HVAC filters and MERV ratings

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:

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.

A typical case

A school upgraded its air handler filters to a higher MERV rating after a smoky summer. A year later, maintenance staff notice that cooling coils are dirty and some classrooms seem dustier than before, which is the opposite of what was intended.

An inspection of a case like this usually starts at the filter rack. Common findings are filters a little smaller than the rack opening, worn or missing gaskets, and a gap where the last filter in a row does not quite fit, so a share of the air goes around the filters. The pressure gauge across the bank may be disconnected or reading zero, so nobody knows when the filters are loaded. In some units, the higher-efficiency filters have been pushed so far past their change-out point that the media has started to tear.

The fixes are practical: correct the filter sizes, replace gaskets and add blank-off panels where needed, reconnect or replace the gauges, and set change-out based on pressure drop. Once air is going through the filters rather than around them, the higher rating starts to deliver what was expected.

What to record at each change

• The unit identifier and the date.
• Filter size, type and rating installed, and the number of filters.
• Pressure drop reading before and after the change, if a gauge is fitted.
• The condition of the old filters: evenly loaded, torn, wet or bypassed at the edges.
• The condition of the rack, gaskets and access door seals.
• Anything unusual, such as dirty coils downstream or water in the filter section.

Over a year, these records show whether the change interval suits each unit and catch fit problems early.

Choosing filters for your building

1. Find out what the equipment can handle from the manufacturer's data or a mechanical designer.
2. Decide what you are trying to achieve: equipment protection, general air quality, smoke protection or a specific requirement.
3. Choose the highest efficiency that meets that goal within the equipment's limits, preferring deeper media to keep resistance down.
4. Standardize filter sizes and types across similar equipment where possible.
5. 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