Furnace filters explained: What they do and why they matter

If you’ve ever stood in front of a wall of furnace filters at the store and had no idea which one to buy, you’re not alone. Between the numbers, sizes and types, it can feel more complicated than it should be.

But don’t worry. Understanding furnace filters is easier than you think. And once you do, you’ll breathe easier (literally) knowing your air is cleaner and your furnace is running efficiently.

If anyone in your household has allergies, asthma, or other respiratory conditions, upgrading your furnace filter can make a noticeable difference. Filters with higher MERV ratings can catch smaller particles, like pollen, mould spores and even bacteria, keeping the air you breathe cleaner and healthier.

A clean filter = a more efficient system. And a more efficient system = less energy used to heat your home.

Filter types 101: What are the differences?

Here’s a quick breakdown of the most common types:

• Fiberglass filters are the most affordable option, usually for less than $5 each. They trap larger particles like lint or dust, which is just enough to protect your furnace from large debris, but don’t do much for air quality.
• Pleated filters are made from polyester or cotton folds. They trap more dust and allergens and are ideal for most homes, typically costing $15-$50, depending on size and MERV rating. Pleated filters should be replaced every 1-3 months.
• Electrostatic filters use static electricity to attract particles. Many are reusable and washable (just rinse with water, let dry and reinstall). They’re often more rigid with a plastic or metal frame and cost more upfront ($75+) but can last several years with proper care.
• HEPA filters are super-efficient, but not usually compatible with standard residential furnaces unless the system is specifically designed to handle it.

MERV vs. MPR: What do they mean?

Most filters are rated using MERV (Minimum Efficiency Reporting Value), on a scale from 1-16. The higher the number, the better it traps particles like dust, pollen, mould and bacteria.

MPR (Microparticle Performance Rating) is specific to 3M’s Filtrete filters and focuses on capturing smaller particles under 1 micron. Here’s a rough comparison:

• MERV 8 ≈ MPR 600
• MERV 11 ≈ MPR 1,000-1,200
• MERV 13 ≈ MPR 1,500-1,900

Most homes do well with a MERV rating between 8-13. But here’s the key: don’t assume the highest MERV rating is the best choice. Filters with higher MERV ratings are denser, which can reduce airflow, especially in older furnaces.

How do you know what size filter you need?

Check the label on your current filter. Most furnace filters have the size printed along the edge–something like 16x25x1 (length x width x thickness in inches).

If it’s missing or unreadable, don’t guess. Check your furnace manual or ask your HVAC technician. The right fit makes all the difference for airflow, efficiency and clean air.

How often should you change them?

Generally, every 1-3 months. If you have pets, allergies, or live in a dusty area (like a new development), check monthly. A clogged filter makes your furnace work harder and uses more energy.

Reusable electrostatic filters can be washed every 1-3 months instead of being replaced. Just make sure to follow the manufacturer’s cleaning instructions.

Pro tip: Set a calendar reminder or change your filter with the seasons.

A small task with a big payoff

Swapping your furnace filter is a quick task that helps your system run smoothly, keeps your air clean, and supports better energy use. Not bad for five minutes of effort.

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