A boiler leaking water from underneath is usually a failed seal, pump, pressure relief valve or heat exchanger inside the casing, or a weeping joint on the pipes directly below it. Switch the boiler off and call a Gas Safe registered engineer for anything inside the unit. If the boiler is dry but pressure keeps dropping, the leak is somewhere on the heating circuit instead.
You open the kitchen cupboard for the washing-up liquid and find the base of it damp, with a tide mark spreading out from under the boiler. Or the airing cupboard floor has gone soft. It is a horrible moment, partly because water and a gas appliance seem like a bad mix and partly because a new boiler is not a small purchase. The good news is that most drips from beneath a boiler have an ordinary explanation, and many of them are repairable parts rather than the end of the boiler. This guide walks through the likely sources, what is safe for you to check, and how to tell a boiler fault from a leak on the heating system that only happens to show up at the boiler.
First, make it safe
Water collecting inside a boiler can reach the electrics and the printed circuit board. Before investigating:
- Switch the boiler off at the fused spur or the switch on the front, not just the thermostat.
- Turn off the water to the boiler if there are isolation valves on the pipes beneath it and you can reach them. On a combi, closing the cold mains valve stops fresh water feeding the leak.
- Put a tray or towel down and photograph the puddle before you mop it. The size and colour of the water are useful clues later.
- If you smell gas, leave the room, do not operate switches, and ring the National Gas Emergency Service on 0800 111 999. A water leak on its own does not cause a gas leak, but it is not worth guessing.
What the water looks like tells you a lot
Have a proper look at what you mopped up.
| What you notice | Most likely source | What to do |
|---|---|---|
| Clear water, boiler runs normally, drip during or after heating cycles | Condensate trap or pipe | Check the plastic condensate pipe for a loose joint or a blockage; engineer if inside the casing |
| Clear water from a copper pipe that runs outside, pressure gauge high | Pressure relief valve lifting | Check the gauge; if above about 2.5 bar the expansion vessel or the valve needs an engineer |
| Dirty, dark or rusty water | Heat exchanger, pump or an internal seal on the heating side | Switch off; Gas Safe engineer; may mean a part or a boiler replacement |
| Water only at one pipe joint below the boiler | Weeping compression or soldered fitting | A plumber can remake the joint; no need to open the boiler |
| Boiler bone dry but pressure keeps falling | A leak elsewhere on the heating circuit | Book leak detection for the system |
The usual suspects inside the boiler
Condensate trap and pipe
A modern condensing boiler produces a mildly acidic liquid as it runs, collected in a trap and piped away to a drain. The trap can crack, its seal can perish and the pipe can come loose or block. In a Cornish winter the external condensate pipe can also freeze, which backs the liquid up until it finds another way out. The water is clear and slightly acidic, and it appears in time with the boiler firing. A blocked or frozen external pipe is something you can often clear yourself with warm (not boiling) water; anything inside the casing is an engineer’s job.
Pressure relief valve
The PRV is the boiler’s safety outlet. If pressure in the sealed system climbs too high, it opens and sends water down a copper pipe that usually ends outside the wall. A dripping PRV therefore points to over-pressure, most often a failed expansion vessel, or to a valve that has passed debris and no longer seats properly. Check the gauge: normal is about 1 to 1.5 bar when the system is cold. If it sits far higher, or swings wildly between cold and hot, the vessel is the likely cause. Repeated topping up to compensate makes the problem worse.
The pump
The circulating pump pushes water around the radiators. Its seals harden with age, and when they go the water runs down the inside of the casing and out of the bottom. A pump leak is often a steady weep rather than a gush, and it may come with a change in the noise the boiler makes.
Heat exchanger
This is the one nobody wants. The heat exchanger is the core of the boiler, and if it corrodes through, the water is usually dirty and the boiler may lock out or lose pressure fast. On an older boiler the cost of the part can approach the cost of a replacement, so an engineer will usually talk you through both options.
Internal seals and diverter valve
Rubber seals on the diverter valve, the flow sensor and various push-fit connections inside a combi all age. These leaks tend to be small, intermittent and dependent on whether hot water or heating is running. They are usually an inexpensive fix once found.
