Microbore heating pipes: problems, fixes and when to replace them

How to tell if you have microbore

Look at the pipes coming out of the floor or wall to a radiator. Standard radiator pipework is 15mm copper or plastic, about the width of a finger. Microbore is noticeably thinner, often about the width of a pencil, and usually comes up as soft, slightly bendy copper rather than rigid straight lengths.

Microbore systems are usually fed in one of two ways. Some have a manifold, a short fat pipe with several small pipes coming off it, hidden under a floor or in a cupboard, with each radiator run back to it. Others have 15mm or 22mm main pipes with microbore branches teed off to each radiator. Knowing which you have matters, because a blocked manifold can affect several radiators at once.

Why microbore gives more trouble than larger pipe

The advantage of microbore was speed and neatness. The soft coils could be threaded under floorboards and around joists with few joints, so installs were quicker and less disruptive. The downside is the small internal bore.

Sludge is the main problem. Steel radiators corrode slowly from the inside, and the black magnetite that forms has to go somewhere. In 15mm or 22mm pipe it tends to settle in the radiators. In microbore it can build up in the pipe itself until the flow to a radiator is restricted or stops.

Kinks and damage come next. Soft microbore bends easily, so a pipe squashed under a floorboard, trodden on in a cupboard or bent too tightly during an earlier job can restrict flow permanently. Nails and screws through floors find microbore more easily than people expect.

Finally, a small pipe can only carry so much heat. A large radiator on a long 8mm run may never get properly hot, and the higher water speed needed to push heat through narrow pipes can make the system noisy.

• One radiator cold or lukewarm while the rest of the house is warm
• Radiators cold at the bottom or cold all over after bleeding
• Several radiators affected at once, often those on the same manifold
• Rushing, whistling or ticking in the floors when the heating is on
• Damp patches in ceilings or floors below where microbore runs

Can microbore be power flushed?

Sometimes, with limits. Power flushing pushes water and cleaning chemical around the system at high flow, but a microbore pipe that is badly restricted may not let enough flow through for the flush to shift the blockage. Where there is still some circulation, a flush combined with a cleaning chemical often improves things. Where a pipe is completely blocked, flushing alone is unlikely to clear it.

An engineer will usually work radiator by radiator on a microbore system, isolating each one to force flow through its own pipes, and may flush individual runs separately. Some blockages sit in the manifold or at a tee rather than along the pipe, and those can be opened and cleaned. Our guide to radiator balancing or power flushing explains how to tell whether a flush is the right first step.

After cleaning, protect the system. A central heating inhibitor slows the corrosion that makes sludge, and a magnetic filter fitted on the return to the boiler catches what is left. Without both, a cleaned microbore system tends to block again.

Avoid adding leak sealers to a microbore system. Products designed to plug small leaks can also settle in narrow pipes and valves and make a partial blockage worse.

Keep, clean or replace: how to decide

Most people should start with the least disruptive option and only move to replacement when it has failed or when other work makes it cheap to do.

Microbore, lower flow temperatures and heat pumps

Running a heating system cooler is more efficient, but at a lower temperature each litre of water carries less heat to the radiator. To deliver the same warmth, the system has to move more water, and that is exactly what narrow pipes are bad at. A condensing boiler with a lower flow temperature can often cope on microbore, especially if the radiators are generous. A heat pump, which works at lower temperatures still, usually needs larger radiators and pipework, and its designer will look closely at any microbore runs. Our guide to lower boiler flow temperature explains how to try it safely.

What replacing microbore involves

Replacement means running new 15mm pipe, in copper or plastic, from the heating circuit to each radiator. The disruption depends on where the old pipes run. On suspended timber floors, boards are lifted along the route and relaid. On solid concrete floors, the old pipes may be buried in the screed, so new pipes are usually run along walls, in boxing or through ceilings from the floor above.

It does not have to be all or nothing. Replacing the runs to the radiators that cause problems, or taking the opportunity during a kitchen or bathroom renovation, spreads the cost and disruption. Our guides to copper or plastic pipework and radiator pipework in solid floors cover the choices that come up. Old radiators are often replaced at the same time if they are corroded; our article on how long radiators last explains how to tell.

When the new pipework is in, the whole system should be cleaned, refilled with inhibitor and balanced so that each radiator gets its share of the flow.

Get your microbore system assessed

Loyal Plumbers repairs and upgrades heating systems across London and everywhere inside the M25, including central heating repairs, cleaning with power flushing and full or partial pipework installation. We can find which runs are blocked, clear what can be cleared and replace only what needs replacing.

Call us or send an enquiry saying which radiators are affected, whether the floors are timber or concrete, how old the system is and whether it has been flushed before. Photos of the pipes at a cold radiator help.