UK Plastic Pipe Sizes — Push-Fit & PEX

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.

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.

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.

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.