# Outlet check valve (terminal unit check valve)

An outlet check valve, as HTM 02-01 (2026), the NHS England guidance, describes it, is the integral isolating valve or check valve of a terminal unit that operates automatically when the socket assembly is removed; with it, work on that terminal unit does not usually require interrupting the supply to adjacent units.

Part B also refers to an automatic isolating valve, or check plate, within the first-fix assembly, as in BS 5682 and BS EN ISO 9170-1 terminal units; with these, a live repair of a terminal unit needs no physical break in the pipeline. Part B adds that some earlier terminal units include a manual isolating valve, and that termination blocks should not be left unattended with the socket removed unless a blocking plate has been attached. ISO 9170-1:2017 is intended to ensure the gas-specific assembly, mechanical resistance, flow, leakage and pressure drop of terminal units.

The 2012 practice guide on fluides médicaux coordinated by OMEDIT PACA-Corse lists double-valve outlets (prises à double clapet) among the characteristics of wall outlets, allowing work on the outlet without cutting off the whole supply. HTM 02-01 also mentions check valves elsewhere: [NIST connectors](https://library.ultracontrolo.com/glossary/nist-connector/) incorporate a check valve for all pressure gases, and AGS terminal units should be checked to confirm that their check valve operates satisfactorily.

## Sources

- [NHS England, HTM 02-01 (2026) Part B, paragraphs 7.21 and 7.78](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)
- [NHS England, HTM 02-01 (2026) Part A, paragraphs 15.124 and 19.171](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)
- [ISO 9170-1:2017, Terminal units for medical gas pipeline systems, Part 1 (iso.org abstract)](https://www.iso.org/standard/67451.html)
- [OMEDIT PACA-Corse, Guide interrégional de pratiques pharmaceutiques en matière de fluides médicaux en établissement de santé (December 2012), wall outlets](https://www.omeditpacacorse.fr/wp-content/uploads/2018/05/Guide_Fluides_medicaux_OMIT-1.pdf)

Reviewed: 2026-10-23. Ultra Controlo engineering. https://library.ultracontrolo.com/glossary/outlet-check-valve/