Who may close a zone valve?

In an emergency, HTM 02-01 (2026) expects users to gain access quickly without a key; other isolations are planned work controlled by the Authorised Person (MGPS) under a permit to work.

How is an AVSU tested?

HTM 02-01 records it on form B23: valve closure, zoning, NIST, identification and signage, schematic drawing and overall condition.

What is a medical gas zone valve box (AVSU)?

Q: What is a medical gas zone valve box or area valve service unit, and what is inside it?

A medical gas zone valve box, called an area valve service unit (AVSU) in NHS England's HTM 02-01, holds the isolating valve for one gas to one zone; HTM 02-01 (2026) describes a lockable, gas-specific enclosure that staff can open quickly without a key in an emergency, or for maintenance.

What is a medical gas zone valve box or area valve service unit, and what is inside it?

Each zone valve box serves one gas pipeline to one clinical area, so a ward or theatre suite has one box per gas, mounted together in an array. The 2021 edition text of NFPA 99 calls them zone valve box assemblies (NFPA now lists the 2027 edition as current); HTM 02-01 (2026 edition), published by NHS England, calls them AVSUs and describes them as line valve assemblies installed in an enclosure for user access in an emergency or for maintenance. The short glossary entry is area valve.

What is inside

A lever-ball isolating valve with flanged O-ring seal connections that opens and closes with a 90-degree turn; the handle is in line with the pipe when open.
NIST connectors with self-sealing check valves upstream and downstream of the valve, used for testing, emergency backfeeding and connecting hoses. Inside an AVSU they do not need to be locked, but they should not be reachable through the break-glass access.
A set of spades for physical isolation: a white through spade and a red blanking spade that blanks both the pipeline and the valve port and is visible when fitted.
Optional provision for pressure gauges, pressure switches or transducers on separate bosses with minimum bleed points; the area alarm sensor may sit here, downstream of the valve.
Gas identity and flow direction arrows inside the enclosure, fixed so that they cannot be reversed without tools or a permit to work.

The enclosure

HTM 02-01 asks for an enclosure with a lockable door that can be closed with the valve open or closed, and for emergency access without a key, for example through break-glass panels or plastic push or pull-out inserts; float glass should not be used, and the method of emergency access should be obvious, labelled and evident once used. The bottom of the enclosure should be ventilated so that leaking gas cannot accumulate, and pipe entries should be sealed so that gas escapes only through the vents or openings into the user space. Each AVSU is gas-specific and serves a single gas pipeline.

Labels and drawings

Every AVSU should carry its identification number, the number of rooms or terminal units it serves, the gas, the flow direction and the key cabinet number, and should be accompanied by a plan showing the terminal units it feeds. In critical care areas with dual circuits, terminal units are marked with the circuit of their AVSU, for example 1 and 2 or A and B. Portugal's ACSS specification ET 03/2006 similarly asks for quarter-turn ball valves without lubrication, to ISO 7396-1 and ISO 7396-2, each identified with the gas or symbol and the branch or area it controls.

Ultra Controlo supplies valves, second stage regulators, automatic changeovers, line regulators and pressure sensors among its accessories for medical gas pipeline systems (ULTRAOX SC datasheet, p. 4).