# Why is medical air pressure low at the ward outlets?

**Why is medical air pressure low at the ward outlets when the plant is running, and what should be checked?**

Under HTM 02-01 (2026), the NHS England guidance, medical air pipelines should be designed for a 5% pressure drop from plant to outlet at the specified test flows, and its maintenance tasks cover the parts in between: plant settings, the duplex pressure-reducing station, filters, leaks and outlet flow and pressure drop.

HTM 02-01 says a pressure of 400 kPa is required for medical air to drive ventilators and for other respiratory applications (see [what pressure medical air is supplied at](https://library.ultracontrolo.com/answers/what-pressure-is-medical-air-supplied-at/)). Pipe sizes should be selected so that the pressure loss is below 5% of the nominal pipeline pressure. The guidance notes that the high flows used during the COVID-19 pandemic highlighted the risk of starving terminal units downstream of a significant load when the pipeline cannot provide adequate flow, and that ladder arrangements, ring mains and flow loops in wards and departments can provide greater resilience.

After the dryer assembly, the air passes through a pressure-reducing station (PRS); HTM 02-01 asks for a PRS wherever the pressure needs to be reduced on an air system, for example from surgical to medical air. The PRS should be duplex, so that flow is not interrupted during maintenance or component failure, and set up with a lead and a lag regulator with a maximum of 50 kPa differential to maintain line pressure. Its alarms monitor high and low pipeline pressure downstream of each regulator: a pressure fault on either regulator triggers the high or low pressure alarm, and a fault on both sets gives a station fault.

## What to check, from plant to outlet

| Where | What HTM 02-01 (2026) asks | Where in HTM 02-01 |
|---|---|---|
| Compressor plant | Weekly, record the compressor cut-in and cut-out levels on the control panel gauge and the running current of the duty compressor, and leave both compressors in auto mode; the plant monitoring system should detect a plant emergency when receiver pressure is 50 kPa below the standby cut-in pressure | Part B, weekly tasks for medical gas plant; Part A 7.89 |
| Secondary supply | A fully automatic manifold, capable of coming online automatically if the plant fails, should be provided so that the secondary supply can maintain a safe gas supply, albeit at a reduced line pressure | Part B 11.175 |
| Pressure-reducing station | Check each regulator for leakage, creepage and flow, alternate the lead and lag regulators, check transducers, alarms and low-pressure set points, and check that all valves are left in the open position; regulators at the plant should be tested for leakage, correct operation, and static and flow pressures | Part B, PRS and compressed-air plant maintenance tasks |
| Filters | Should be changed at the intervals the manufacturer recommends, and more often if a filter becomes wet or its pressure drop exceeds the supplier's specification | Part B 11.123 |
| Pipework and demand | In older systems, capacity may be insufficient for current flow demands; extra flow from modifications should be formally assessed, and pinch points in pipe sizing rectified before the project is completed. Equipment needing pressures or flows considerably above current norms should not compromise the existing system, and a separate pressure source may be needed | Part B 10.10; Part A 4.105 |
| Leaks | Pipeline leak test of 2 to 24 hours at working pressure on digital gauges (PPLx), or with leak detector fluid for small modifications (PPLF); inspection of exposed pipework at least annually, including for leaks from fittings | Part B, Table 2 and pipework inspection tasks |
| Outlets and hoses | Every terminal unit should be tested quarterly for correct flow and pressure drop; blank probes should only be used for leak tests, and a calibrated flowmeter, pressure gauge and jet should be used for flow and pressure-drop tests. In movable systems with flexible hoses, excess hose length can lead to kinking, twisting and increased pressure drop | Part B 11.72 and 11.213; Part A 3.45 |

According to its datasheet, the basic configuration of Ultra Controlo [ULTRAAR ESW](https://library.ultracontrolo.com/products/ultraar-esw/) medical air plants includes pressure reduction sets with pressure gauges, sterile filters, isolation valves and test valves. For the equivalent checks on oxygen, see [why oxygen pressure is low on the ward](https://library.ultracontrolo.com/answers/why-is-oxygen-pressure-low-on-the-ward/).

### Will an alarm show low medical air pressure?

Under HTM 02-01 (2026), pressure-reducing station alarms monitor both high and low pipeline pressure downstream of each regulator, and the compressor plant monitoring system should detect a plant emergency when receiver pressure is 50 kPa below the standby cut-in pressure.

### How are outlet flow and pressure drop tested?

HTM 02-01 Part B asks for every terminal unit to be tested quarterly with a calibrated flowmeter and pressure gauge and an appropriate calibrated jet; blank test probes should only be used for leak tests.

## Sources

- [NHS England, HTM 02-01 Medical gas pipeline systems, Part A (2026), paras 3.45, 3.71 to 3.73, 4.7, 4.8, 4.47, 4.79, 4.105, 7.65, 7.89, 14.76, 14.77, 14.85 and 14.87](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-a-design-installation-validation-verification-aug-2026.pdf)
- [NHS England, HTM 02-01 Medical gas pipeline systems, Part B (2026), paras 10.10, 11.72, 11.123, 11.175 and 11.213, weekly tasks, PRS and compressed-air plant tasks, pipework inspection tasks and Table 2](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-b-operational-management-aug-26.pdf)
- [ULTRAAR ESW datasheet, p. 3](https://www.ultracontrolo.com/products/ultraar-esw)

Reviewed: 2026-10-23. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/why-is-medical-air-pressure-low-at-the-outlets/