Testing Guides

Cleanroom Test Failed? Common Causes and How to Fix Each One

By QV TEST engineering team · Published 27 Oct 2026 · Updated 27 Oct 2026 · 7 min read

Summary table

Confirm the result first

A failed result is sometimes caused by the measurement conditions rather than the system. Before fixing anything, check four things.

• Instruments: ISO 14644-3:2019 expects each instrument to have a valid calibration certificate, for example the pressure gauge (B.1.3) and temperature sensors (B.5.2).
• System state: before measuring room pressure differences, the air system should be running and stable, supply airflow within specification, doors and pass boxes closed, and extraction running as agreed (B.1.2).
• Measuring points: do not measure close to supply diffusers, return grilles or doors, where local air velocity distorts the reading (B.1.2).
• Occupancy state: check that the test was done in the as-built, at-rest or operational state that matches the acceptance criteria.

Airflow or air velocity failure

Airflow underpins almost every other result. If there is not enough air, room pressure and particle counts usually suffer too. The common causes fall into four groups.

1. Filters: clogged pre-filters or HEPA filters, or filters of the wrong specification, raise resistance above design. Check the pressure drop across the filters.
2. Fan and AHU: a slack or slipping belt, dirty blades, a motor running in reverse, or a VFD frequency set too low.
3. Ductwork: duct leakage, dampers stuck closed or set wrongly, or obstructions in the duct.
4. Controls and layout: a drifting pressure sensor that controls the fan, or new partitions added without rebalancing the air system.

Once the cause is fixed, rebalance every diffuser before re-testing to ISO 14644-3 clause B.2. If duct leakage is suspected, a duct leakage test can confirm it.

HEPA filter leaks

ISO 14644-3:2019 clause B.7.2.4 treats a reading above 0.01% of the upstream concentration as a leak, and uses 0.1% for filters with an MPPS efficiency from 99.95% up to less than 99.995%.

Repair: clause B.7.6 allows leaks in the filter media, sealant or grid to be repaired. The repair method must be agreed between the customer and the supplier and should follow the filter manufacturer’s instructions. Repair materials should be chosen with outgassing and deposition on products in mind. Once the repair has cured, the spot must be rescanned with the same method.

Leaks at the frame: check whether the gasket or gel seal is worn, broken or not fully seated, and tighten the clamps evenly on every side.

Replacement: if the media is damaged or leaks in several places, stop the air supply, clean the housing, install a new filter and leak-test it again after installation.

Room pressure difference failure

PIC/S GMP Annex 1 in PE 009-16 (clause 53) recommends 10–15 Pa between rooms of different grades; PE 009-17 (clause 4.14) recommends at least 10 Pa. Both are guidance values. In Thailand, the 2026 MOPH notification allows Annex 1 of PE 009-16 to be used until 31 December 2029.

Room pressure comes from the difference between supply air and return or exhaust air, so confirm that airflow passes first, then adjust the return or exhaust for each room. Check for leaks at door edges, penetrations and ceilings. Where the measured difference is below the agreed value, ISO 14644-3 clause B.1.2 recommends confirming the direction of flow between rooms with flow visualization.

Humidity failure

ISO 14644-3 clause B.6 does not set a fixed humidity value. It tests whether the system holds humidity within the limits and over the period agreed between the customer and the supplier. The ASHRAE Design Guide for Cleanrooms, section 9.2, gives an example of a specification written as a set point with a tolerance, such as 45±5% RH, or as a dew point.

Too humid: check that the cooling coil is cold enough to remove moisture, look for outside air leaking in and blocked or wet drain pans, and consider adding a dehumidifier.

Too dry: check that the humidifier works and whether too much dry outside air is being brought in.

Temperature out of specification

As with humidity, ISO 14644-3 clause B.5 tests against the agreed control limits, using sensors with the accuracy given in ISO 7726 and a valid calibration certificate.

Too warm: dirty coils, a refrigerant leak, a faulty expansion valve or compressor, or extra heat load from newly installed equipment.

Too cold: an unsuitable set point or deadband, a drifting sensor, a reheat that is not working, or a chilled-water valve stuck open.

Calibrate the control sensors first. If a sensor reads wrong, adjusting the cooling system will not solve the problem.

Particle counts too high

Particle counts reflect several systems at once, so check them in this order.

1. HEPA leaks, with an installed filter leak test
2. Clogged or worn pre-filters that overload the final filters
3. Room pressure lower than the next room, which lets air flow in from a dirtier area
4. Airflow or air change rate below design
5. Gowning, staff movement, and cleaning methods and materials
6. Leaks in walls, ceilings, penetrations and door edges

If the room passes at rest but fails in operation, look at people and activity first, since the air system and filters already meet the limits when the room is empty.

Re-testing after the fix

The failed tests must be repeated, and tests that the fix may affect should be repeated too. For example:

• HEPA filter replaced or repaired: leak test, airflow and particle count
• Air rebalanced or fan settings changed: airflow, room pressure difference and particle count
• Cooling or humidity system fixed: temperature and humidity over the agreed period

The NEBB Procedural Standards for Certified Testing of Cleanrooms (clause 9.5) list airflow velocity and volume, HEPA installation leak, particle count classification and room pressurization as the primary tests, performed in the order that suits the project. In practice, confirming airflow and leaks before counting particles reduces repeat tests.

FAQ

References

• ISO 14644-3:2019 Cleanrooms and associated controlled environments — Part 3: Test methods, clauses B.1, B.2, B.5, B.6, B.7.2.4 and B.7.6
• PIC/S GMP Guide PE 009-16, Annex 1 clause 53, and PE 009-17, Annex 1 clause 4.14
• Thai Ministry of Public Health Notification on Rules and Procedures for Drug Manufacturing, B.E. 2569 (2026)
• ASHRAE Design Guide for Cleanrooms: Fundamentals, Systems, and Performance, section 9.2 Humidity
• NEBB Procedural Standards for Certified Testing of Cleanrooms (CPT), Section 9, clause 9.5