MERV 8: Good
MERV 11: Better
MERV 13: Best
A MERV 8 filter removes fine particles from the air. MERV 11 and 13 filters capture even finer particles. As the table below shows, your specific circumstances help determine what quality furnace filter to buy. And the quality you choose determines how often it should be replaced.
If you live in an older home or a city with dirtier air, the highest quality furnace filter will help protect you from adverse health effects. If you live in a brand new home in a small town with cleaner air, a MERV 8 or 11 might be just fine. Similarly, it might be more important for people with allergies or asthma to use higher quality filters.
The bottom line: You should replace low quality filters, sometimes labeled 30-day filters, every month. Higher quality filters should last 2-3 months. Follow the manufacturer’s recommendations or ask your HVAC contractor.