Boiler Leaking Water: Causes and What to Do

First steps when you spot water

Start by keeping water away from anything electrical. If water is running near the boiler's wiring, a socket or the fused spur that powers the boiler, switch the boiler off at that spur if you can do so safely without touching anything wet. Put a bowl or towels down to catch drips and protect the floor.

If the leak is steady and you can reach the cold water feed, or the filling loop valves, make sure they are closed. Do not take the casing off the boiler. Everything behind it is work for a Gas Safe registered engineer, and the casing is part of what keeps the boiler safe. If you smell gas at any point, leave the boiler alone and call the National Gas Emergency line on 0800 111 999.

Where the water is coming from

Taking a careful look before you ring helps the engineer a great deal. Note where the drips land, whether they come from the boiler itself or the pipes beneath it, and whether the leak happens all the time or only when the heating is running.

• Pressure relief valve pipe: a copper pipe that runs outside. If water drips from it, the system pressure has been too high and the valve has let some out.
• Pump: the pump seals can wear, leaving drips or a damp patch inside or under the boiler.
• Heat exchanger: corrosion can cause leaks from inside the boiler, and this is the most serious of the common causes.
• Pipe joints and valves: fittings below the boiler can weep over time, especially after the system has been heating and cooling for years.
• Condensate pipe: the plastic pipe carrying acidic water to a drain can come loose, crack or block.

How a leak links to pressure loss

On a combi or system boiler, the heating circuit is sealed and holds water under a little pressure. Any water lost through a leak lowers that pressure, so a slow leak often first shows up as a gauge that needs topping up every few days. Eventually the boiler locks out to protect itself.

The reverse also happens. If pressure rises too far, the relief valve opens and dumps water outside. So a dripping outside pipe and a dropping gauge can be two sides of the same fault. Our boiler losing pressure guide explains how to read the gauge and top it up safely.

What a Gas Safe engineer checks

When Chris comes out to a leak, he traces the water back to its source rather than drying it off and guessing. That means checking the joints and valves under the boiler, the pump, the expansion vessel, the relief valve and, with the casing safely removed, the inside of the boiler.

He will then explain what has failed and what the repair involves before any work starts. Many leaks come down to a seal, a valve or a joint. You can read more about the faults we handle on our boiler repairs page, and an annual boiler service is a good way to catch weeping joints and tired seals early.

Repair or replace?

A leaking valve or pump seal is normally a straightforward repair, even on a boiler that has been around a while. A leaking heat exchanger is a bigger decision, because it is one of the most expensive parts and on an older boiler it can be the point where repair stops making sense.

Chris will give you an honest view based on the boiler's age, its history and whether parts are still easy to get. Our guide to how long a boiler lasts covers the other signs that a boiler is reaching the end of its working life.

When to treat it as urgent

A drip into a bowl can usually wait for a booked visit. Treat it as urgent if water is reaching electrics, if the leak is getting faster, or if you have lost heating and hot water altogether. Call 07429 632804 and Chris will tell you honestly whether and when he can attend; our emergency boiler repair page has more on what to do in the meantime. You can also send photos by WhatsApp or use the form on our contact page.