A boiler usually leaks water because pressure is too high and the pressure relief valve is releasing it, or because of a worn pump seal, a cracked heat exchanger, corrosion, loose joints or a blocked condensate pipe. Switch the boiler off, isolate the water if you safely can, and call a Gas Safe registered engineer. Never try to fix a boiler leak yourself.
Finding water under your boiler is unsettling, especially when you can't see where it's coming from. The good news is that most boiler leaks trace back to one of a handful of causes, and many are straightforward for an engineer to put right. This guide explains what's most likely leaking and why, how to tell a genuine leak from harmless condensate, and the safe steps to take before help arrives.
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First things first: is it actually a leak?
Before assuming the worst, it's worth ruling out condensation. Modern condensing boilers produce a small amount of acidic water vapour as they run, which drains away through a plastic condensate pipe. A little moisture near that pipe can be normal, or a sign it's partly blocked. A genuine leak, by contrast, tends to be a steady drip or a small pool that keeps coming back, often paired with the boiler losing pressure over time.
A quick check: wipe the area dry, then watch it over an hour or two. If water returns from a specific point on the boiler or its pipework, rather than just around the condensate outlet, you're dealing with a leak that needs investigating.
6 common reasons a boiler leaks water
Boiler manufacturers and heating engineers consistently point to the same short list of culprits. Here are the six you're most likely to be facing.
1. Pressure too high, releasing through the relief valve
This is the most common reason a healthy-looking boiler suddenly drips water. Most boilers sit at around 1 to 1.5 bar when cold, and if the pressure climbs too high (typically towards 3 bar) the pressure relief valve (PRV) opens to let the excess out, venting water through an outlet pipe that often runs to the outside wall. The valve is a safety device doing its job, but the underlying high pressure still needs sorting. A frequent cause is a faulty expansion vessel: when the vessel can no longer absorb the expansion of heated water, pressure spikes and forces the valve open, which is why some boilers only leak once the heating has been running.
2. A worn pump seal
The circulation pump moves heated water around your radiators, and it's sealed to keep that water inside. Over years of heating and cooling, those seals can wear or perish, allowing water to seep out. It often shows up as a leak from underneath or around the pump body. Replacing a pump seal (or the pump) is routine work for a heating engineer.
3. A cracked or corroded heat exchanger
The heat exchanger is the component that transfers heat into your water, and it endures constant heating and pressure. If it cracks or corrodes, water can escape internally or from the body of the boiler. This is one of the more serious causes: because the heat exchanger is expensive, an engineer may advise whether a repair or a new boiler is the more sensible option, especially on an older unit.
4. Corrosion inside the system
Where iron components meet water and oxygen, rust forms. Over time corrosion eats away at pipes, tanks and the joints between them, and can perish the rubber seals around fittings. Older systems are the most prone to it. Once corrosion has spread widely, patching one spot may simply move the problem along to the next weak point, so an engineer will assess how far it has gone.
5. Loose joints and worn fittings
Every time your heating fires up and cools down, the metal pipework expands and contracts slightly. Years of that movement can loosen joints and connections, letting water weep from a fitting rather than from a major component. These are often among the simpler leaks to resolve, but they still need a competent engineer to reseal or replace the affected joint correctly.
6. A blocked or damaged condensate pipe
As mentioned above, condensing boilers drain acidic condensate through a plastic pipe. If that pipe becomes blocked, commonly by debris or by freezing in cold Cornwall and Devon snaps, water can back up and overflow from the boiler. A split in the pipe causes a similar pooling. It's usually a straightforward fix once identified.
What the leak's location tells you
Where the water appears is a useful clue, though only an engineer opening the casing can confirm the cause for certain.
| Where you see the water | Most likely cause(s) |
|---|---|
| From the external outlet pipe / outside wall | Pressure relief valve releasing high pressure (often a faulty expansion vessel) |
| Pooling underneath the boiler | Worn pump seal, internal corrosion, cracked heat exchanger, or condensate pipe |
| Around visible pipe connections | Loose joints or perished fittings from thermal movement |
| Only when the heating is running | Over-pressure venting through the relief valve as water heats and expands |
| Near the plastic condensate pipe | Possibly normal condensate, or a blocked / split condensate pipe |
If your boiler is also dropping in pressure, that's worth understanding alongside the leak. Our guide on a boiler that keeps losing pressure explains why pressure falls and what it points to, and if you simply need to bring the pressure back up safely, see how to increase boiler pressure.
What to do right now
You can make the situation safer without touching the boiler's internals. Work through these steps in order.
