Pipes Under the Boiler Leaking: Which One Is It?

What each pipe under the boiler does

Look underneath a typical combi boiler and you will see a row of copper pipes dropping out of the base, plus one or two others heading off to the wall. The order varies by make and model, so do not rely on a diagram from a different boiler. The installation manual, or a sticker on the connection bracket, usually labels them.

On most combis you will find:

• Heating flow: hot water going out to the radiators. Part of the sealed system.
• Heating return: cooler water coming back from the radiators. Also part of the sealed system.
• Hot water outlet: goes to your taps and shower. It sits at mains pressure, so it can drip even when no tap is open.
• Cold mains inlet: incoming water from the street supply, always live.
• Gas supply: usually in the middle, often with a yellow-handled or lever gas cock. Never touch it to stop a water leak.
• Pressure relief discharge: a thin copper pipe that goes out through the wall.
• Condensate: the plastic pipe, not copper, carrying waste water to a drain.

If you smell gas anywhere near the boiler, open the windows, turn the gas off at the meter if it is safe to reach, avoid switches and flames, then leave and call the National Gas Emergency Service on 0800 111 999. Water dripping onto the gas pipe from above is not a gas leak, but do not try to tighten anything on the gas line.

How do I find which boiler pipe is leaking?

Start by drying every pipe and joint with kitchen roll, then lay a dry sheet of paper or a tray underneath. A fresh drip will show you where it lands. Then work through these in order, because each one rules a group of pipes in or out.

Check one is the pressure gauge. Note the reading and come back in an hour. If it has fallen, the leak is on the sealed heating side: the flow, the return, a valve on either, or water escaping through the pressure relief pipe. If it has not moved, the heating circuit is probably sound and you are looking at the mains side or the condensate pipe.

Check two is a hot tap. Run it for a couple of minutes. A drip that starts or gets faster is almost always on the hot outlet or the cold inlet, because a combi only moves mains water when a tap is open.

Check three is the heating. Turn it on and let the radiators warm up. Joints expand as they heat, and a weeping compression fitting on the flow or return often shows itself within ten or fifteen minutes of the heating running.

Check four is the boiler switched off at the spur. With the burner off, condensate stops forming and the heating joints cool and contract, so a drip that slows or stops points to the heating side or the condensate pipe. A drip that carries on unchanged, with the gauge steady, points to the cold inlet or hot outlet, which stay at mains pressure all the time.

Where on the pipe is the water coming from?

Knowing which pipe is only half the answer. Where along it the water forms makes a big difference to the fix.

• At a nut on a joint: a compression fitting has loosened or its olive has worn. Common, and usually a straightforward repair.
• At the spindle of a valve: each water pipe normally has an isolating valve with a slot or small lever. The seal around the spindle can weep, especially after the valve has been turned for the first time in years.
• From the pipe itself: pinhole corrosion in older copper, usually with a green or white crust. The section needs cutting out.
• Running down from the base of the casing: the water is coming from inside the boiler and following the pipe down. It might be the pump, a seal, the heat exchanger or the relief valve, and it is not a pipe fault at all.

What is safe to do tonight

Put a tray or towels under the drip and move anything electrical out of the way, as in the ten-minute routine for a leaking boiler. If water is reaching the boiler controls or the fused spur, which is when a leak turns into a safety risk, do not touch the spur: switch the boiler circuit off at the consumer unit, from a dry floor, and leave it off.

If the leak is on the cold mains inlet or the hot outlet and it is more than a drip, you can close the isolating valve on the cold inlet with a flat screwdriver, turning the slot so it sits across the pipe. That stops water reaching the boiler, which means no hot water until it is fixed. If the valve will not move easily, do not force it: close the main stopcock instead.

For a leak on the heating flow or return, closing valves does not help much, because the water is already in the sealed system. Turn the boiler off, catch the drip and keep an eye on the pressure. When it falls to around 0.5 bar or lower, most boilers lock out on their own.

Avoid tightening nuts yourself. It is tempting, but an overtightened compression fitting can crush the olive and turn a drip into a spray, and joints near the boiler are close to the gas connection and the electrics. Never open the casing to look for the source.

When to call someone out now

A slow drip into a tray, with the pressure holding, can wait for a daytime visit. Call out that night if the water is spraying rather than dripping, if it is coming through a ceiling below, if it is landing on electrics, or if the gauge keeps falling after you have topped it up. A sealed system that keeps losing water will also pull in fresh oxygenated water every time you refill it, which speeds up corrosion inside radiators and the boiler.

If the gauge is climbing rather than falling, and the relief pipe outside is dripping, that is a different fault with its own guide: pressure relief valve leaking.

What the repair involves

The engineer dries everything, repressurises the system and watches each joint under working pressure until the drip shows. A loose compression joint is usually remade with a new olive. A weeping isolating valve is replaced, which means draining down just enough of the system or isolating the mains. Corroded pipe is cut out and a new section soldered or fitted with compression joints.

If the water turns out to be coming from inside the casing, the job becomes a boiler repair rather than pipework, and anything inside the case is gas work. Gas work is carried out by Gas Safe registered engineers.

As a typical market guide, remaking a joint or swapping an isolating valve under a boiler commonly falls between £100 and £250 including labour, more if the system has to be drained. Leaks from inside the boiler cost more, and leaking boiler repair cost breaks those down. Our £60 call-out gets an engineer to you, and the repair price is agreed with you on site before any work starts. See leaking boiler repair for how the visit works.