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15mm or 22mm copper pipe: a branch versus the trunk

Updated: October 2026

You are at a boiler, a cylinder, or a radiator valve, and the offcut in the toolbox may be the wrong diameter. Choosing 15mm or 22mm copper pipe is a branch-versus-trunk decision: 15 mm to one tap or one radiator, 22 mm on the flow and return that feeds several of them. This page is not the full buying guide and not a brake-diameter page. The listings are on the [copper pipe comparison](/hub/rods-and-pipes/copper-pipe). Products Compared used UK trade notes and those amazon.co.uk listings. We did not measure flow in a lab. A qualifying purchase through our links can earn a commission at no extra cost to you.

You are at a boiler, a cylinder, or a radiator valve, and the offcut in the toolbox may be the wrong diameter.

Our top picks

PlumbUniverse PlumbUniverse 15 mm copper pipe, 1 m

Best overall

PlumbUniverse PlumbUniverse 15 mm copper pipe, 1 m
7.9/10about £9.49
View offer
sourcing map sourcing map 15 mm copper tube, 100 mm

Alternative

sourcing map sourcing map 15 mm copper tube, 100 mm
6.1/10about £5.39
View offer
BroLuka BroLuka 28 mm copper tube, 100 mm

Alternative

BroLuka BroLuka 28 mm copper tube, 100 mm
1.9/10about £10.49
View offer
On this page
  1. 1.15 mm is the branch
  2. 2.22 mm is the trunk
  3. 3.28 mm is the exception
  4. 4.Clip spacing follows the size

15 mm is the branch

In a mains-pressure or small pumped house, 15 mm outside diameter is the pipe to a single basin, a kitchen tap, or a short radiator tail. Table X, the light wall, is 15 × 0.7 mm in the half-hard R250 temper trade sticks use. This comparison has one listing in that family: PlumbUniverse 15 mm × 1 m at £9.49, rated 4.4 from 164 reviews. It does not state BS EN 1057 or a 0.7 mm wall. A merchant 15 mm × 3 m Wednesbury tube was £15.60 including VAT on the Screwfix page checked here. Use the 1 m for a short tail. Marketplace length traps are on the copper pipe buying guide. A 1 mm craft wall, and the sourcing map pieces at 100 mm and 200 mm, will not make a radiator tail. Imperial 1/2 inch is not 15 mm.

22 mm is the trunk

22 mm is the trunk: boiler flow and return, a pumped cylinder primary, and a run that splits into several 15 mm branches. BPEC describes a pumped primary as often 22 mm on the flow, the vent, and the return, with a 15 mm cold feed. Joule’s cylinder manual asks for at least 22 mm on the primary. Table X here is 22 × 0.9 mm. There is no 22 mm stick in this amazon set. TradeCalculator, citing an HVCA quick table at an 11 °C difference, puts about 6 kW on 15 mm and about 13 kW on 22 mm. BuildWiz’s page talks about 10 kW on 15 mm and about 22 kW on 22 mm. Treat those two pages as rules of thumb. Which short length is worth buying is on best copper pipe. A Screwfix category snippet showed 22 mm × 3 m around £28.68.
PlumbUniverse PlumbUniverse 15 mm copper pipe, 1 m

Testsieger

PlumbUniverse PlumbUniverse 15 mm copper pipe, 1 m
7.9/10about £9.49
View offer

28 mm is the exception

28 mm is the step up. BPEC puts gravity primaries at often 28 mm. Table X 28 mm is 28 × 0.9 mm. The only 28 mm item in this set is BroLuka’s offcut, 100 mm long, 1 mm wall, £10.49, with no reviews. Screwfix category snippets showed 28 mm × 3 m around £33.88.

Clip spacing follows the size

Lawton’s table shows 15 mm clips at 1.2 m horizontal and 1.8 m vertical, and 22 mm at 1.8 m horizontal and 2.4 m vertical. A 15 mm fitting will not start on 22 mm pipe.

Common questions

Do I use 15 mm or 22 mm for one radiator?
Use 15 mm for the branch to that radiator, which is the usual tail in a pumped system. Use 22 mm for the trunk and for the pipes on the boiler. Dropping the whole circuit to 15 mm because one valve is 15 mm starves the far ends. The 1 m listing can cover a short 15 mm tail. It cannot replace a 22 mm flow.
What size leaves the boiler?
Often 22 mm on a pumped system. BPEC describes pumped primaries as 22 mm flow, vent, and return. TradeCalculator notes that boiler tappings are often 22 mm. Read the boiler manual before you reduce the connection. A 15 mm reducer on a 22 mm tapping is the mistake this page is trying to stop, unless the manufacturer designed that boiler for 15 mm.
When is 28 mm the right copper?
When the circuit is gravity, or the load is large enough that 22 mm is no longer the trunk. BPEC often shows 28 mm on a gravity primary. It is not the normal size for a mains-pressure tap or a small pumped house. Our only 28 mm amazon listing is 100 mm long, so the pipe you want is a merchant length.
What is the wall on Table X 15 mm and 22 mm?
Table X is 15 × 0.7 mm and 22 × 0.9 mm, with 28 × 0.9 mm if you step up. Straight sticks are usually R250. A 1 mm craft wall at 15 mm outside diameter is thicker than Table X and is not the same tube, even if a 15 mm olive slides onto a round end. None of the amazon rows here states the Table X wall.
How far apart are the clips?
Lawton’s table gives 15 mm at 1.2 m horizontal and 1.8 m vertical, and 22 mm at 1.8 m horizontal and 2.4 m vertical. Some contract specifications use 1.2 m horizontal for both sizes. If the job has a specification, follow it. If it does not, the shorter spacing is the cautious choice on a long hot run.
Is microbore 8 mm or 10 mm the same as 15 mm?
No. Microbore is a smaller outside diameter with its own fittings. The 10 mm craft pack in this comparison is three 300 mm tubes for models and fabrication, not a microbore heating system. The 5 mm pack is thinner still. A 15 mm radiator valve needs 15 mm pipe, or an adapter you have identified, not a 10 mm tube in the wrong olive.

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Sources

Public UK sources and named merchant pages used with the amazon.co.uk listings; last checked on 16 October 2026.

  1. BSI BS EN 1057, seamless round copper tubes for water and gas (retrieved 16 October 2026)
  2. Water Supply (Water Fittings) Regulations 1999 (retrieved 16 October 2026)
  3. WaterRegsUK guidance on solders and fluxes (retrieved 16 October 2026)
  4. Gas Safety (Installation and Use) Regulations 1998 (retrieved 16 October 2026)
  5. HSE gas safety for employers (retrieved 16 October 2026)
  6. MOT inspection manual: brakes (retrieved 16 October 2026)
  7. Yorkshire Copper technical guide (retrieved 16 October 2026)
  8. Screwfix Wednesbury copper pipe 15 mm x 3 m (retrieved 16 October 2026)