BoilerBuddy · symptom guide
Boiler leaking — the five things that cause it.
A puddle under the boiler is water leaving somewhere it shouldn't. On a combi or system boiler, the five most-likely paths are a pressure-relief-valve discharge pipe dripping outside, a worn seal or internal-corrosion leak dripping under the boiler, a loose pipe joint on the flow or return, a condensate pipe dripping or frozen, or the expansion vessel itself starting to weep. Each one is registered-engineer territory — UK Gas Safety (Installation and Use) Regulations reserve appliance work on a gas boiler for a Gas Safe registered engineer. The cards below rank them most-likely-first.
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Why this happens
Five causes, ranked by how often we see them on a UK boiler.
If the boiler is leaking, water is leaving a sealed system somewhere it shouldn't. On a combi or sealed system the five common culprits, most-likely-first, are the pressure-relief valve on the safety side (a small copper or plastic pipe outside the house dribbling because the expansion vessel has lost its air charge), worn seals or corrosion on internal components (a slow drip directly under the boiler that gets worse over weeks), a loose pipe joint on the flow or return (often after recent plumbing work), condensate pipe leakage (the white plastic pipe that exits to an outside drain or soak-away), and the expansion vessel itself starting to weep from its mounting bracket. Each has a different physical sign and a different urgency; the cards describe each, what it looks like, and the line for when to stop.
Pressure-relief valve discharging
The most common reason to find a puddle under or near the boiler is the pressure relief valve — a small safety device the system uses to dump water if pressure climbs too high. On a healthy combi it should never drip. When the expansion vessel (the football-sized silver tank that absorbs expansion water) loses its air charge, the system pressure rises on every heating cycle and the PRV starts dribbling out the discharge pipe to relieve it. If the vessel itself starts to weep from its mounting bracket instead, that is a separate cause covered further down the page. Walk outside and look at the small copper or plastic pipe that exits the wall near the boiler: if it is dripping even when the gauge inside reads under 1.5 bar, that is the symptom. Do not cap or block the pipe — it is there to protect you from over-pressure. Engineer territory; the fix is usually a new expansion vessel or a PRV head. Persistent pressure loss alongside a discharge leak points to a pressure-side story covered on the boiler losing pressure page.
Engineer territory · PRV / expansion vessel
Run this in the triage →Worn seals or internal corrosion (under-casing leak)
Worn seals and slow internal corrosion is the catch-all cause for any leak that appears from inside the boiler casing — staining around the casing seam at the bottom, water marks on the floor directly under the boiler, drips from beneath the case without any pipe joint near them. The most common sub-causes are a heat-exchanger body starting to corrode or micro-crack (telltale kettle-like noises on older boilers in hard-water areas), a pump shaft seal failing on the heating flow pipe (a slow drip that gets noticeably worse when the pump is running), and ferrule or o-ring fatigue on internal connections. Once any of these starts it rarely stops by itself — the leak gets worse over weeks. UK Gas Safety (Installation and Use) Regulations reserve any work on a sealed component inside a gas boiler for a registered engineer, which includes replacing a heat-exchanger body, a pump seal, or an internal o-ring.
Engineer territory · pump seal / heat exchanger / o-rings
Run this in the triage →Loose pipe joint at a fitting
A loose pipe joint shows up as a drip concentrated at a single compression fitting on the flow or return pipe — usually close to the boiler, often after recent plumbing work, a system refill, or a radiator replacement. The joint should be a dry olive-onto-copper seal; when it weeps, you can usually see a thin water track running down the pipe from one fitting and onto the floor. A knocking noise when the pump starts (a draught knock) is often the same joint moving under pressure. The fix is for the engineer to dismantle the joint, clean the olive, and refit it; a temporary turn of the cap from the front can stop a slow weep but rarely holds for long. UK Gas Safety (Installation and Use) Regulations reserve work on joints that tie into a sealed gas-boiler system for a registered engineer.
Engineer territory to refit
Run this in the triage →Condensate pipe leakage
A combi boiler produces a small amount of slightly acidic condensate (the water vapour from combustion, cooled back to liquid). It carries it out through a condensate pipe — usually white plastic, around 22 mm in diameter, exiting the wall to an outside drain or soak-away. The common leaks are at the joint where the pipe meets the boiler (a push-fit that has slipped) or from a split along the pipe itself, and the very common winter-only leak where the pipe has frozen solid and is now overflowing at the boiler-end every time the boiler fires. If you can see water dripping from the joint at the boiler end or the pipe is split or iced up outside, that is the symptom. Safe to thaw carefully with warm water from inside the house end if it is blocked with ice — do not apply direct heat — but the joint repair and any split pipe replacement are engineer territory.
Safe at home if frozen · engineer territory if the pipe is split
Run this in the triage →Leaking expansion vessel
On an older sealed-system boiler, the expansion vessel itself can start to leak — a slow drip from its mounting bracket onto the floor below, sometimes with the football-shaped silver tank visibly wet on the outside. A waterlogged vessel also tends to make the circulation pump noisier because the system no longer has the air cushion the vessel used to provide, and the pressure gauge will start climbing on every firing cycle (which is why a leaking vessel usually shows alongside the PRV discharge on the first card). Engineer territory entirely — the vessel is bolted to a flow or return pipe, pressurised on the water side, and any replacement involves depressurising the system and recharging the new vessel with air to the right bar before refilling. A leaking vessel is the most common single reason a sealed system slowly loses pressure on an older boiler.
Engineer territory · expansion vessel
Run this in the triage →Try this first
Before anything else, find out where the water is coming from.
The single most useful diagnostic move on a leaking boiler is to find out exactly where the water appears, and where it is coming from. Walk around the boiler with a torch — is the drip on the floor under the boiler itself, on a pipe union close to the casing, or only just visible on the small discharge pipe outside through the wall? That one observation narrows the cause from three possibilities to one and tells you which of the three cards above is most likely. You can also read the pressure gauge at the same time — if it has dropped below 1.0 bar, the system has lost water and book the engineer today instead of watching.
8 steps. Yes / no at each. Stops the moment a step says it should.
Know when to stop
Stop if you notice
- A smell of gas, or a yellow, orange or sooty flame.
- A carbon monoxide alarm is sounding, or someone has possible CO symptoms.
- Smoke, scorching, visible damage, overheating or a leak around the boiler.
- The boiler keeps locking out or will not stay lit.
Where the DIY line stops
Stop and call a Gas Safe engineer if any of these apply.
These are the lines that mean the leak has moved past the safe-at-home boundary. Any of them — leave the boiler in its current state, don't keep topping up the pressure to mask it, and book a registered engineer today, not next week:
- Water is visibly leaking from inside the boiler casing — a pump seal, a heat-exchanger body, or a fitting that was disturbed; sealed-component work, registered engineer territory entirely.
- The outside PRV discharge pipe is dripping and the boiler's pressure gauge keeps climbing back above 1.5 bar — the pressure side is no longer holding and the expansion vessel almost certainly needs attention; engineer territory.
Next step
Run the full triage in about a minute.
Tell BoilerBuddy what your boiler is doing. We'll return a ranked list of likely causes, the safe checks to do first, and the exact line that means 'call a Gas Safe registered engineer'.
FAQ
The five questions this symptom raises.
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