Microbore pipe leaks: finding a leak in thin heating pipes

What microbore pipe is, and where you find it

A microbore pipe leak is a leak on the narrow pipe, usually copper, that carries heating water to radiators in some homes. Because the pipe is thin and often runs under floors, you rarely see any water. What you notice is the boiler pressure falling. The leak itself can be traced from above with the heating warm, then checked with a second test, before anyone lifts a floor.

Microbore is usually 8 mm or 10 mm across, against 15 mm for standard heating pipe, and soft enough to bend by hand. To check, compare the pipes rising from the floor to each radiator with the pipes at the boiler. The thin pipes often start at a manifold, a small metal block where one larger pipe splits into several thin ones, usually under a floorboard or in a cupboard. Two thin pipes then run to each radiator, one in and one back.

Microbore was widely fitted when central heating spread through ordinary homes, so it turns up in some houses in the post-war new towns of Hertfordshire and Essex, on the estates built as Peterborough expanded and in older houses that were modernised. In bungalows and on ground floors it is often laid in the concrete floor. Upstairs it usually runs between the joists under the floorboards.

Why a microbore leak is hard to see

The pipe is thin, so a pinhole lets out only a little water. That water can soak into a screed floor (the smooth cement layer over the concrete) or along a joist without reaching the surface. With most modern boilers the heating is a sealed loop, so lost water is only replaced when you top up the boiler, and nothing at the taps changes to warn you. Older systems fed by a small tank in the loft refill themselves, so there may be no warning at all.

When it does leak, the cause is often simple. A nail from a carpet gripper or a screw through a floorboard can pierce it. Copper laid straight into a cement floor without a protective sleeve can corrode from the outside over the years. Joints at the manifold can weep. Each of these hides under a floor covering, which is why the boiler gauge is usually the first clue.

Signs of a leak on thin heating pipes

The clearest sign is the pressure gauge on the boiler. It rises a little when the heating is hot and settles as it cools, which is normal. If it keeps falling below where you set it, day after day, water is leaving the system. Write down how often you top up and how far it drops each time, because that rate tells us a lot about the size of the leak before anyone visits.

Rule out the visible places first: radiator valves, the joints under each radiator, and the copper pipe that runs from the boiler to the outside wall. That pipe drains the pressure relief valve, a safety valve that lets water out when the pressure gets too high. If it drips, or the valve lets water by, the pressure falls with no hidden leak at all, so a heating engineer usually checks the boiler first.

How a microbore pipe leak is found

It starts with a phone call. The first questions are about your top-ups, meaning how often and how far the needle falls, then where the pipework runs and whether anyone has put leak sealer in. Sealer matters because it can stop testing working, so mention it even if it went in a while ago. On the day, where valves allow it, parts of the system are pressure tested separately to narrow down which part is losing water.

With the heating warm, thermal imaging shows the thin pipes as warm lines under the floor, and a wider warm patch where leaking water has spread. To confirm the point, the heating water can be let out and tracer gas put into the pipe. It is a hydrogen and nitrogen mix. It slips out through the split and works its way up through the screed, where we pick it up at the floor. When both readings point to one place, we mark it on the floor and photograph it.

Repairs to discuss

Once the leak is marked, it is worth talking through the options before the floor is opened. The simplest is to open a small area at the mark and joint in a new piece of pipe. Where a concrete floor has corroded the pipe, it can make more sense to run a new length along a skirting or under the floorboards than to put copper back into the same concrete.

We can usually fix the pipe as well, and we price that work apart from the search. If we give you that price beforehand, it stands only when the repair goes ahead while we are there. Repairs are guaranteed for 3 months from the repair day, and the guarantee covers only the pipework and fittings we put in. Thin copper that has worn in one place may be weak elsewhere, and testing or repairing it can show other leaks, which are quoted separately.

Questions to ask before anyone lifts a floor

Lifting a tiled or wooden floor on a guess is slow and hard to put right, and a microbore run can be several metres long. Before anyone starts, ask questions that show the leak has really been found and the repair has been thought through, whoever is doing the work.

If the floor has insulation under it, or a membrane (a plastic sheet laid under the screed to keep damp out), or the pipes run inside a protective sleeve, ask how that affects the search. Those layers can limit what any survey can show, and it is better to know at the start than after hours of testing.