HVAC Air Filter Buying Guide

Choosing the right furnace filter is crucial for protecting central air equipment and improving indoor air quality.  

However, one cannot simply pick any furnace filter off the shelf. The filter rating, size, and proper installation all contribute to ensuring proper performance. 

How to Pick an HVAC Filter 

The following sections outline the necessary steps for choosing and installing the appropriate HVAC filter for a home. 

Measure Your Furnace Filter 

First, the filter slot must be located in the furnace or air handler. Some homes also have filters located behind their return vents. 

Furnace filter size is measured using length, width, and depth. For example, a furnace filter that measures 18" x 16" x 1" is 18 inches long, 16 inches wide, and 1 inch deep. These measurements can be made with a simple tape measure. The easiest method, however, is to inspect the filter already in place and purchase the same size. 

Often, home air filters will have a nominal size, which is a rounded figure typically printed on the filter, and an actual size, which is the precise measurement obtained with a tape measure. It is important to check the actual size before buying and round up to the next whole number (sometimes by a 1/2 inch or more) to determine the nominal size. 

If the location is unclear, the furnace filter slot is typically located between the return air duct and the furnace assembly, sometimes behind a removable door. There may also be a filter slot at the return air vent in the home. 

Choose Your MERV Rating 

The Minimum Efficiency Reporting Value (MERV) measures how effective a furnace air filter is at capturing particles.

Furnace filter MERV ratings use a scale of 1–20. The lower the MERV number, the less effective the filter is at capturing particles (measured in microns) from the air. As the value increases, the filter captures smaller particles at a higher rate but also restricts airflow. 

For example, a MERV 4 filter can capture less than 20% of large ($3.0 - 10.0µm) airborne particles. Upgrading to a MERV 6 filter allows for the capture of up to 35% of large airborne particles. For comparison, the thickness of a human hair ranges from 50 to 100 microns. 

For even higher performance, a MERV 8 filter can capture up to 70% of large airborne particles and up to 20% of medium-sized ($1.0 - 3.0µm) airborne particles. 

Air Quality and MERV 

When choosing a MERV filter rating, the quality of indoor air and specific air quality needs must be evaluated. 

Indoor air quality is impacted by various factors: 

• Location: The surrounding environment makes a significant difference. Areas surrounded by factories and construction sites often have more polluted air than rural areas. The more polluted the air is outside a home, the more pollution will be present inside. 
• Household Materials: Heavy carpeting, adhesives, wood preservatives, air fresheners, and paints are all volatile organic compounds (VOCs) that can pollute a home. 
• Occupants: A smoker in the house can drastically pollute the air with cigarette smoke. Pets release significant amounts of microscopic dander, which is a common allergen. 

A heavily carpeted urban home with multiple pets will have poorer indoor air quality than one with no pets or carpeting. A higher-MERV filter may be needed to effectively protect the system and treat the air. 

If occupants suffer from allergies or asthma, the air filter likely needs to go beyond the minimum requirement of protecting the HVAC system. To capture tiny germs and allergens, a MERV 8 filter or higher is typically preferred. 

Before doing so, however, check with an HVAC professional to ensure the furnace and A/C unit can adjust to the reduced airflow resulting from a high-MERV filter. Otherwise, the system may work inefficiently and sustain permanent damage. 

If energy efficiency is a higher priority, a lower-MERV filter is suitable, as it prevents the largest particles from entering the system without reducing airflow. While the upfront cost is lower, these filters require more frequent replacement. 

Choose the Filter Type 

Once the necessary filter dimensions and desired MERV rating are known, the type of air filter can be selected. Choosing a MERV rating will automatically narrow down the available filter media options. 

Fiberglass Air Filters 

Fiberglass filters are the least expensive but have the lowest MERV ratings, typically 1–4. If indoor air quality is a critical concern, these are not the ideal solution. 

However, if the priority is HVAC energy efficiency, they work well because they place the least restriction on airflow. Fiberglass filters may allow dust to pass around them once they fill to capacity, so they must be changed frequently—approximately once a month. 

Pleated Air Filters 

Pleated filters are best for moderate to high MERV ratings and capture smaller particles and allergens. While the ability to improve indoor air quality is beneficial, it is important not to simply buy the highest-MERV pleated filter available without verifying system compatibility. 

If a system is not designed to compensate for the reduced airflow of a MERV 12 or 13 filter, the evaporator coil could freeze, or the heat exchanger could crack. In such cases, an additional indoor air quality system may be required. 

For those requiring the highest levels of filtration for allergies and other conditions, dedicated air purification equipment—such as electronic air cleaners or UV air purifiers—can remove the tiniest particles without straining the HVAC system. 

Install Your Filter 

Once the correct filter size, MERV, and type are chosen, installation can begin. First, the old filter is removed from the slot to make room for the new one. 

The new filter must be installed in the direction of airflow. All filters feature an arrow indicating the proper direction. Simply insert the filter into the slot with the arrow pointing toward the airflow (typically toward the fan), ensuring it is snug with no gaps in the filter rack. 

Replace Your Filter Regularly 

A filter cannot simply be installed and forgotten. Eventually, it will clog with dust and particulates, reducing airflow to the furnace and becoming ineffective. 

The frequency of furnace filter replacement depends on the filter type and the home's air quality. Homes with pets, heavy carpeting, or nearby construction will require more frequent replacements.  

In a home with average air quality, fiberglass filters should be replaced at least once per month, and pleated filters every 30–90 days. Filters should be inspected regularly; if the media appears dark with dust, it is time for a change. 

Shop with Confidence 

When purchasing a replacement filter at a big-box store, it is important to look for official MERV scores rather than arbitrary ratings. If a MERV score is not present, the filter has not been formally rated for efficiency.