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Filter packaging says "every 90 days" year-round. That's oversimplified for AC season. In July and August, your system runs 3-5x more than in April. Your filter fills up accordingly.
Why summer loads filters faster
- Runtime multiplier — during a heat wave, AC can run 18+ hours a day vs 2-3 hours on a mild spring afternoon. Filter-hours skyrocket.
- Open doors, open windows — summer activity brings more outdoor dust, pollen, and pet hair into circulation.
- Humidity loading — moist air deposits more readily on filter media.
A filter that lasts 90 days in March may only last 45 days in July.
The season-aware schedule
| Season | 1" Filter | 4" Filter |
|---|---|---|
| Spring (mild use) | 75-90 days | 8-12 months |
| Summer (peak AC) | 30-60 days | 6-9 months |
| Fall (mild use) | 75-90 days | 8-12 months |
| Winter (peak heat) | 45-75 days | 6-9 months |
In practice: check monthly in summer and winter, every two months in spring and fall.
Peak heat wave considerations
During an extended heat wave (multiple days above 95°F), check your filter weekly. Your system is under extreme stress, and a clogged filter during these conditions is the most common cause of AC failure calls.
If a heat wave is forecast, install a fresh filter BEFORE it hits. This single habit prevents roughly half of summer AC breakdowns.
Frequently asked questions
Should I change the filter before AC season starts?
Yes, absolutely. A fresh filter in late May sets you up for a problem-free summer.
What if my AC runs constantly?
Replace every 2-3 weeks even for MERV 11 during a prolonged heat wave.
Does the filter matter for dehumidification?
Yes. A clogged filter reduces airflow across the coil, reducing moisture removal. Your house feels "cold but damp."
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