How the interval is worked out
Three things move the number, and only three:
base by filter depth 1" = 90 days 2" = 120 4" = 180 5" = 270
one cat or dog × 0.7
two or more × 0.5
allergies or asthma × 0.7
ENERGY STAR anchors the top of that: check the filter every month, change it at least every three months, and change it sooner if it looks dirty after one. The 90 days for a one-inch filter is that figure. The scaling by depth and the pet and allergy multipliers are this site’s own extrapolation from it — reasonable, widely used in the trade, and not published by anyone official.
Depth dominates because it is really a proxy for surface area. A five-inch media cabinet holds several times the pleated area of a one-inch panel, so it loads several times more slowly at the same airflow.
The result is rounded to a fortnight, because a schedule that says “every 63 days” is a schedule nobody follows.
None of this beats looking at the filter. Hold it up to a light. If light does not come through, it is finished, regardless of what any calendar says. The schedule exists so that the looking happens at all — not so that you stop looking.
Why the published intervals disagree
You will find thirty days on a filter box, ninety days from a technician, and a year in an equipment manual, all for what looks like the same situation. They are answering different questions.
| Source | Typical figure | What they are optimising |
|---|---|---|
| Filter manufacturer | 30–90 days | Filtration performance — and filter sales |
| HVAC technician | 90 days for 1”, 6–12 months for media | Airflow and equipment life |
| Equipment manual | Varies, often vague | Warranty conditions |
| Air quality guidance | More often, higher MERV | Particulates, not the machine |
A loaded filter genuinely does trap more particles for a while — the cake of trapped dust is itself a filter — and it genuinely does restrict airflow. Both things are true, which is why the advice splits along whoever is giving it.
MERV, and where higher stops being better
MERV measures how small a particle the filter catches. Higher sounds better and is not automatically so, because catching more also means resisting airflow more.
- MERV 8–11 — the sensible range for a one-inch filter in a residential system. Catches dust, pollen, mould spores and most pet dander.
- MERV 13 — meaningfully better on fine particles and smoke. Fine in a four- or five-inch cabinet; genuinely risky in a one-inch slot on an older blower.
- Above 13 — hospital territory. In a house it mostly starves the system.
The failure mode is worth knowing: too much restriction in cooling drops airflow across the evaporator coil, the coil gets too cold, and it freezes into a block of ice. The symptom is no cold air and, once it thaws, water everywhere. People replace a perfectly good filter with a much more expensive one and cause exactly this.
The condensate drain
If you fix one thing on this page, make it this.
An air conditioner pulls water out of the air continuously — a humid day can produce several gallons — and all of it runs down a small drain line. Algae grows in that line because it is dark, wet and warm. When it blocks, the pan overflows, and the pan is usually above a ceiling.
Prevention is a cup of white vinegar poured into the access tee at the start of the cooling season, and a look at the line once a month while it is running. That is the entire maintenance procedure for the most common air conditioning failure there is.
What to write on the unit
Two things, in permanent marker, on the side of the furnace or air handler:
- The filter size, including thickness. You will buy it dozens of times.
- The install date, if you know it.
Everything else — model, serial, refrigerant type — is on the data plate, but the data plate is often in a bad position with small text, so photograph it once and keep the photo.
The service record at the bottom of the sheet is the part with resale value. A buyer’s inspector who sees a documented service history reads the whole house differently, and it costs you nothing but writing down what you already paid for.