A boiler leaking water, whether it is a drip from the underside or a small puddle on the worktop or floor below it, is one of the more common heating calls. It looks alarming because the boiler is a gas appliance full of hot water and electrics. In practice, the water almost always comes from one of a handful of places, and several of them sit outside the casing where you can look at them safely. The ones inside the casing are a different matter, and the law is clear about who may open it.
This guide covers the likely sources, how to narrow them down without touching anything you should not, and how a leak at the boiler differs from a hidden leak somewhere else in the heating system.
Why is my boiler leaking water from the bottom?
Water runs downhill, so anything leaking inside or directly under the boiler tends to appear at the lowest point: the bottom of the casing, the pipes below it, or the surface underneath. The usual sources are:
- The pipe connections under the boiler. A combi usually has five pipes below it: heating flow and return, cold mains in, hot water out, and the gas supply. The water pipes join the boiler through valves and compression or union fittings, any of which can weep.
- The condensate trap and pipe. Condensing boilers produce slightly acidic water that drains through a trap inside the casing and then a plastic pipe to a drain. A cracked trap, a loose joint or a blockage can send that water out of the bottom of the boiler.
- The pressure relief valve. The valve on the heating side can weep at its connection, or discharge water down its pipe when the system pressure is too high.
- The pump. The circulating pump inside the casing has seals and a connection to the pipework that can leak as they age.
- The automatic air vent. A small valve that releases air from the system, usually on top of the pump. It can let water past when debris sits in it, and the water then runs down inside the casing.
- The heat exchanger. The part that transfers heat from the burner to the water. Corrosion or a crack here lets water escape inside the boiler.
- The filling loop. The connection used to top up the pressure, often a braided hose or a built-in key under the boiler, can drip at its fittings.
Condensation is worth ruling out too. A cold mains pipe running into a warm cupboard can form droplets that collect at the lowest bend and drip steadily.
Which boiler leaks can you safely check yourself?
By law, anyone carrying out work on gas appliances must be on the Gas Safe Register. That means the boiler casing stays on unless a Gas Safe registered engineer removes it. What you can do is observe what is happening outside the casing, which is often enough to tell the engineer where to start.
| Source | What you might see | Who deals with it |
|---|---|---|
| Water pipe joints and valves below the boiler | A wet fitting, green or white crusting, a drip hanging from one pipe | A plumber or heating engineer; the boiler side of the valves is for a Gas Safe engineer |
| Condensate pipe outside the casing | Water at a joint on the plastic pipe, often only while the boiler is firing | A plumber for the external pipe; the internal trap needs a Gas Safe engineer |
| Filling loop | Drips at the hose connections, pressure creeping up with the boiler cold | A heating engineer |
| Pressure relief valve | Wet copper discharge pipe, dripping outside, high gauge reading | A Gas Safe engineer |
| Pump, air vent or heat exchanger | Water coming from inside the casing with every pipe below it dry, rust marks or staining on the casing | A Gas Safe engineer |
What should you do first if your boiler is leaking?
- Switch the boiler off. Use the boiler controls, then the fused spur or switch that feeds it, provided the switch and the area around it are dry. If water has reached a socket or switch, turn the circuit off at the consumer unit instead, as long as the consumer unit is dry.
- Contain the water. A tray, bowl or towels under the drip, and move anything stored below the boiler.
- Stop the water if it is more than a drip. On a combi, closing the main stopcock stops mains water reaching the boiler and the hot water side. The heating circuit is sealed, so the stopcock will not stop a heating leak, but the leak will slow as the system pressure falls.
- Leave the gas valve and heating valves alone. Do not try to tighten fittings on the boiler itself or remove any panel.
- Note the pressure. Write down the gauge reading and the time. It tells the engineer how fast water is leaving.
- Photograph it. Pictures of where the water is, the pipes below the boiler and the discharge pipe outside are useful to the engineer and to an insurer.
If you smell gas at any point, the situation is different. Do not operate electrical switches, open doors and windows, turn off the gas at the meter if you can do so safely, leave the property and call the National Gas Emergency Service on 0800 111 999.
How can you tell which pipe under the boiler is leaking?
Water that appears under the boiler often runs along a pipe before it drips, so the wettest point is not always the leak. A simple towel test separates them.
- With the boiler switched off, dry every pipe and fitting you can reach below the casing with a cloth.
- Wrap a strip of dry kitchen towel loosely around each pipe, just below the casing, and another around each valve.
- Leave them for a while and check which strip darkens first.
- If a strip on one pipe gets wet but the others stay dry, the leak is on that pipe or above it. If every strip stays dry but water still gathers under the boiler, the water is coming out of the casing itself, which points to an internal part.
