A MERV rating tells you how well an HVAC filter captures particles of different sizes, on a scale where higher numbers capture more fine particles. The best filter for a building is not always the highest-rated one: it is the most efficient filter the equipment can handle, installed with no gaps and changed before it loads up.
What MERV means
MERV stands for Minimum Efficiency Reporting Value. It comes from ASHRAE Standard 52.2, which tests filters against particles in three size ranges, from about 0.3 to 10 micrometres. A filter's MERV reflects the lowest efficiency it achieves in those ranges during the test, as the filter loads with dust.
In general terms:
| MERV range | Typical use | Captures well |
|---|---|---|
| 1 to 4 | Basic protection for equipment, older residential furnaces | Large particles such as lint and coarse dust |
| 5 to 8 | Common commercial and residential filtration | Pollen, dust mites, larger mould spores |
| 9 to 12 | Better commercial filtration, many offices and schools | Finer dust and a meaningful share of smaller particles |
| 13 to 16 | Higher-performance buildings, healthcare support areas, wildfire smoke concerns | A large share of fine particles, including many that carry bacteria and smoke |
HEPA filters sit beyond the MERV scale and are used where very high efficiency is needed, usually in dedicated equipment rather than in standard air handlers. Other rating systems also exist, such as ISO 16890, used on many filters sold internationally, so check which rating a product is quoting.
The trade-off: efficiency and airflow
Higher-efficiency filters generally have more resistance to airflow, measured as pressure drop. Every fan is designed to move a certain amount of air against a certain resistance. If the filter adds too much, airflow falls. Results can include:
- Reduced heating and cooling capacity, and uncomfortable spaces.
- Frozen cooling coils or overheated heat exchangers on furnaces and rooftop units.
- Higher fan energy on systems that speed up to compensate.
- Filters bowing, tearing or being pulled out of their frames.
Deeper pleated or extended-surface filters reduce this problem by adding media area, which lowers resistance at the same efficiency. Before upgrading, check what the equipment manufacturer allows and whether the filter rack can take a deeper filter.
Installation details that matter
A high-rated filter does little if air goes around it. Bypass around filters is a common and easily missed problem.
- Filters are the correct size for the rack, with no gaps along the sides or between filters.
- Gaskets and seals in the rack are present and intact.
- Access doors close tightly.
- Airflow arrows on the filter point the right way.
- Filters are seated fully and not crushed or bent.
- Pre-filters and final filters are both in place in multi-stage systems.
When to change filters
Changing on a fixed calendar is simple, but it does not reflect how quickly filters actually load. A better approach is to measure pressure drop across the filter bank with a gauge, and change filters when they reach the change-out pressure recommended by the manufacturer. Many air handlers have gauges installed for this purpose, and building automation systems can alarm on high pressure drop.
Change filters more often during construction, wildfire smoke events, high pollen seasons and in dusty locations. Do not leave them so long that they collapse or let dust through.
Handle used filters carefully. Used filters can hold dust, mould and other contaminants. Turn the equipment off before changing them, wear suitable protection such as gloves and a respirator where needed, bag the old filters and do not shake them out indoors.
Choosing filters for your building
- Find out what the equipment can handle from the manufacturer's data or a mechanical designer.
- Decide what you are trying to achieve: equipment protection, general air quality, smoke protection or a specific requirement.
- Choose the highest efficiency that meets that goal within the equipment's limits, preferring deeper media to keep resistance down.
- Standardize filter sizes and types across similar equipment where possible.
- Record filter types, sizes and change dates for each unit.
Filtration works alongside ventilation and source control. Our guide to ventilation and indoor air quality explains how they fit together. For rooftop equipment, see rooftop unit maintenance.
How ehZee can help
Our air and water filtration system inspections check filter selection, fit, bypass, pressure drop and change practices across your equipment. We report what we find and recommend filters and routines that suit both your equipment and your air quality goals.
Common questions
Is MERV 13 always better than MERV 8?
It captures more fine particles, but only helps if the equipment can move enough air through it and if it is installed without bypass. In older equipment, it may cause more problems than it solves.
Do washable filters work?
Some do, but many have low efficiency. They must be cleaned thoroughly and dried completely before reinstalling, or they can grow mould. Disposable pleated filters are usually more practical in commercial buildings.
What should we do during wildfire smoke?
Use the highest-efficiency filters your equipment can handle, check them more often, and follow public health advice on ventilation settings during smoke events.