Boiler Leaking Water? Causes, Fixes & When to Call an Engineer

A boiler leaking water is never a good sign. 

Sometimes it can be a sign of (or cause) a boiler break down. 

But other times there may be a simple and cheap fix.

Common causes boiler leaks at a glance

If you notice your boiler is leaking, the issue is typically caused by one of a few common culprits.

Several (but not all) of these might also appear with strange noises from your boiler.

High system pressure

This causes the pressure relief valve to leak water to prevent the system from rupturing.

Corroded components

Inside older boilers, oxygen in the water can mix with metal to cause rust and corrosion.

Worn pump seals

The seals around your central heating pump can perish over time, causing water to drip from underneath the pump casing.

Loose pipe connections

If your boiler was recently installed, pipe joints may have been left slightly loose or settled.

Faulty heat exchanger

A cracked heat exchanger can leak water internally and is one of the most serious boiler faults.

Important note

Only a Gas Safe registered engineer should carry out internal repair work on your boiler. Never attempt to dismantle your boiler or repair gas components yourself, as this can endanger you and your property.

Diagnosing a leaking boiler

The location and timing of a leak can give you important clues about what is wrong with your heating system.

Boiler leaking water when the heating is on

If you only notice your boiler leaking when the heating is active, the cause is usually thermal expansion. 

This is when, as your central heating water heats up, it expands, causing the parts and water to swell.

This increased volume and pressure can force water past components. Once the heating turns off and the system cools, these parts contract and the leak may temporarily stop.

Boiler leaking from the top

A leak coming from the top of your boiler is usually caused by one of the following:

• A failing pressure relief valve
• A faulty pressure gauge
• A loose pipe joint at the top of the heat exchanger.

This is different from bottom leaks and requires an engineer to inspect the top casing components to locate the source of the water.

Boiler leaking from the bottom or underneath

Internal corrosion 

In older boilers, oxygen in the water mixes with the metal pipework over time. This chemical reaction creates metal oxides, which corrodes pipes and components. 

If you notice brown water leaking from the bottom of your boiler, this is a clear sign of severe internal corrosion. 

Brown water means rust has collected inside. This changes the urgency: You must call an engineer immediately.

Leaking pump seal

The central heating pump works hard to push water around your radiators, and its rubber seals naturally wear down and perish over time. 

If an engineer finds a leaking pump seal on an older pump, they will usually advise replacing the entire pump. 

This is because old pumps are highly likely to break down soon anyway, making a full replacement more cost-effective.

Leaking pressure valve

High pressure

Your boiler needs stable water pressure to run efficiently. Normal boiler pressure should be between 1 and 1.5 bar (around 14.5 to 22 PSI). 

If the pressure exceeds 2 bar, the system is over-pressurised. This can happen if the filling loop was accidentally left open after repressurising. 

When pressure gets too high, the safety relief valve will leak. It does this to release excess pressure and stop your boiler from rupturing.

Limescale and sludge

Limescale and sludge can restrict water flow and increase internal pressure. If this is the case, an engineer will need to remove the obstructions before resealing the valves.

You can often fix high pressure yourself by bleeding your radiators. To do this, first make sure the filling loop tap on your boiler is tightly closed. 

Then, locate a radiator key and a small bucket. Use the key to loosen the nut on the end of the radiator to let out trapped air and a little water, catching it in your bucket, before tightening the nut back up.

Boiler leaking water from the temperature valve

If the temperature valve is leaking water, it usually means the boiler’s internal temperature is too high. 

This is often caused by a faulty internal temperature probe that is no longer regulating the heat. A qualified gas engineer will need to replace the probe to resolve this.

Water dripping from boiler pipes

This is usually a sign of an installation fault. 

Small leaks can be hard to spot initially as pipes settle after installation. You should contact the engineer who installed it. 

They will usually return to tighten the fittings and resolve the issue.

Overflow pipe leaking

This means there is likely a problem with the float valve in your loft tank. 

The float valve controls the water level in the tank. If it fails, the water level will rise too high and escape through the overflow pipe. 

You will need a new float valve or washers to fix this, which is a straightforward job for a heating engineer.

Gas smells and carbon monoxide safety

Water is not the only thing that can leak from a boiler. Gas leaks are extremely dangerous and require immediate action.

If you smell gas when your boiler is running, or if there is a musty odour, you may have a gas leak or a flue issue. 

Switch off your gas supply at the meter, open all your doors and windows, and leave your home immediately.

Call the National Gas Emergency line on 0800 111 999 from outside and do not turn on any electrical switches or light naked flames.

Don’t confuse a gas leak with carbon monoxide 

Carbon monoxide is an odourless, tasteless, and colourless gas. It is highly poisonous and can lead to brain damage or death. 

Because you cannot smell it, you must ensure you have a working carbon monoxide alarm installed in any room with a gas appliance.

What to do when a boiler is leaking

You should turn off your boiler if it is leaking water. It prevents further water damage to your home and stops water from dripping onto internal electrical boards.

Most leaking boilers can be repaired if the problem is caught early

Timeframes and costs

The timeframe for repairing the issue naturally depends on what the issue is.

Minor problems, like loose pipe fittings can usually be repaired on the same day. They might typically costs between £150 and £400, depending on your boiler model and location. 

But replacing a complex part like a heat exchanger typically takes 1 to 3 days and cost £500 or more. . 

If the leak is caused by widespread corrosion, you will need a full boiler replacement, which can take several days to organise. A completely new boiler usually costs between £1,800 and £3,500 (to buy and install).

Prevent future leaks with regular servicing

The most effective way to prevent boiler leaks is to have your heating system serviced annually.

During a professional service, a Gas Safe-registered engineer will inspect your boiler and fix small issues before they turn into bigger ones. 

Regular servicing also ensures your boiler runs as efficiently as possible, keeping your monthly energy bills as low as possible.

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