Air filter classes in HVAC equipment: ISO 16890, MERV and what to specify

A filter grade affects air quality, fan pressure and maintenance. Name the standard before comparing grades.

Filters are easy to overlook in an equipment quotation because they are cheap compared with the unit. Yet the filter grade decides how clean the supply air is, adds pressure the fan must overcome and sets how often someone must open the casing. Chinese datasheets often describe filters as "primary", "medium" or "high efficiency", which is not a test result.

The classification systems

ISO 16890 groups general ventilation filters by their efficiency against particle-size ranges: ISO ePM1, ePM2.5 and ePM10, plus ISO Coarse for filters that do not reach 50 percent on ePM10. A filter needs at least 50 percent efficiency in the relevant range to be classed as ePM1 or ePM2.5, and results are reported in 5 percent steps, for example ePM1 55%.

In North America, ASHRAE Standard 52.2 rates filters with a MERV number. NAFA publishes a rough correspondence between the two systems, but the tests differ, so treat any conversion as approximate.

High-efficiency HEPA and ULPA filters are tested to their own methods; ISO 29463 has replaced the older EN 1822 approach in many specifications.

What a filter grade changes

A finer filter has a higher initial pressure drop, and its resistance rises as it loads with dust. The fan must be selected for the design final pressure drop, not only the clean filter, or airflow will fall as the filter ages. Our duty-point guide explains how to state the pressure the fan must deliver.

Access matters as much as grade. Ask how filters are removed, whether a pressure gauge or alarm is fitted and what happens to airflow when the alarm trips.

Filters on this site

Air handling units are where filter choices are largest. The Dunham-Bush DMA, DAHU and CAHU air-handling units are configured section by section and list filter alarms among their controls, the Dunham-Bush DKFP cabinet units mention filter grades and filter-block alarms, and the Hanwei direct-expansion air handler needs its internal filters counted in the external pressure. Hanwei lists filtration among its air-handling sections.

Fan boxes and terminals often include a basic filter. The Tongtian TY-B initial-filter fan box is defined by its filter stage, and Tongtian makes filter fan boxes in several grades. The Yilaide eight-outlet cassette fan coils describe a washable synthetic filter, a common choice for fan coils.

Process and data-centre equipment adds service questions. HBO's intake-filter hot-air packages and its ultra-clean filter box both show why filter resistance belongs in the thermal duty, and HBO builds these around its own heaters. Envicool's EMA cabinet air conditioners and the AIRSYS DATACOOL package raise filter service and removal-route questions.

Questions to ask

Which standard and class do the filters meet, for example ISO 16890 ePM1 50% or MERV 13? What are the initial and recommended final pressure drops at design airflow, and was the fan selected for the final value? Is a filter frame leakage class stated? How are filters accessed and replaced, and are spare sets included? Is a differential pressure gauge or alarm supplied?

Common questions

Is a higher filter class always better? It removes more fine particles but costs fan energy and more frequent changes. Choose the class the application needs.

Can a "G4/F7" label be compared with ISO 16890? Those are classes from the older EN 779 standard, which ISO 16890 replaced. Ask the supplier for the ISO 16890 result.

By HVACDeck Editorial · Updated 2026-11-06