# Zone valve box vs line isolation valve

Under NHS England's HTM 02-01 (2026), a zone valve box (AVSU) is a line valve assembly in a lockable enclosure whose valves users can operate without a key in an emergency; a line valve assembly (LVA) isolates pipeline sections such as branches and risers and is lockable, with its own key.

HTM 02-01 says AVSUs are provided for user access in an emergency or for maintenance, and that they should follow the requirements for LVAs, except that security is achieved by installing them in an enclosure with a lockable door. The 2021 edition text of NFPA 99 calls the same devices zone valve box assemblies.

| HTM 02-01 (2026) | Zone valve box (AVSU) | Line valve assembly (LVA) |
|---|---|---|
| Purpose | User access in an emergency, or for maintenance; each AVSU is designed for a single gas pipeline and is gas-specific | Isolation at the connection to any source of supply, at the emergency inlet port, where the pipeline enters or leaves a building, at branches from the main run, at risers, and upstream, downstream and in the branch connection to a ring main |
| Access | Enclosure with a lockable door that closes with the valve open or closed; the user can reach the valves quickly and simply without a key, for example through break-glass panels or plastic push or pull-out inserts | Lockable, and capable of being locked with the valve open or closed; each lock has an independent key kept in the medical gas key safe and listed on the key register |
| Valve and isolation | Built to the LVA requirements | Lever-ball valve that opens and closes with a 90-degree turn, NIST connector bodies with self-sealing check valves upstream and downstream, and white through and red blanking spades |
| Position | AVSUs mounted at a height between 1 m and 1.8 m; departmental AVSUs outside the departments they serve, on the hospital street side of the entrance doors | Not downstream (patient side) of the AVSU; no LVA or other isolation device between the final AVSU and the terminal unit, except MSU isolation valves |
| Keys (Part B) | AVSU key sets held in one key cabinet | Line valve padlock keys held in another; in the sample procedure, LVAs in ducts, risers and ceiling spaces are locked in their normal operating position |

## Alarms and the patient side

Area alarm pressure sensors may sit inside the AVSU enclosure, on the downstream side, and HTM 02-01 asks for a local alarm for each individual AVSU. Downstream of an AVSU, line valves are only permitted immediately before the NIST connections of a medical supply unit containing flexible hoses, where multiple MSUs are fed from a single AVSU; these MSU isolation valves are not monitored by the location pressure alarm system. HTM 02-01 uses line valves (LVs) without NIST components for isolations where NISTs are not required, such as these MSU valves.

## Working on the pipeline

Part B of HTM 02-01 says an AVSU and an LVA are both designed to provide a physical break: the valve is closed, the section to be worked on is depressurised and a blanking spade is fitted. In its sample procedure, keys to AVSUs and other locked LVAs are used under the control of a permit to work. In Portugal, ACSS ET 03/2006 (v.2024) 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 lists an area valve and shut-off box for isolating medical gas zones, allowing maintenance shutdown of individual ward sections without disrupting supply to other areas, and a shut-off line isolation valve for main risers, plant rooms and distribution networks carrying oxygen, compressed air or vacuum. See [what is a zone valve box](https://library.ultracontrolo.com/answers/what-is-a-zone-valve-box/) and [what is a line valve](https://library.ultracontrolo.com/answers/what-is-a-line-valve/).

### Does a zone valve box need a key in an emergency?

HTM 02-01 (2026) says the user should be able to gain access to operate the AVSU isolating valves quickly and simply, without the need for a key.

### Are line valves allowed after the zone valve box?

HTM 02-01 (2026) says LVAs should not be installed on the patient side of the AVSU; line valves are only permitted there immediately before the NIST connections of a medical supply unit with flexible hoses, where multiple MSUs are fed from a single AVSU.

## Sources

- [NHS England, HTM 02-01 (2026) Part A, paragraphs 3.58, 3.63, 3.66, 3.83, 4.33, 4.36, 4.38 and 15.133 to 15.179](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 (2026) Part B, paragraphs 6.74 and 7.22 and Appendix B sample SOP](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)
- [NFPA 99, 2021 edition text of 5.1.9.4 (zone valve box assemblies), reproduced in NFPA committee input report HEA-PIP](https://docinfofiles.nfpa.org/files/AboutTheCodes/99/99_A2023_HEA_PIP_CIReport.pdf)
- [ACSS ET 03/2006 v.2024, sections 7.1 and 7.2](https://www.acss.min-saude.pt/wp-content/uploads/2016/10/ET-03_2006_v2024-Gases-medicinais_VF.pdf)
- [Ultra Controlo, Area Valve and Shut-Off Box for Medical Gas Zones](https://www.ultracontrolo.com/accessories/area-valve-and-shut-off-box)
- [Ultra Controlo, Shut-off Line Isolation Valve for Medical Gas Systems](https://www.ultracontrolo.com/accessories/shut-off-line-isolation-valve)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/compare/zone-valve-box-vs-line-valve/