The filter is the cheapest part of your HVAC system and the one most responsible for expensive failures. Here is how often to change it, and what else belongs on the maintenance log.
Filter intervals, realistically
The “every 90 days” printed on the box is a compromise for an average that describes almost nobody.
| Situation | Interval |
|---|---|
| 1" filter, typical home, moderate use | 60–90 days |
| 1" filter, heavy cooling or heating season | 30 days |
| Pets | 30–45 days |
| Multiple pets or allergies | 20–30 days |
| Recent construction or renovation | Weekly during work |
| Vacant or lightly used property | 6–12 months |
| 4"–5" media filter | 6–12 months |
The honest test: pull it and look. If light does not pass through it, it is overdue regardless of the calendar.
MERV rating: higher is not automatically better
MERV measures how fine a filter is. Higher catches more, but also restricts more airflow, and residential systems have limited static pressure to spare.
- MERV 5–8 — standard residential, good airflow, catches dust and lint
- MERV 9–12 — better filtration for allergies, acceptable in most modern systems
- MERV 13+ — captures very fine particles, but can strain systems not designed for it
A filter too restrictive for your blower reduces airflow, which lowers efficiency and can freeze the evaporator coil, the same symptom as a filter left in too long. If you want high filtration, a thick media cabinet filter gives you surface area without the pressure drop.
What an annual tune-up should include
Twice yearly is ideal: cooling in spring, heating in fall. Expect $80–$180 per visit, less on a maintenance plan.
A real tune-up covers refrigerant pressure and temperature split, evaporator and condenser coil condition and cleaning, blower wheel and motor amp draw, capacitor and contactor condition, condensate drain clearing and pan check, tightened electrical connections, thermostat calibration, heat exchanger inspection for cracks on gas furnaces, and combustion analysis on gas.
Ask for the numbers, not just “everything looks good.” A capacitor reading near the bottom of tolerance is a $15 part now and a $400 emergency call in August.
Things you can do yourself
- Keep two feet clear around the outdoor unit and rinse the condenser fins with a hose from the inside out, gently, with the power off.
- Pour a cup of distilled vinegar down the condensate drain line a couple of times a season in humid climates. A clogged drain is one of the most common summer service calls and can dump water into a ceiling.
- Check the return vents are not blocked by furniture.
- Watch the thermostat differential. If the system runs continuously and cannot reach setpoint on a mild day, call before the first heat wave.
Why the log matters
Warranty. Most manufacturer warranties require documented annual professional maintenance. When a compressor fails at year seven, the claim asks for service records. No records, no coverage, and a compressor is a large number.
Diagnosis. A tech who can see that the same capacitor was replaced twice in three years is looking at a different problem than one seeing it fresh.
Resale. A documented HVAC history is one of the few things that reliably calms a buyer’s inspection anxiety about the most expensive system in the house.
Your own sanity. When was the last refrigerant charge? Which company came out in 2023? Is this the original coil? All answerable, if someone wrote them down.
What to record each time
Date, company and tech name, what was done, parts replaced, readings if they gave you any, cost, and the invoice. Plus the filter size, because the one thing you never have when the filter needs changing is the filter size.