
Look at the size printed on the edge of your current filter, such as 16x25x1. That is the nominal size to buy. If it is missing, measure the filter slot or grille opening.
Why is my filter smaller than the size printed on it?
The printed size is nominal. Filters are made slightly smaller so they slide into the slot.
What MERV rating should I use?
MERV 8 is a sensible default for many homes. MERV 13 captures much more fine smoke, but check that your system can handle the airflow restriction, especially with a 1 inch filter.
Can a high MERV filter damage my HVAC?
It can restrict airflow on systems with undersized returns, leading to frozen coils, overheating or noise. A deeper media filter reduces that restriction.
How often should I change my air filter?
Every 1 to 3 months for 1 inch pleated filters, and every 6 to 12 months for thick media filters. Check weekly during wildfire smoke.
Does the filter direction matter?
Yes. The arrow on the frame points toward the furnace or air handler, in the direction of airflow.