Where should a medical gas zone valve box be installed?

The guidance below is from HTM 02-01 (2026 edition), published by NHS England, which sets out detailed public rules on where to place area valve service units (AVSUs); other countries set their own, and Portugal's ACSS asks that shut-off valve locations follow the risk analysis of ISO 14971.

Position in the building

• Departmental AVSUs should be external to the departments they serve, at the hospital street side of the entrance doors, to aid isolation of the department in an emergency.
• Large departments, such as an operating department whose clean corridor crosses fire compartment walls, may need additional AVSUs aligned with the fire strategy, under the advice of the healthcare organisation's Fire Safety Manager.
• If a department extends over more than one floor, a set of AVSUs should be provided on each floor, fed by separate mains from the riser or by a ring main.
• The mains that feed the AVSUs should not run inside the area the AVSU isolates, unless the distribution pipework has additional protection.
• Each different room or department type should be isolated by its own AVSU; special ventilated isolation rooms may have their own AVSUs fed from the departmental one.

Height, layout and access

HTM 02-01 asks for AVSUs between 1 m and 1.8 m above the floor so they can be operated without stooping or overreaching, with 1 m as the optimum; in critical care areas with dual circuits the bottom may drop to 800 mm to avoid too many columns. Valves in an array follow the order of the terminal units, for example oxygen, nitrous oxide, medical air, surgical air, vacuum and carbon dioxide. AVSUs should not sit within the swing of a door or behind partitions, should be kept clear of signage, and their position should be risk-assessed so that the glass panels are not easily damaged, if needed by recessing them further into the wall.

Alarms, sensors and signage

Each individual AVSU should have its local (area) alarm, and the pressure sensor for that alarm sits downstream of the AVSU and before any terminal unit, either inside the AVSU enclosure or above the alarm panel. The 2021 edition of NFPA 99 likewise placed area alarm sensors on the patient side of each zone valve box, with an option for the source side of the room valve box in anaesthetizing locations, and set the area alarm at a nurse's station or similar point of surveillance; NFPA now lists the 2027 edition as current. Specific signage and a schematic drawing at each AVSU should show the area and the terminal units it isolates. HTM 02-01 also says line valve assemblies should not be installed downstream (on the patient side) of an AVSU.