A forced-air system is one of the most popular ways to heat or cool a house. Yet as they run, these systems can collect a lot of dust, allergens, and other airborne particles that circulate throughout your home. In this guide, we’ll explain how to choose the right furnace air filter and cover the various air filter options. In the video above, This Old House plumbing and heating expert Richard Trethewey demonstrates the importance of air filtration and reviews different filter types.
Types of Furnace Air Filters
There are several types of furnace air filters available, each with its own set of advantages and drawbacks. Here are the basics:
- Electrostatic filters: Use an electric charge to attract and trap particles
- Fiberglass filters: Basic, disposable filters that offer minimal filtration
- HEPA filters: High-efficiency filters that capture the smallest particles
- Pleated filters: Provide better filtration due to increased surface area
- Washable filters: Reusable filters that you can clean and reinstall
MERV Ratings Explained
The Minimum Efficiency Reporting Value (MERV) rating system is based on the filter’s efficiency in trapping particles. MERV ratings range from 1 to 20. The higher the number, the better the filtration. Below is a quick overview:
- MERV 1–4: Basic filtration for large particles
- MERV 5–8: Medium filtration for smaller particles
- MERV 9–12: High filtration for very fine particles
- MERV 13–16: Superior filtration for microscopic particles
- MERV 17–20: Hospital-grade filtration (not typically used in residential settings)
For most homes, a filter with a MERV rating between 8 and 13 provides enough filtration without restricting airflow.
HEPA Filters and Their Importance
High-efficiency particulate air (HEPA) filters are different from other filter types because they are very efficient in catching particles as small as 0.3 microns. They can also capture up to 99. 97% of particles—this is especially helpful for homeowners with severe allergies or asthma. However, due to their high filtration capabilities, you may need the help of a professional to install a HEPA filter so that it doesn’t disrupt the airflow in your HVAC system.
Choosing the Right Filter Size
It’s important to get the correct filter size because an ill-fitting one can allow unfiltered air to bypass the system, which reduces its effectiveness. We’ve summed up filter measuring and sizes below:
How To Measure Your Filter
Most residential furnace filters are 1 inch thick, but some systems may require thicker filters. To determine the right size filter for your furnace:
- Remove the old filter from your furnace.
- Look for size markings on the filter frame.
- If no markings are present, measure the length, width, and thickness of the filter slot.
- Round down to the nearest whole inch for each measurement.
Common Filter Sizes
While furnace filter sizes can vary, some common dimensions include:
- 12-by-24 inches
- 14-by-25 inches
- 16-by-20 inches
- 16-by-25 inches
- 20-by-25 inches
You can double-check your furnace’s specifications or ask a professional to confirm that you’re using the correct filter size.
Custom and Odd-Sized Filters
If you’ve found that the standard filter sizes don’t fit, you may need a custom or odd-sized filter. We recommend you consult an HVAC professional who can help you find a filter that fits while providing efficient filtration. Keep in mind that custom filters may be more pricey, but they’re worth the cost to keep your system working efficiently.
Evaluating Air Filter Effectiveness
Once you’ve determined the right size, you’ll want to find one that effectively traps particles. There are a few things to consider during your search.
Particle Capture Efficiency
When you start shopping for air filters, you’ll learn that they come in a variety of materials, so they don’t all collect the same size particles. We’ve included a breakdown of filter types and what size particles they can capture:
- Electrostatic filters: Attract and hold fine particles, such as smoke and bacteria
- Fiberglass filters: Capture large particles, such as dust and lint
- HEPA filters: Capture microscopic particles, including some viruses
- Pleated filters: Trap smaller particles, such as pollen and mold spores
Airflow Considerations
If you use a filter with a higher MERV rating to improve infiltration, it could end up restricting airflow, and your HVAC system may not run as efficiently. When you’re selecting your filter, remember to balance filtration needs with airflow requirements.
Energy Efficiency
Your HVAC system will need to work harder if you use a more efficient filter, leading to higher utility bills. Again, be mindful of balancing filtration efficiency with adequate airflow.
Health Benefits
The higher the MERV rating, the more allergens, pet dander, and bacteria and viruses it traps. Consider this when you pick out your air filter and pick one that can contribute to a healthier living environment.
Disposable vs. Reusable Furnace Filters
Another decision to make is whether you use disposable or reusable filters. Each has its own advantages and challenges.
Disposable Filters
Disposable filters are convenient and widely available. They include:
- Fiberglass filters: These are inexpensive but offer minimal filtration
- High-efficiency disposable filters: You’ll have to replace these more frequently but they offer superior filtration
- Pleated filters: Pleated filters provide better filtration at a moderate cost
Reusable Filters
Reusable filters can be a cost-effective and environmentally friendly option. Here are a few options:
- Electronic air cleaners: Offer high-efficiency filtration and you can run them through the dishwasher when it’s time to clean them
- Electrostatic filters: Provide good filtration, and you can clean them with water
- Washable fiberglass filters: Fiberglass filters have basic filtration but with a longer lifespan
Professional Filter Options
If you want an advanced filtration solution, you can have a professional install a whole-house air filtration system. These systems provide stellar filtration for your entire home and include:
- Electronic air cleaners: An electric air cleaner uses an electric charge to attract and trap particles.
- Media air cleaners: This option has a thick, pleated filter that captures a high percentage of airborne particles.
- UV light purifiers: A purifier uses ultraviolet light to neutralize bacteria and viruses.
Benefits of Professional Installation
Below are a few advantages to professional installation:
- Optimal placement for maximum effectiveness: Experts can find the right location in your HVAC system to install advanced filters.
- Proper sizing and integration with your existing HVAC system: A professional has a lot of experience fitting filters correctly and making sure they function properly.
- Regular maintenance and filter replacements: An HVAC professional can provide ongoing support to keep your system running smoothly.
Enhanced Air Quality Solutions
A whole-house system with a UV light purifier works well if you have significant air quality concerns, such as severe allergies or a respiratory illness. Consider a professional installation if you or a family member suffers from a medical condition.
Maintenance and Replacement of Furnace Filters
Regular maintenance will keep your system in good condition and continue to filter the air effectively. Here are a few tips:
When To Replace Your Filter
How often you will need to replace your filter depends on the following factors:
- Filter type and quality
- Home size and occupancy
- HVAC system usage
- Local air quality
- Presence of pets
Many HVAC professionals advise homeowners to check their filters monthly and replace them every one to three months or as recommended by the manufacturer.
Signs Your Filter Needs Replacement
During your monthly check, if you find any of the indicators below, it’s time to change your filter:
- Increased dust accumulation in your home
- Increased energy bills
- More frequent cycling of your HVAC system
- Reduced airflow from vents
- Visible dirt and debris on the filter surface
We recommend you perform these checks and change your filters often, because a clogged filter could lead to decreased air quality and increased strain on your HVAC system. These problems may result in higher energy bills and potential damage to the system components.
Do-It-Yourself (DIY) vs. Professional Maintenance
Changing an air filter is a basic DIY project, so most homeowners can perform this task. However, we recommend you hire a professional if you want a more comprehensive approach. They check to make sure all the HVAC system’s components are in optimal condition.