AC and Furnace Filter Guide

What MERV Ratings Mean

MERV (Minimum Efficiency Reporting Value) rates HVAC filters from 1-20. Higher numbers catch smaller particles.

• MERV 1-4 — Large particles only: dust bunnies, carpet fibers, pollen over 10 microns. These flat fiberglass panels protect the equipment more than you.
• MERV 5-8 — Mold spores, dust mite debris, hair spray. A reasonable minimum for any home.
• MERV 9-12 — Fine dust, lead dust, auto emissions. The sweet spot for most homes. Removes most allergens without restricting airflow.
• MERV 13-16 — Bacteria, tobacco smoke, particles down to 0.3 microns. MERV 13 is the max most residential systems handle. Approaches HEPA.
• MERV 17-20 — True HEPA. Hospitals and clean rooms only. Not compatible with standard home HVAC.

Recommended Rating for Homes

MERV 10-13 is ideal for most homes — captures the majority of allergens while allowing adequate airflow.

Before installing a higher-rated filter, check two things:

1. Static pressure tolerance. A filter that is too dense starves the system of airflow, forcing it to work harder and potentially freezing the evaporator coil.
2. Return duct size. Undersized ductwork cannot handle dense filters. Stick with MERV 10 or lower if ducts are small.

If unsure, start with a MERV 11 pleated filter. If the blower runs louder or your home heats/cools slowly, step down to MERV 8.

Fiberglass vs Pleated Filters

Fiberglass filters ($1-$3 each) are flat, thin, and rated MERV 1-4. They protect the equipment but do very little for air quality.

Pleated filters ($10-$25 each) use folded synthetic media with far more surface area. Available from MERV 5-16, they catch finer particles without choking airflow. Worth the upgrade for any home where air quality matters.

How Often to Change Your Filter

The interval depends on your household:

• Every 30 days — multiple pets, allergies/asthma, or high-pollen/dust areas
• Every 60 days — one pet or moderate dust
• Every 90 days — no pets, average conditions
• Check monthly regardless — hold it to a light. If you cannot see through it, replace it.

A dirty filter is worse than a low-rated clean one. It restricts airflow, raises energy bills, and can cause equipment failure.

Signs Your Filter Needs Changing

• Dust accumulating faster than usual
• Reduced airflow from vents
• Longer HVAC cycles to reach set temperature
• Musty or stale smell when the system runs
• Visible dust or discoloration on the filter
• Increased allergy symptoms

Tips

• Write the install date on the filter frame with a marker.
• Buy multi-packs so a replacement is always on hand.
• Point the airflow arrow toward the blower (toward the furnace, away from the return duct).
• If your system has both a furnace filter and a return grille filter, change both on the same schedule.