Glossary · Faults, servicing and gas safety
Why is my boiler leaking water?
A boiler leaking water means water is escaping from the boiler or its connections. Common causes are high system pressure forcing the pressure relief valve to discharge, corroded parts, worn seals or joints, and a poor installation. Whatever the cause, finding and repairing the leak is gas work for a Gas Safe registered engineer.
Key points
- First check the water is coming from the boiler and not a nearby washing machine, sink or pipe.
- Ideal Heating says pressure should be 1 to 1.5 bar with the heating off. A reading of 2 bar or higher could be the cause.
- A leak from the pressure relief pipe can point to high pressure or a faulty expansion vessel.
- Contain the water, and if you cannot, turn off the mains water and the boiler's electrical supply, as Glow-worm advises.
- Do not take the boiler casing off. Repairs must be done by a Gas Safe registered engineer.
What causes a boiler to leak
Most modern central heating systems are sealed. Vaillant explains that if the pressure rises too high, the pressure relief valve automatically releases water to bring it down, and excess pressure can also make boiler parts fail and leak.[1]Vaillant, Why is my boiler leaking water? Ideal Heating says the relief valve is designed to leak if the pressure gets too high.[2]Ideal Heating, My boiler is leaking: what should I do?
The other causes the manufacturers list are below. Which one applies can only be settled by looking at the boiler.
| Cause | What the manufacturers say |
|---|---|
| High pressure | The pressure relief valve discharges. Ideal Heating says a faulty expansion vessel or a filling loop left open can be behind it. |
| Corrosion | Air in the system corrodes metal parts, which weaken and can crack. Glow-worm says brown water can point to sludge. |
| Worn seals and joints | Joints and seals loosen or wear with age. Glow-worm says damage to seals is usually a sign of long-term decay. |
| Pump seals | Glow-worm says weak seals on the boiler pump let water out. |
| Poor installation | Vaillant says a brand new boiler that leaks was probably fitted incorrectly, and the installer should return to put it right. |
| Heat exchanger | Vaillant says a cracked heat exchanger is less common but the most expensive part to deal with. |
What you can check yourself
Confirm the source first. Ideal Heating says to make sure the water is coming from the boiler and not from a washing machine, dishwasher, sink or nearby pipework, because calling out a boiler engineer for a plumbing leak does not solve it.[2]Ideal Heating, My boiler is leaking: what should I do?
Then look at the pressure gauge on the front of the boiler. Ideal Heating says the pressure should be between 1 and 1.5 bar with the central heating off, and a reading of 2 bar or higher could be the cause.[2]Ideal Heating, My boiler is leaking: what should I do? Glow-worm also treats a reading above 2 as high.[3]Glow-worm, Is your boiler leaking water? What to check Your boiler’s user guide gives the figures for your model.
Contain the water with a bucket or towels. Glow-worm says that if you cannot contain the leak, you should turn off the mains cold water stop tap and switch off the electrical supply to the boiler. If it is a small leak, isolating may not be needed.[3]Glow-worm, Is your boiler leaking water? What to check
When to call an engineer
Always, for a leak from the boiler itself. Vaillant says not to try to fix it yourself because handling boilers is dangerous, and not to remove the casing.[1]Vaillant, Why is my boiler leaking water? HSE states that anyone employed to work on gas appliances in domestic premises must be a Gas Safe registered engineer.[4]Health and Safety Executive, Domestic gas: frequently asked questions
Example
Harpreet in Leeds finds a small puddle under his combi. He wipes the pipework near the boiler and finds the wet patch is a joint on the pipe from the washing machine. A plumber fixes the joint. If the boiler gauge had read above 2 bar with the heating off, he would have called a Gas Safe engineer to find out why.
What people get wrong
Many people assume a leaking boiler is a boiler problem. Ideal Heating says the puddle may come from a washing machine, a sink or nearby pipework. The reverse mistake is also common: topping up the pressure and leaving it. If water keeps leaving the system, the cause is still there.
Ideal Heating says fixing a water pipe is usually a job any plumber can do.[2]Ideal Heating, My boiler is leaking: what should I do? Another mistake is opening the casing to look. Both Vaillant and Glow-worm say only a Gas Safe engineer should check inside the boiler.[1]Vaillant, Why is my boiler leaking water?[3]Glow-worm, Is your boiler leaking water? What to check
The bottom line
A boiler leak usually comes from high pressure, corrosion, worn seals or a poor installation. You can check where the water is coming from, read the gauge and contain the water. You cannot repair the boiler yourself. Call a Gas Safe registered engineer, and if the boiler is new, call the installer first.
Common questions
Should I turn my boiler off if it is leaking?
For a small leak you may not need to. Glow-worm says that if you cannot contain the leak, turn off the mains cold water stop tap and the electrical supply to the boiler, then call a Gas Safe engineer.
Why is water dripping from the pipe outside the boiler?
Ideal Heating says boilers have a pressure relief valve designed to leak if pressure gets too high. A discharge from that pipe often points to high pressure, and Ideal says a faulty expansion vessel can be linked to it.
Can I fix a boiler leak myself?
No. Vaillant, Ideal Heating and Glow-worm all say not to repair it yourself and not to remove the casing. Work on a gas appliance must be done by a Gas Safe registered engineer.
Is a leaking radiator a boiler problem?
Not necessarily. Ideal Heating says to check the water is coming from the boiler and not nearby pipework, because fixing a water pipe is usually a job any plumber can do.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Vaillant. Why is my boiler leaking water?. Checked 11 November 2026. ↩
- Ideal Heating. My boiler is leaking: what should I do?. Checked 11 November 2026. ↩
- Glow-worm. Is your boiler leaking water? What to check. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