On this page
- What should you do if your boiler is leaking?
- Where is the water coming from?
- Why is water coming out of the pressure relief pipe?
- Can seals and joints cause a boiler leak?
- What if the pump or heat exchanger is leaking?
- Can you keep using a leaking boiler?
- Getting a leaking boiler repaired in Stoke-on-Trent
- Frequently asked questions
Quick answer
A boiler leaking water is usually caused by the pressure relief valve discharging, a failed seal or joint, a leaking pump, or corrosion in the heat exchanger. Turn the boiler off, catch the water, keep it away from electrics and never open the casing. Then call a Gas Safe registered engineer to find and fix the source.
Key takeaways
- Where the water appears is the best clue to the cause.
- Water from the outside copper pipe usually means system pressure is too high.
- Turn the boiler off and keep water away from electrics.
- Never open the boiler casing to find a leak.
A small puddle under the boiler, a damp patch on the cupboard floor, or drips from a copper pipe on the outside wall: boiler leaks start in different places and for different reasons. None of them are worth ignoring, because water and a gas appliance's electrics do not mix, and even slow leaks tend to get faster.
What should you do if your boiler is leaking?
Turn the boiler off at its controls and at the fused spur, put a bowl or towels under the drip, and keep water away from any sockets or wiring. Check the pressure gauge and note any fault code. Do not open the boiler casing. Then call a Gas Safe registered engineer.
- Switch the boiler off at the controls, then at the fused spur if it is dry and safe to reach.
- Catch the water with a bowl, tray or towels and move anything that could be damaged.
- Check the pressure gauge. A falling reading confirms water is leaving the system.
- If a pipe near the boiler is spraying, turn off the mains water at the stopcock.
- Take a photo of where the water is coming from. It helps with the diagnosis.
Water and electrics. If water is running into the boiler's wiring, a socket or the fused spur, do not touch them with wet hands or while standing in water. Never remove the boiler casing to trace a leak. If you also smell gas, call the National Gas Emergency Service on 0800 111 999.
Where is the water coming from?
Where the water shows up is the best clue to the cause. Water from inside the casing points to a seal, pump or heat exchanger. Drips from joints underneath point to fittings. Water from the small copper pipe through the outside wall points to the pressure relief valve.
| Where you see water | Likely cause | How urgent |
|---|---|---|
| Copper pipe on the outside wall | Pressure relief valve discharging | Book soon; check pressure |
| Joints on pipes under the boiler | Loose or failed fitting or valve | Book soon; catch drips |
| Dripping from the bottom of the casing | Internal seal, pump or condensate trap | Turn off; book a repair |
| Water with rust staining or corrosion | Heat exchanger or component corrosion | Turn off; book a repair |
| Plastic pipe dripping outside | Condensate pipe, normal while running | Usually not a leak |
That last row catches people out. The plastic condensate pipe is meant to drip while the boiler runs, and is covered in what gets checked on the condensate pipe.
Why is water coming out of the pressure relief pipe?
The pressure relief valve opens to dump water when system pressure gets too high, on most domestic boilers at around 3 bar. Water from the outside copper pipe usually means the system has been overfilled, the filling loop is letting water through, or the expansion vessel has lost its charge.
Check the gauge with the heating off and cold. If it is well above the level your manual gives, the cause needs finding. Letting some water out from a radiator bleed valve may bring the pressure down temporarily, but if the expansion vessel is the problem it will climb again when the heating warms up. Once a relief valve has discharged a few times, it may not reseal properly and can keep dripping even at normal pressure.
Can seals and joints cause a boiler leak?
Yes, and they are among the most common causes. Rubber seals inside the boiler harden and shrink with age and heat, and fittings on the pipework underneath can loosen as pipes expand and contract. A slow weep can go unnoticed for weeks, often showing first as a boiler that keeps losing pressure.
If your pressure drops without an obvious puddle, our guide to why your boiler keeps losing pressure explains where hidden leaks tend to be. Topping up repeatedly without finding the leak brings in fresh water and oxygen, which speeds up corrosion inside the system.
What if the pump or heat exchanger is leaking?
A leaking pump usually shows as drips from its body or seal, sometimes with a crusty white or rust-coloured trail beneath it. A leaking heat exchanger is more serious, and is usually the result of long-term corrosion from sludge or dirty system water. Both need a Gas Safe registered engineer.
Pump seals and pumps can be replaced on most boilers. A corroded heat exchanger is one of the more expensive parts, and on an older boiler the engineer may suggest comparing the repair against a replacement. Either way, ask what caused it. If the answer is sludge, cleaning the system and fitting a filter protects whatever comes next.
Can you keep using a leaking boiler?
It is best not to. Even a small leak inside the casing can reach electrical components, and the boiler may lock out on low pressure anyway. In early summer you may manage with the heating off, but a combi supplies your hot water too, so it is worth getting the repair done rather than waiting.
Only a Gas Safe registered engineer should open the boiler to trace and fix the leak.
Getting a leaking boiler repaired in Stoke-on-Trent
Our boiler repair service is £90 per hour for diagnosis and repair, and the engineer will find the source of the leak and explain the options before fitting parts. If water is pouring out and cannot be controlled, our emergency boiler repair page explains how call-outs work.
We work across Staffordshire and beyond, including boiler service in Cannock and boiler service in Lichfield. Call 07896 677083 to arrange a visit.
Frequently asked questions
Is a leaking boiler dangerous?
A water leak is not the same as a gas leak, but it can still be a hazard. Water reaching the boiler's electrics can cause a short circuit, and leaks can damage floors and ceilings. A leak caused by corrosion can also be a sign of wider wear. Turn the boiler off and have it checked promptly.
Will boiler leak sealer fix the problem?
Leak sealer products added to heating water are sometimes used for tiny weeps in the system, but they are not a fix for a failed seal, pump or relief valve, and the wrong product can cause blockages inside the boiler. Some manufacturers advise against them. Get the leak diagnosed before adding anything.
Why is my boiler leaking only when the heating is on?
Water expands as it heats, which raises system pressure and puts more strain on seals and joints. A weak seal may hold when the system is cold and weep once it warms up. If pressure climbs high when hot, it may also point to a problem with the expansion vessel, which an engineer can check.
Sources and further reading
Gasmaster Plumbing & Heating has served Stoke-on-Trent since 2005. Every article is written by our Gas Safe registered engineers (reg. 220932) from day to day work on boilers, heating and gas appliances across Staffordshire, Cheshire and Derbyshire.