- Smell for gas first. If you smell gas, don't operate any switches. Leave the property and call the National Gas Emergency line on 0800 111 999.
- Turn the boiler off. Switch it off at its controls and, if you're confident which one it is, at the fused spur or mains switch to avoid water reaching electrics.
- Isolate the water. If you can safely reach the boiler's water isolation valve, or your home's stopcock, turn it off to limit the flow. Not sure where it is? Our guide on finding your water shut-off valve walks you through it.
- Contain and protect. Catch drips with a bowl and towels, and move anything water-sensitive clear of the area.
- Don't top up or tinker. Avoid repressurising or tightening fittings until the cause is known, because it can mask or worsen the underlying fault.
- Call a Gas Safe registered engineer. Have the leak diagnosed and repaired properly.
When to call a Gas Safe registered engineer
For any boiler leak, the answer is straightforward: a boiler is a sealed gas appliance, and by law only a Gas Safe registered engineer should open it up or carry out a repair. Manufacturers and heating specialists are unanimous that you should never attempt a boiler repair yourself, because the risks involve gas, electricity and pressurised water all in one unit.
Call without delay if the leak is heavy, if water is anywhere near electrical parts, if pressure keeps dropping despite topping up, or if you've lost heating and hot water in cold weather. Acting quickly almost always works out cheaper, because a small leak left alone can corrode components, cause damp, and turn into a far bigger job.
When the leak isn't the boiler at all
Sometimes the water near a boiler is the symptom, not the source. A leak elsewhere in the central heating circuit, whether under floorboards, behind a wall, or beneath a solid floor, can track back and pool near the unit, or quietly drain your pressure with no obvious wet patch in sight. These hidden heating leaks are exactly what we find without ripping rooms apart.
At DCI, we use thermal imaging, acoustic sensors and tracer gas to pinpoint central heating leak detection across the system, so the repair is targeted rather than guesswork. If your boiler keeps losing pressure but a Gas Safe engineer has cleared the boiler itself, a hidden leak in the pipework is the usual next suspect, and finding it is our job. Where the leak has caused damage you may be able to claim for, we produce an insurer-ready report, and it is worth knowing what trace and access cover includes before you contact your insurer.
Frequently asked questions
Is a leaking boiler an emergency?
Treat it seriously. Turn the boiler off, isolate the water if you safely can, and keep water away from any electrics. It becomes an urgent emergency if you smell gas, see water near electrical parts, or the leak is heavy. If you smell gas, leave the property and call the National Gas Emergency line on 0800 111 999, then book a Gas Safe registered engineer.
Can I fix a leaking boiler myself?
No. A boiler is a sealed gas appliance, and by law only a Gas Safe registered engineer should open it up or repair it. You can safely turn the boiler off and isolate the water, but topping up pressure or tightening fittings without knowing the cause can make things worse. Always have the leak diagnosed by a registered engineer.
Why is my boiler leaking water from the bottom?
Water pooling underneath a boiler usually points to a failed pump seal, internal corrosion, or a cracked heat exchanger. It can also be condensate from a blocked or split condensate pipe. Because several of these sit inside the casing, a Gas Safe engineer needs to open the boiler to confirm exactly which part is leaking.
Why does my boiler leak only when the heating is on?
A leak that appears only when the system heats up often means pressure is climbing too high and the pressure relief valve is releasing water. This is frequently caused by a faulty expansion vessel. As the water heats and expands with nowhere to go, the valve opens and water escapes through the outlet pipe. A Gas Safe engineer can check the vessel and valve.
Is the water under my boiler from a leak or condensation?
Modern condensing boilers produce acidic condensate that drains through a plastic pipe, so a small amount of water near that pipe can be normal or a sign it's blocked. A steady drip, rising damp around the unit, or pressure that keeps dropping points to a genuine leak that needs investigating rather than condensate.
Will home insurance cover a boiler leak in Cornwall or Devon?
Many home insurance policies include trace and access cover, which pays to find a hidden leak and repair the damage caused by reaching it, though usually not the boiler repair itself. If a heating leak has soaked into floors or walls, an insurer-ready report helps your claim. Always check your policy schedule and call your insurer before work begins.
Boiler cleared, but still losing water? Let us find the hidden leak
If a Gas Safe engineer has signed off the boiler but pressure keeps falling, the leak is likely hidden in your heating pipework. We trace central heating leaks across Cornwall & Devon with non-invasive equipment, so you get minimal damage and an insurer-ready report.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)