microbore
A kind of narrow tubing with an outside diameter of 10 millimetres or less.
microbore: narrow copper tube for tight domestic heating
Microbore tubing is copper pipe with an outside diameter of 10 mm or less, commonly 8 mm or 6 mm, used primarily in central heating systems and water supply installations. The small bore allows it to fit through walls, floors and joists with minimal disruption to the building fabric, making it far more practical than the 15 mm or 22 mm pipework that preceded it in domestic applications.
The two standard sizes dominate UK plumbing practice. 8 mm (outside diameter) microbore typically carries flow rates up to about 2 litres per minute and handles system pressures of 3 bar or more. 6 mm bore is used for branch runs to individual radiators or outlets where flow demand is lower. Wall thickness remains adequate, usually around 0.8 to 1 mm, preserving structural integrity despite the narrow internal diameter.
Friction and velocity matter
The chief constraint of microbore is friction loss. Water velocity through 8 mm microbore at typical domestic flow rates generates measurable pressure drop over distance, usually 100 to 200 pascals per metre depending on flow. This is why microbore systems require careful design and sometimes a faster pump to drive water through the network than a conventional system would need. Poorly designed layouts with excessive pipe runs or undersized diameters can leave distant radiators cold.
Microbore also demands cleaner system water than larger pipe. Corrosion debris and magnetite particles that a 22 mm pipe might pass cause blockages in 8 mm bore. Modern heating systems employ magnetic filters and inhibitor chemicals to manage this risk, but the requirement for system cleanliness remains a defining feature of microbore design.
Manufacturers typically supply microbore as straight coils of 25 m to 100 m. This flexibility allows the pipe to be run through cavities and under floors without excessive joinery and soldered joints, reducing leak points and labour time. Installation speed and concealment are the practical advantages that keep microbore standard in new build housing despite its design constraints.