Boiler Leaking Water? What It Means and What to Do Now

What Should I Do If My Boiler Is Leaking Water?

Switch the boiler off at the controls, put a container or towels under the leak, and keep water away from anything electrical. If the water is running near the boiler's electrical connection or fused spur, do not touch the switch with wet hands or while standing in water; turn off the power at the consumer unit instead. Then look at where the water is coming from before you call an engineer.

If the leak is heavy, turning off the main stop tap helps when the water is coming from the hot water side of a combi or from the filling loop, since both are fed from the mains. It will not stop water already in a sealed heating circuit from draining out of a leak, but that circuit holds a limited amount and the flow slows as the pressure falls. Most leaks from a boiler are a slow drip rather than a flood, which gives you time to do this calmly.

Take a photo of where the water is, note the pressure gauge reading and write down any fault code on the display. An engineer can often tell from those three things which part is leaking before opening the boiler.

Is a Leaking Boiler an Emergency?

A leaking boiler is urgent but not usually dangerous in itself, as long as water is kept away from electrics. It becomes an emergency if water is getting into electrical fittings, running through a ceiling below, or if you can smell gas at the same time. For a gas smell, leave the boiler off, open windows and call the National Gas Emergency Service on 0800 111 999.

A slow drip from a pipe outside is often less urgent than it looks, while a small leak from inside the boiler case needs looking at soon, because water inside a boiler can reach the controls and speed up corrosion. Our page on emergency boiler repair in London explains which faults need a same-visit fix and which can wait for a booked appointment.

Where the Water Is Coming From

The location of the leak narrows the cause down more than anything else. Dry the area, then watch where the first drip reappears.

• A copper pipe through the outside wall: this is usually the pressure relief valve discharge pipe. Water here means the valve is opening, either because it is worn or because the system pressure is rising too high.
• A plastic pipe running to a drain: this is the condensate pipe. A little water here is normal on a condensing boiler. Water pooling or the boiler locking out in cold weather points to a blocked or frozen pipe.
• The pipes and valves under the boiler: a loose or corroded joint, a washer on a service valve, or the filling loop connection.
• From inside the case, dripping from the bottom: an internal part such as the pump seal, a pressure relief valve, a seal on the heat exchanger or, less often, the heat exchanger itself.

The Most Common Causes

Most boiler leaks come from one of a small number of parts. The pressure relief valve is the most common, and our guide to why a pressure relief valve drips explains how to tell a worn valve from one reacting to high pressure. Pump seals wear with age and weep where the pump joins the pipework. Joints and seals under the boiler loosen as pipework expands and contracts every time the heating runs.

A failed expansion vessel causes leaks indirectly: it can no longer absorb the extra volume of hot water, so the pressure spikes and the relief valve dumps water outside. Corrosion is the cause to worry about most, because it is often a sign of sludge or poor water treatment in the whole system, and a corroded heat exchanger is the most expensive part in the boiler to replace.

Why a Leak Makes the Pressure Drop

A sealed heating system holds a fixed amount of water, so every drip that leaves it lowers the pressure on the gauge. Eventually the pressure falls below the level the boiler needs, and it locks out with a low pressure fault. That is often the first sign people notice, before they find the water.

Topping the pressure back up will get the boiler running, but it also pushes fresh, oxygenated water into the system, which feeds corrosion. If you are topping up every few weeks, the leak needs finding. Our guide on why a boiler keeps losing pressure covers the leaks you cannot see.

What Not to Do

Do not take the front casing off the boiler to find the leak. On a modern boiler it seals the combustion chamber, and any work behind it must by law be done by a Gas Safe registered engineer. Do not keep resetting the boiler or topping it up while water is visibly escaping. And be cautious about leak sealer products: they are designed for small, slow weeps in radiators and pipework, not for a failing valve or seal inside the boiler, and can make a later repair harder.

Repair or Replace?

Most boiler leaks are repairs, not a reason for a new boiler. A relief valve, a pump seal, a washer or a joint is a routine job on almost any boiler. The honest exception is a leaking main heat exchanger on an older boiler, where the cost of the part and labour can approach the cost of replacement, and other parts of similar age are likely to follow.

If you are told the boiler needs replacing because of a leak, ask exactly which part is leaking and why it cannot be replaced. A clear answer to that question is the difference between advice and a sales pitch.