Intervals by filter type
Thickness matters more than anything else, because it determines how much surface area is available to load up before airflow suffers.
- One-inch disposable: 1–3 months. Closer to one with pets or allergies.
- Four- to five-inch media cabinet: 6–12 months.
- Washable filters: rinse monthly, dry completely before reinstalling.
- During regional smoke events: check monthly regardless of type.
A higher MERV rating is not automatically better
A denser filter catches more, and it also restricts more airflow. Dropping a high-MERV one-inch filter into a system designed for a basic filter can starve the blower, which shows up as a frozen coil in summer or a furnace tripping its high-limit switch in winter.
If you want serious filtration — and in Orinda and the wooded parts of Lafayette, during smoke season, that is a reasonable thing to want — the right answer is usually a thicker media cabinet sized for the system, not a denser filter jammed into the existing slot. More surface area gets you filtration without the pressure drop.
How to tell you are late
Hold it up to a light. If you cannot see light through it, it stopped being a filter and became a restriction some time ago.
Write the date on the cardboard edge in marker when you install it. It is the only method anyone reliably sticks with.
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