When the boiler is not the problem
Here is the case we see most often. The boiler has been dripping, an engineer has replaced a valve or a seal, everything looks fine, and a fortnight later the pressure has dropped to zero again with no water under the boiler at all. The boiler was only ever the messenger. The pressure loss was always coming from the heating circuit, and the earlier drip was the PRV doing its job while someone kept topping up.
The heating circuit in a typical Cornish house runs under floorboards, through solid screed and inside walls that may be granite, cob or block, and a pinhole on that circuit can lose a litre or two a day without a single visible sign. The pressure gauge is the only witness. If the boiler is dry and the gauge still falls, the next step is central heating leak detection rather than another boiler part. Our guide to why a boiler keeps losing pressure explains how to read the gauge over a week and what the different patterns mean.
A quick test: with the system cold, note the pressure, then check it again after twenty-four hours with the boiler off. A fall with the boiler not running is a leak, not a boiler fault.
Who to call, and in what order
- Anything inside the boiler casing: a Gas Safe registered engineer. Gas appliances must legally be worked on by one, and opening the case yourself can void the warranty and the manufacturer’s cover.
- A weeping joint on the pipes below the boiler: a plumber, or the same heating engineer while they are there.
- Boiler dry, pressure still dropping, no visible leak anywhere: a leak detection survey of the heating circuit. This uses thermal imaging to map the pipe runs and tracer gas to pinpoint the failure, so a single floorboard or tile comes up rather than the whole floor.
If the drip has already damaged a floor or a cupboard, most buildings policies include trace and access cover, which pays towards finding the leak and making good after getting to it, though not the repair of the failed part. Our article on what trace and access insurance covers sets out what to expect.
Small habits that stop this happening again
- Note the pressure gauge monthly. Write it on the boiler with a marker if you like. A slow downward trend is the earliest warning you will ever get.
- Lag the external condensate pipe before winter, and make sure it falls continuously to the drain.
- Get the boiler serviced annually. Seals, the expansion vessel charge and the PRV are all checked on a proper service.
- Do not keep topping up. If you are repressurising more than once or twice a year, something is wrong. Find it rather than feeding it.
- Fit a leak sensor on the floor beneath the boiler if it lives in a cupboard or a holiday let that stands empty for weeks.
We handle heating and water leak detection across Cornwall, and the boiler is one of the most common places a hidden leak first announces itself.
Frequently asked questions
Is it safe to use a boiler that is leaking water underneath?
It is best not to. Water inside the casing can reach the electrics and the circuit board, turning a cheap seal repair into an expensive one. Switch the boiler off at the spur, isolate the water if you can, and arrange a Gas Safe registered engineer before running it again.
Why is my boiler leaking but the pressure is high?
High pressure usually means the expansion vessel has lost its air charge, so the system has nowhere to expand when it heats. The pressure relief valve then opens and discharges water, which is the leak you see. Recharging or replacing the vessel and checking the valve is an engineer’s job.
Can a boiler leak be repaired or does it need replacing?
Most can be repaired. Condensate traps, pumps, seals, expansion vessels and relief valves are all replaceable parts. A corroded heat exchanger is the exception, because on an older boiler the part can cost nearly as much as a new unit. Your engineer will price both.
My boiler is dry now but the pressure still drops. What is going on?
The leak is on the heating circuit, not the boiler. Radiator valves, pipes under floors and joints inside walls can lose water slowly with no visible sign. A leak detection survey using thermal imaging and tracer gas finds the point so only a small area needs opening.
Will home insurance pay for a leaking boiler?
The boiler part itself is normally treated as wear and tear and is not covered, though a boiler care plan might cover it. Damage the water caused to floors and cupboards, and the cost of finding a hidden leak on the system, are often covered under escape of water and trace and access. Check your own wording.
Boiler dry, pressure still falling?
If the engineer has done their part and the gauge still creeps down, the leak is on the system. We find it with thermal imaging and tracer gas, keep the disruption to one spot and give you a report your insurer will accept.
