Cleanroom pressure alarm: how to find the cause

By ZY Cleanroom · Updated 2026-11-01 · Sources at the end of the page

First steps

Treat the alarm as real until the sensor is proved wrong. Annex 1 (para 4.16) says the warning signal should not be overridden without assessment, and a written procedure should say what to do.

FDA asks that all alarms are documented and deviations investigated. If the overpressure to an unclassified room falls below its minimum, FDA says the aseptic room's air quality should be restored and confirmed. Start from the limits.

Find the cause

Example: door leakage

Example only, from HTM 03-01 Appendix 4. A single door leaks 0.05 m³/s at 10 Pa, with gaps of 4 mm at the bottom and 3 mm at top and sides. A room with two such doors loses 0.10 m³/s, or 360 m³/h, before any other leak. Supply minus extract must cover that loss.

HTM notes that cold smoke seals cut door leakage. If the relief path is not sized for that, the room can whistle and air pressure can hold doors partly open. A seal change can shift the balance, so rebalance the air after it.

Stabilisers and containment rooms

HTM 03-01 (para 8.19) says pressure stabilisers set the room difference with the doors shut. They close when a door opens, which sends air through the doorway. Add them to the check list where they are fitted.

In a containment room the direction can be reversed. Annex 1 (para 4.14) allows modified pressures, including negative airlocks, for hazardous materials. Check that the alarm set point follows the intended direction.

What the design must provide

Annex 1 (para 4.16) asks for pressure indicators between rooms. Critical differences should be monitored and recorded continuously. Set points, criticality and any alarm delay belong in the contamination control strategy and must be justified.

HTM 03-01 (para 7.24) puts gauges beside the entry door, 1.5 m above the floor, with the normal range marked. FDA says alarms should catch an emerging low pressure before unclassified air enters.

After the fix

Annex 1 (para 4.32) includes the pressure difference between rooms in requalification. Record the cause and the action. If alarms repeat with no fault, review the design: supply capacity, door count and leakage paths.

Who approves the room

Your regulator or customer auditor judges the facility, not the contractor. The contractor builds to your room data sheet and supplies test data. To have a room list reviewed, send it to ZY Cleanroom, a cleanroom design-and-build contractor.

Related guides

• Cleanroom failed particle count: causes and fixes
• HEPA and ULPA filters: EN 1822, ISO 29463 and integrity testing
• Cleanroom pressure differentials: cascades, values, monitoring
• Class 100, 1,000, 10,000 and 100,000 Cleanrooms Explained
• Turnkey cleanroom contractor
• Room list builder

Sources

• EudraLex Volume 4, Annex 1: Manufacture of Sterile Medicinal Products (published 25 August 2022, applied from 25 August 2023; checked 31 October 2026)
• FDA, Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing, Current Good Manufacturing Practice (September 2004, nonbinding; checked 31 October 2026)
• NHS England, Health Technical Memorandum 03-01, Specialised ventilation for healthcare premises, Part A (published 22 June 2021; checked 31 October 2026)