After 52 years in this business, we can tell you with certainty: a dirty filter is the #1 cause of HVAC problems we see. It's also the easiest and cheapest thing to fix.
Yet we regularly find systems with filters that haven't been changed in a year or more. This guide will help you understand why filters matter and how to choose the right one.
Why Your Filter Matters
Your HVAC filter does two important jobs:
- Protects your equipment. It keeps dust, debris, and particles out of the blower motor, heat exchanger, and evaporator coil.
- Improves air quality. It traps airborne particles so you're not breathing them.
When a filter gets clogged:
- Airflow decreases, making your system work harder
- Energy consumption increases (up to 15%)
- Your system may overheat and shut down
- Dust bypasses the filter and coats internal components
- Your home gets dustier and air quality suffers
💡 The Dirty Filter Story
A customer recently called us because their furnace kept shutting off. A competitor quoted them $2,000+ for a new system. Our diagnosis? A $5 filter that was completely black with dirt. Changed the filter, problem solved. Total cost: $150 for the service call.
Understanding MERV Ratings
MERV stands for Minimum Efficiency Reporting Value. It's a scale from 1-20 that measures how effectively a filter captures particles. Higher numbers = finer filtration.
| MERV Rating | What It Catches | Best For |
|---|---|---|
| 1-4 | Large dust, pollen, carpet fibers | Basic protection (not recommended) |
| 5-8 | Dust mites, mold spores, pet dander | Most residential homes |
| 9-12 | Fine dust, legionella, humidifier dust | Homes with allergies/asthma |
| 13-16 | Bacteria, smoke, sneeze particles | Superior filtration (check system compatibility) |
| 17-20 | Viruses, carbon dust, microscopic particles | Hospitals, clean rooms (not for residential) |
What MERV Rating Should You Use?
For most homes, MERV 8-11 is the sweet spot. It provides good filtration without restricting airflow.
Important: Higher isn't always better. MERV 13+ filters are denser and can restrict airflow in systems not designed for them. This makes your system work harder and can cause problems. Check your system's specifications or ask us what's appropriate.
Filter Types
Fiberglass Filters (MERV 1-4)
The cheap blue or white filters you see at hardware stores. They're better than nothing, but just barely. They protect your equipment but don't do much for air quality.
Cost: $1-5 | Change: Monthly
Pleated Filters (MERV 5-13)
The most common choice for residential use. The pleats increase surface area, allowing better filtration without as much airflow restriction. This is what we recommend for most homes.
Cost: $5-20 | Change: Every 1-3 months
Media Filters (MERV 11-16)
Thicker filters (4-5 inches) that fit in a special cabinet. They last longer and provide excellent filtration. Great for homes with allergies or pets.
Cost: $30-80 | Change: Every 6-12 months
HEPA Filters (MERV 17-20)
Hospital-grade filtration. Most residential systems can't handle these—they're too restrictive. If you want HEPA-level filtration, you typically need a separate whole-home air cleaner.
Finding Your Filter Size
Filter sizes are listed as Length x Width x Depth (e.g., 16x25x1). The easiest way to find yours:
- Pull out your current filter
- Look at the size printed on the frame
- Write it down somewhere you'll remember (phone notes, tape it inside the furnace door)
Common residential sizes include 16x20x1, 16x25x1, 20x20x1, and 20x25x1. If you have a media filter cabinet, you'll likely need a 4" or 5" thick filter.
How Often to Change Your Filter
The real answer is: when it's dirty. But here are general guidelines:
- 1" fiberglass: Every 30 days
- 1" pleated: Every 60-90 days
- 2" pleated: Every 3-4 months
- 4" media: Every 6-12 months
Change More Frequently If:
- You have pets (pet hair clogs filters fast)
- Someone in your home has allergies or asthma
- You live in a dusty area or near construction
- You run your system constantly
- Wildfire smoke is affecting air quality
💡 The Visual Test
Hold your filter up to a light. If you can't see light through it, it's definitely time to change it. But don't wait until it fails this test—check monthly and change when it looks gray or dirty.
Common Filter Mistakes
Installing It Backwards
Filters have an airflow direction—look for the arrow on the frame. It should point toward the furnace/air handler (the direction air flows). Installing it backwards reduces effectiveness and can damage the filter.
Using the Wrong Size
A filter that doesn't fit properly lets air (and dust) bypass around the edges. Make sure you're using the exact size your system requires.
Going Too Long Between Changes
Set a reminder on your phone. A $10 filter changed on time is a lot cheaper than a $500 repair caused by a clogged one.
Buying the Cheapest Option
Fiberglass filters are cheap but offer minimal protection. Spend a few extra dollars on a MERV 8+ pleated filter—your lungs and your equipment will thank you.
The Bottom Line
Your filter is cheap insurance. Check it monthly, change it when it's dirty, and use a quality pleated filter in the MERV 8-11 range. This simple habit prevents most of the HVAC problems we see.
Not sure what filter is right for your system? Give us a call—we're happy to recommend the best option for your setup.