A practical guide to choosing, sizing, and replacing AC filters for residential HVAC systems. Includes MERV rating comparisons, filter sizing charts, replacement schedule recommendations, and a cost comparison tool for common filter types.
Your AC filter is the single most impactful maintenance item you can control as a homeowner. A clogged or undersized filter:
- Increases energy bills by 5-15% — restricted airflow forces the blower motor to work harder
- Reduces system lifespan — the evaporator coil freezes when airflow drops below design specs
- Degrades indoor air quality — saturated filters stop trapping particles and can release captured dust back into the air
- Voids warranty coverage — most manufacturers require proof of regular filter changes
In hot, humid climates like South Florida, filters clog faster due to higher runtime hours (your AC runs 2,000-3,000+ hours per year vs. 1,200 in temperate climates) and elevated dust, pollen, and humidity levels.
MERV (Minimum Efficiency Reporting Value) rates a filter's ability to capture particles. Higher isn't always better — your system needs adequate airflow.
| MERV | Captures | Best For | Pressure Drop |
|---|---|---|---|
| 1-4 | Pollen, dust mites, carpet fibers | Minimal filtering (builder grade) | Very low |
| 5-8 | Mold spores, pet dander, dust | Most residential systems | Low |
| 9-12 | Legionella, humidifier dust, auto emissions | Homes with allergies or pets | Moderate |
| 13-16 | Bacteria, tobacco smoke, sneeze droplets | Hospital-grade, special needs | High |
| 17-20 | Viruses, carbon dust, sea salt | Clean rooms, surgical suites | Very high |
- Standard home, no pets: MERV 8
- Home with pets: MERV 11
- Family member with allergies/asthma: MERV 11-13
- South Florida coastal home: MERV 11 (salt air + humidity = faster buildup)
Warning: Don't install MERV 13+ without checking your system's static pressure rating. High-MERV filters in a system designed for MERV 8 will restrict airflow and can freeze your evaporator coil.
Residential AC filters come in standard sizes. Measure your existing filter or check the return air grille dimensions. The size printed on the filter is the nominal size — actual dimensions are typically 0.25-0.5 inches smaller.
| Nominal Size | Actual Size | Common In |
|---|---|---|
| 16x20x1 | 15.5 x 19.5 x 0.75 | Small homes, condos |
| 16x25x1 | 15.5 x 24.5 x 0.75 | Standard homes |
| 20x20x1 | 19.5 x 19.5 x 0.75 | Standard homes |
| 20x25x1 | 19.5 x 24.5 x 0.75 | Larger homes |
| 20x25x4 | 19.5 x 24.5 x 3.75 | Media filter cabinets |
| 24x24x1 | 23.5 x 23.5 x 0.75 | Commercial/large residential |
| 25x25x1 | 24.5 x 24.5 x 0.75 | Large residential |
- Turn off your AC system
- Locate the return air grille (usually on a wall or ceiling)
- Remove the existing filter
- Measure length × width × depth in inches
- Round up to the nearest standard size
If your filter slot accepts 4-inch or 5-inch deep filters, strongly consider upgrading from 1-inch. Deeper filters have more surface area, last longer, and create less pressure drop at the same MERV rating.
How often you need to replace depends on filter type, MERV rating, and environmental factors.
| Filter Type | Thickness | Typical Lifespan | South Florida Lifespan |
|---|---|---|---|
| Fiberglass (MERV 1-4) | 1" | 30 days | 21-30 days |
| Pleated (MERV 8) | 1" | 90 days | 45-60 days |
| Pleated (MERV 11) | 1" | 90 days | 30-45 days |
| Deep pleated (MERV 8) | 4" | 6-9 months | 4-6 months |
| Deep pleated (MERV 11) | 4" | 6-9 months | 3-4 months |
| Deep pleated (MERV 13) | 4" | 6-9 months | 2-3 months |
Replace sooner if: you have pets, run the AC 24/7, live near construction, have a dusty attic, or notice reduced airflow from vents.
See filter-comparison.md for a detailed breakdown of fiberglass vs. pleated vs. electrostatic vs. HEPA filters — including cost per year, pros, cons, and best use cases.
See cost-calculator.py for a simple Python script that estimates your annual filter cost based on your filter size, MERV rating, and replacement frequency.
python3 cost-calculator.py --size 20x25x1 --merv 11 --pets true --climate hot-humid- Stock up on filters before the season — supply chains get disrupted after storms
- After a hurricane, replace your filter immediately even if it's not due — debris in the air clogs filters fast
- If your home flooded, have the entire duct system inspected before running the AC
- Check your filter monthly during peak cooling months
- Set a phone reminder — "out of sight, out of mind" is the #1 reason filters go unchanged
- Keep 2-3 spare filters at home so replacement is frictionless
- If you leave your home vacant, set the AC to 78°F and install a fresh filter before you leave
- Consider a smart thermostat that alerts you remotely when airflow drops
Replace the filter yourself, but call a licensed HVAC technician if:
- Airflow is still weak after a fresh filter
- You hear whistling or unusual noise from the return air grille
- Ice is forming on the refrigerant lines or evaporator coil
- Your energy bills spike despite regular filter changes
- You see mold on or around the filter slot
Professional AC repair and maintenance in South Florida: AC Repair Today — licensed (FL CAC1824118), same-day service available in Miami-Dade, Broward, and Palm Beach counties.
Found an error or want to add information about a specific filter brand? Pull requests welcome.
MIT — see LICENSE
Created by AC Repair Today — Licensed AC repair serving South Florida since day one.