Air filters protect HVAC equipment from dirt and protect occupants from airborne particles. A filter that is too coarse lets dust coat coils and duct surfaces and does little for indoor air quality. A filter that is more efficient than the system can handle can choke airflow and raise fan energy. Choosing well means understanding how filters are rated, what particle sizes matter, and how pressure drop changes over a filter's life.
Interest in filtration has grown in Canada because of wildfire smoke events, which can push outdoor fine particle levels very high for days, and because of attention to airborne infection control.
How filters are rated
ASHRAE Standard 52.2 and MERV
ASHRAE Standard 52.2 tests filters against particles in three size ranges and assigns a Minimum Efficiency Reporting Value (MERV) from 1 to 16. The efficiency used is the minimum measured as the filter loads with test dust, so the rating reflects worst-case performance in the test.
| Range | Particle size | Examples of particles |
|---|---|---|
| E1 | 0.3 to 1.0 µm | Smoke, combustion particles, some bacteria, virus-carrying droplet nuclei |
| E2 | 1.0 to 3.0 µm | Fine dust, mould spores, some bacteria |
| E3 | 3.0 to 10 µm | Pollen, larger dust, lint |
Typical minimum efficiencies for common MERV levels:
| MERV | E1 (0.3 to 1 µm) | E2 (1 to 3 µm) | E3 (3 to 10 µm) | Typical use |
|---|---|---|---|---|
| 8 | Not rated | Low or not rated | At least 70 percent | Minimum for most commercial systems; prefilters |
| 11 | At least 20 percent | At least 65 percent | At least 85 percent | Better commercial buildings |
| 13 | At least 50 percent | At least 85 percent | At least 90 percent | Offices, schools, recommended for smoke and infection control where feasible |
| 14 | At least 75 percent | At least 90 percent | At least 95 percent | Healthcare final filters, laboratories |
| 16 | At least 95 percent | At least 95 percent | At least 95 percent | Critical applications |
ASHRAE 62.1 requires a minimum filter efficiency, MERV 8 in recent editions, upstream of cooling coils and other wet surfaces. Healthcare facilities follow CSA Z317.2 or ASHRAE 170, which specify higher efficiencies and multiple filter banks for many spaces.
ISO 16890
ISO 16890 is the international standard used widely in Europe and increasingly in Canada. It classifies filters by efficiency against particulate matter fractions: ePM1, ePM2.5 and ePM10, with a percentage, for example "ePM1 50%." It also includes a conditioning step that removes electrostatic charge, so ratings reflect performance after a charged filter loses its charge in service. MERV and ISO 16890 classes are not directly interchangeable, although manufacturers often publish both.
HEPA and ULPA filters
High-efficiency particulate air (HEPA) filters are a different category, tested individually rather than by MERV.
- In North America, HEPA usually means at least 99.97 percent efficiency at 0.3 µm, following IEST recommended practices.
- Under EN 1822 and ISO 29463, filters are classified at their most penetrating particle size, for example H13 (at least 99.95 percent) and H14 (at least 99.995 percent).
- ULPA filters are even more efficient and are used in some semiconductor and pharmaceutical cleanrooms.
HEPA filters are used in cleanrooms, isolation rooms, pharmacy compounding, some laboratories and portable air cleaners. They are rarely practical in general commercial air handlers, because of high pressure drop, size, and the need for leak-tight housings. See HEPA and ULPA filter systems for cleanroom applications.
Pressure drop
Every filter resists airflow. Pressure drop depends on filter efficiency, media type, media area, face velocity and dust loading.
Clean and final pressure drop
Filters are specified with an initial (clean) pressure drop at rated airflow, and a recommended final pressure drop at which they should be changed. Indicative ranges for commercial filters at typical face velocities are:
| Filter type | Typical initial pressure drop | Typical change-out point |
|---|---|---|
| MERV 8 pleated, 50 mm (2 in.) | About 50 to 90 Pa (0.2 to 0.35 in. w.c.) | About 175 to 250 Pa (0.7 to 1.0 in. w.c.) |
| MERV 13 to 14 deep pleat or rigid box | About 60 to 150 Pa (0.25 to 0.6 in. w.c.) | About 250 to 375 Pa (1.0 to 1.5 in. w.c.) |
| MERV 13 to 15 bag | About 60 to 125 Pa (0.25 to 0.5 in. w.c.) | About 250 to 375 Pa (1.0 to 1.5 in. w.c.) |
| HEPA | About 250 Pa (1.0 in. w.c.) or more | Often about 500 Pa (2.0 in. w.c.) |
Actual values vary by product; use manufacturer data at your design face velocity.
Face velocity
Most commercial filters are rated at a face velocity around 2.5 m/s (500 fpm). Lowering face velocity by using a larger filter bank, or deeper filters with more media, reduces pressure drop and often extends filter life. When upgrading efficiency, adding media area is often the most effective way to control pressure drop.
Energy impact
Fan power is roughly proportional to airflow times pressure. A filter bank adding 100 Pa (0.4 in. w.c.) of pressure drop to a system running at 500 Pa (2 in. w.c.) total increases fan power significantly if airflow is maintained. In constant-speed systems, airflow drops instead, which can reduce ventilation and cooling capacity. Our guide to reducing fan energy covers this relationship.
Upgrading filtration in existing systems
Before moving from MERV 8 to MERV 13, check:
- Fan capacity: can the fan deliver design airflow with the added pressure drop? Check fan curves and current operating point.
- Filter rack: does the rack accept deeper filters? Is there space for a larger bank?
- Bypass: gaps around filters and leaky racks let air bypass the media and defeat the upgrade. Gaskets and good frames matter.
- Coil and heating capacity: reduced airflow affects coil performance.
- Controls: variable-speed fans will speed up to compensate; constant-speed fans will lose airflow.
- Measurement: after the upgrade, measure airflow and pressure drop.
Monitoring and maintenance
- Install differential pressure gauges or sensors across each filter bank.
- Change filters based on pressure drop and condition, not just a calendar. See filter change strategies.
- Inspect for bypass, damaged media and wet filters.
- During wildfire smoke events, expect faster loading and plan for extra filter stock.
Frequently asked questions
Is MERV 13 always better?
It captures more fine particles, but only helps if the system can handle the pressure drop without losing airflow, and if filters are well sealed. In some small packaged units, a deeper MERV 11 or a larger MERV 13 bank may be the practical limit.
Do electrostatic filters keep their rating?
Some charged synthetic filters lose efficiency as their charge dissipates in service. ISO 16890's conditioning step and ASHRAE 52.2's optional conditioning appendix address this.
Can portable HEPA air cleaners help?
Yes, in individual rooms, especially during smoke events or for infection control. Size them on clean air delivery rate relative to room volume.
Key takeaways
- MERV ratings come from ASHRAE 52.2 and reflect minimum efficiency in three particle size ranges; ISO 16890 uses ePM classes.
- MERV 8 is a common minimum; MERV 13 or higher captures a much larger share of fine particles.
- HEPA filters are a separate class used in cleanrooms, isolation rooms and critical spaces.
- Pressure drop rises with efficiency and loading; extra media area keeps it manageable.
- Check fan capacity, seal filter racks and verify airflow after upgrades with air balancing.
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