Microbore pipework: why older small-bore systems behave differently

Practical notes on microbore pipework limitations

• Microbore pipe is typically 6mm or 8mm rather than 15mm or 22mm
• Low water volume makes sludge and airlocks disproportionately disruptive
• A single blocked microbore run can isolate one radiator almost entirely
• Pump sizing on microbore systems needs care during any boiler upgrade
• Manifolds distributing microbore runs are usually hidden under floors

Microbore pipework limitations: the short technical version

The smaller bore means far less water volume in circulation, which makes the whole system more sensitive to sludge build-up, air locks and pump sizing than a modern equivalent. A single partially blocked microbore run can starve one radiator almost completely while barely affecting the rest of the system, which is why individual cold radiators are more common on this pipework than on 15mm systems.

Where we deal with this

• Chew Magna (BS40) · 8 mi
• Litton (BA3) · 6 mi
• Dundry (BS41) · 11 mi
• Broughton Gifford (BA14) · 11 mi

Questions we get asked

Is microbore pipework outdated and worth replacing?

It still works perfectly well when clean and correctly balanced; replacement is only usually justified during a wider renovation.

Does a new boiler need different settings for microbore pipe?

Pump speed and system bypass settings often need adjusting to suit the lower flow characteristics of microbore compared with standard pipe.