Choosing an air filter: MERV ratings and your system
The most restrictive filter is not always the best choice. Here is how to think about filters for both air quality and equipment health.
By JBrown Mechanical
Changing the furnace filter is one of the few pieces of system care that most homeowners can do themselves. It is also one of the most misunderstood. Walk down a hardware store aisle and you will find filters promising to capture everything from dust to viruses. Choosing well means balancing what the filter catches against how freely air can move through it.
What a filter is for
The filter in a forced-air system has two jobs. It protects the equipment by keeping dust and debris off the blower and coils, and it captures some of the particles circulating in the home's air. The first job is essential. The second varies a lot depending on the filter.
Understanding MERV
MERV stands for Minimum Efficiency Reporting Value. It is a scale describing how well a filter captures particles of different sizes. Higher numbers capture smaller particles more effectively.
Very low-rated filters mainly catch larger dust and lint. Mid-range filters capture finer particles such as pollen and some mold spores. Higher-rated filters capture smaller particles still.
The trade-off: airflow
A filter that captures more particles generally offers more resistance to air flowing through it. Every system is designed to move a certain amount of air, and the blower has limits. If the filter is too restrictive for the system, or becomes clogged, airflow drops.
Reduced airflow can cause:
- Less heating or cooling reaching rooms, especially those farthest from the equipment
- The system running longer to reach the set temperature
- Added strain on the blower
- In cooling mode, problems at the indoor coil
This does not mean high-rated filters are bad. Many systems handle them well, especially those with larger filter cabinets designed for deeper media filters. It means the right filter depends on the system.
Thickness matters too
Filters come in different thicknesses. A thin one-inch filter and a deeper media filter can have similar MERV ratings but very different resistance, because the deeper filter has much more surface area. If your system has a filter cabinet for a thicker filter, use the type it was designed for.
How often to check
There is no single schedule that suits every home. Pets, construction dust, the number of people in the house, and how often the system runs all affect how quickly a filter loads up. A good habit is to check the filter monthly during heavy heating and cooling seasons and replace it when it looks dirty, or according to the filter maker's guidance.
When you replace it, note the size printed on the frame and make sure the airflow arrow points in the correct direction, toward the equipment.
Signs a filter is a problem
Weak airflow at registers, a system that seems to run longer than it used to, or a filter that collapses or bows are all signs worth noticing. Mention them if you have the system checked.
Beyond the filter
If air quality is a real concern, such as allergies, dust, or odors, the filter is only one piece. Humidity levels, ventilation, sources of pollutants in the home, and dedicated air quality equipment may all play a role. A conversation about what you are noticing can help determine whether a filter change is enough or whether something else is worth considering.
If you are not sure which filter suits your system, we can help you choose one that protects the equipment and fits your goals.