Some leaks only show when the boiler runs. If the strips stay dry with the boiler off, an engineer can repeat the test with the heating or hot water running and the casing on, watching the condensate pipe in particular.
Is it the condensate trap or pipe?
A condensate leak has a recognisable pattern. It appears only while the boiler is firing, it tends to be worse in cold weather when the boiler condenses more, and the water may smell faintly acidic. Look for:
- a loose or split joint on the plastic pipe as it leaves the boiler;
- a pipe that has pulled out of its connector where it runs through the wall or into a waste pipe;
- a blockage further down, including ice on an external section in winter, which makes the water back up and escape at the lowest weak point.
A frozen or blocked condensate pipe usually makes the boiler lock out rather than leak badly, but a backed-up trap can overflow inside the casing. The external pipe is ordinary plumbing. The trap inside the boiler is part of the appliance and is for a Gas Safe engineer.
Could it be the pressure relief valve?
The pressure relief valve is a safety device. If the heating system pressure gets too high, it opens and sends water down a copper discharge pipe, usually to the outside. A wet discharge pipe means the valve has been releasing water. Water at the valve connection itself, under the boiler, can mean the valve or its joint is weeping.
Check the gauge with the system cold and again when hot. A reading that climbs towards the valve's opening point when hot points to a system problem, often the expansion vessel, rather than a failed valve. Our guide to a dripping boiler pressure relief pipe goes through that in detail. Do not cap or block the discharge pipe.
What if the pump, air vent or heat exchanger is leaking?
If every pipe below the boiler is dry and water still comes out of the bottom of the casing, the leak is inside. The engineer will normally check, in order of how often they fail:
- The automatic air vent, which can pass water when debris from the system stops it seating.
- The pump and its connections, where seals harden and fittings loosen over time.
- Internal pipe connections and O-rings between components.
- The heat exchanger, where corrosion, often linked to sludge and a lack of inhibitor, can cause a leak that is hard to repair economically.
Water dripping onto electrical parts inside a boiler is one reason not to keep using it. A heat exchanger leak in particular tends to lead to a conversation about repair against replacement, and that is worth having with the engineer once the cause is confirmed.
How is a leak at the boiler different from a hidden system leak?
Both make the pressure fall, which is why they are easy to confuse. The difference is where the water goes.
- A leak at the boiler leaves visible evidence at the boiler: a wet casing, a drip, a wet discharge pipe or a damp worktop.
- A hidden system leak leaves the boiler dry. The pressure still drops, but the water is escaping from pipework under a floor, inside a wall or behind a radiator, where it soaks away or evaporates.
It is also possible to have both. A system that has been losing water and topped up repeatedly can end up overfilled, which pushes the relief valve open and adds a second fault. If the boiler has been repaired and the pressure still falls with the boiler dry, our guide to a boiler that keeps losing pressure explains how the rest of the system is searched, and our article on the signs of a central heating leak lists what to look for around the house.
When should you call a leak detection specialist?
A leak from the boiler or the pipes directly beneath it is a job for a heating engineer, and anything inside the casing is for a Gas Safe registered engineer. A leak detection specialist is useful when:
- the boiler has been checked or repaired and the pressure still drops;
- water is appearing below the boiler cupboard, in the ceiling or wall underneath, and the pipes you can see are dry;
- the boiler pipes run into a floor or wall and the leak could be on a hidden section;
- you need the source of the water found and documented for an insurer or landlord.
Our central heating leak detection uses pressure checks, thermal imaging and acoustic listening. A thermal imaging survey is £150 to £250, and a combined detection package is £250 to £500. Call 020 3912 0200 to talk through what you are seeing.
Frequently asked questions
Is it safe to use a boiler that is leaking from the bottom?
It is best to switch it off until the cause is known. Water inside the casing can reach electrical parts, and a leak that is dumping water can leave the system without enough pressure to run. If you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999.
Can I tighten a leaking fitting under my boiler?
A plumber can deal with water pipework below the valves. Anything on the boiler side, the gas pipe or inside the casing is for a Gas Safe registered engineer. Overtightening old compression fittings can also make a weep worse.
Why does my boiler only leak when the heating is on?
Leaks that appear only while the boiler runs often come from the condensate trap or pipe, from a joint that opens as it heats up, or from the relief valve discharging because the pressure rises too far when hot.
Why is my boiler leaking and losing pressure?
Every drop that leaves the heating circuit lowers the pressure, so a leaking valve, pump or heat exchanger causes both. If the boiler is dry and the pressure still falls, the leak is more likely elsewhere in the system.
Can a leaking heat exchanger be repaired?
It depends on the boiler and the damage. A Gas Safe engineer can confirm whether a repair is possible or whether replacing the part or the boiler makes more sense.
