Can 15 mm pipe be used for a medical gas ring main?

Not under HTM 02-01, which asks for at least 22 mm on ring mains, distribution lines, supply lines and flow loops.

Why do high-flow ventilation areas need bigger pipes?

HTM 02-01 asks for the flow loop to go up one pipe size in critical care areas with high-flow ventilation, to meet the higher demand there.

What are the minimum pipe sizes for medical gas and vacuum lines?

Q: What minimum pipe diameters does HTM 02-01 set for medical gas supply lines, distribution pipework, ring mains, flow loops and vacuum lines?

Under HTM 02-01 (2026), the NHS England guidance, medical gas supply lines, distribution pipework, ring mains and flow loops should be at least 22 mm in diameter, as should vacuum pipework in multi-outlet trunking up to the last terminal unit branch, with a design calculation deciding when a larger size is needed.

What minimum pipe diameters does HTM 02-01 set for medical gas supply lines, distribution pipework, ring mains, flow loops and vacuum lines?

HTM 02-01 Part A (2026 edition), the NHS England guidance for medical gas pipeline systems, sets minimum diameters as a floor and leaves the actual size to a design calculation: there is a limited range of pipe diameters, and where a calculation comes close to the upper limit of one size, the next nominal diameter should be chosen. The calculation itself, based on diversified flows and a pressure loss below 5% of nominal pipeline pressure, is explained in how medical gas pipe is sized.

Minimum diameters in HTM 02-01

Part of the system	Minimum	Notes
Supply line, from the plant to the distribution system	22 mm	Sized for the full design flow of the system plus 25%
Distribution lines, up to the area valve service units	22 mm	Larger where the design calculation requires
Ring mains	22 mm	Larger where the design calculation requires
Flow loops in wards and departments	22 mm	One pipe size larger in critical care areas with high-flow ventilation
Vacuum in in-patient ward and department branches	22 mm in flow loops	HTM 02-01 notes that vacuum pressure losses are greatest near the terminal units
Vacuum in trunking with several terminal units	22 mm up to the last terminal unit branch	More than one feed to the unit may be needed to keep losses acceptable

High-flow areas and vacuum

Where an area is designed for CPAP, BiPAP or non-invasive ventilation, HTM 02-01 prefers enlarging the flow loop over dedicated pipelines, and the pipework down to each terminal unit should be calculated. For vacuum, the design target at the outlet is at least 315.5 mmHg at the front of each terminal unit at a design flow of 40 L/min (see hospital vacuum levels).

The short drops to individual outlets

HTM 02-01 is not consistent about the minimum size of the final drops that feed individual terminal units (compare paragraphs 4.5 and 4.110 with paragraph 15.11, on the carcass pipeline), so this page gives no figure for them. Their size should come from the design calculation, which HTM 02-01 asks the Authorised Person (MGPS) to review with the Authorising Engineer (MGPS) and the Medical Gas Safety Group to approve before work starts.

For reference, HTM 02-01 lists half-hard copper tube in 15, 22, 28, 35, 42 and 54 mm outside diameters for bendable runs, and the European tube standard EN 13348 covers seamless copper tubes for medical gases from 6 mm to 219 mm outside diameter (see copper pipe).