Question 1

What size plastic pipe replaces 15 mm copper?

Accepted Answer

Both 15 mm Speedfit and 15 mm Hep2O are direct replacements for 15 mm copper, using the same nominal size. The outside diameter of 15 mm plastic pipe is 15 mm, matching copper. However, the bore (internal diameter) is slightly smaller due to thicker walls, which can marginally reduce flow rates on very long runs.

Question 2

What is barrier pipe and when do I need it?

Accepted Answer

Barrier pipe has an aluminium or EVOH (ethylene vinyl alcohol) oxygen barrier layer to prevent oxygen permeating through the pipe wall into the heating water. This is essential for sealed central heating systems to prevent corrosion of steel radiators, boiler heat exchangers and other ferrous components. BS 7291 Class S requires a barrier pipe for heating systems. Non-barrier pipe is fine for cold water supply only.

Question 3

Can I use plastic pipe for hot water?

Accepted Answer

Yes. Polybutylene (Hep2O) and cross-linked polyethylene (PEX) are rated for continuous use at 82°C and up to 95°C for short periods. Standard MDPE/HDPE cold water pipe is NOT suitable for hot water. Always check the specific pipe rating — BS 7291 Class S (heating) or Class W (hot and cold water) indicates suitability.

Question 4

Do I need pipe inserts with plastic pipe?

Accepted Answer

Pipe inserts (tube liners/stiffeners) are required inside plastic pipe when used with push-fit fittings to maintain the pipe shape and prevent the grab ring cutting into the pipe. They are NOT required when connecting to compression fittings with a standard olive, or when connecting copper pipe to push-fit fittings. Always use the correct size insert for your pipe brand.

Nominal Size (mm)	OD (mm)	Copper Equivalent	Common Use
10	10	10 mm copper	Micro-bore radiator connections
15	15	15 mm copper	Taps, basins, individual radiators
22	22	22 mm copper	Hot/cold distribution, heating mains
28	28	28 mm copper	Mains supply, boiler connections

Material	Brand Examples	Max Temp	Barrier	Colour
Polybutylene (PB)	Hep2O, PolyPlumb	82°C (95°C peak)	Available	Grey, white
PEX (cross-linked PE)	Speedfit PEX, Rehau	82°C (95°C peak)	Standard	White, red, blue
PERT (PE raised temp)	Underfloor heating pipe	60°C continuous	Yes (EVOH)	White, orange
MDPE	Blue mains supply pipe	20°C (cold only)	No	Blue
HDPE	Below-ground drainage	Cold only	No	Black

Class	Application	Requirements
Class S	Central heating	Oxygen barrier required, 82°C at 3 bar
Class W	Hot and cold water	82°C at 3 bar, no barrier required
Class D	Cold water supply only	20°C at 12 bar

Nominal Size (mm)	OD (mm)	Colour	Use
20	20	Blue	Domestic mains supply (from stopcock)
25	25	Blue	Larger domestic, shared supplies
32	32	Blue	Commercial, multiple dwellings

UK Plastic Pipe Sizes — Push-Fit & PEX

Push-Fit Plastic Pipe Sizes

Pipe Material Types

BS 7291 Pipe Classes

Underground Cold Water Supply (MDPE)

Related Calculators

Frequently Asked Questions

Related Calculators

Related Calculators

UK Push-Fit Compatibility

UK Pipe Sizes

UK Copper Pipe Sizes

UK Cable Sizes

Was this calculator helpful?