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Published by Mo's Plumbing and Heating
Why Is My Boiler Leaking Water From Underneath?
Water under a boiler almost always comes from one of about six places, and where it is coming from tells you a great deal about how serious it is and what it will take to fix. The useful first move is not to mop it up. It is to work out which pipe or component it is running down.
The Short Answer
Turn the heating off, put a container under it and look at where the water is actually emerging rather than where it is pooling. A leak from the pressure relief pipe means the system is overpressurised. A leak from a pipe joint underneath means a fitting has worked loose. A leak from inside the casing means a pump seal, a valve, or the heat exchanger, and those are progressively more serious. Water plus any electrical noise, burning smell or tripping is a stop-and-isolate situation.
First: Isolate and Look
Three things, quickly, before diagnosing anything.
Turn the heating and hot water off at the programmer or the boiler controls. That stops the system cycling and reduces the pressure and temperature behind the leak, which usually slows it.
Put a bowl or a towel under it and note roughly how fast it is filling. That rate is useful information later: a slow drip and a steady run are different problems with different urgency, and it is hard to describe accurately from memory.
If water is anywhere near the electrical supply, the controls, or a socket, turn the boiler off at its isolating switch or at the consumer unit. Water and electricity together stop being a plumbing problem, and a boiler that is tripping the electrics, making a buzzing noise or smelling of burning should be isolated and left alone.
Then look properly with a torch. You are trying to find the highest point that is wet, because water runs down and along before it drips, so the puddle is rarely under the source.
What Each Leak Position Means
Six common sources, roughly in order of how easily they are dealt with.
- The pressure relief pipe. A small pipe leaving the boiler and going outside through the wall. Water here means the system reached about 3 bar and the safety valve released, which is the system telling you it is overpressurised rather than a leak in the usual sense. Check the gauge: high when hot and normal when cold points at the expansion vessel.
- A pipe joint under the boiler. The connections where gas, water, flow and return enter the appliance. These can weep as a compression joint loosens with years of expansion and contraction. Often the simplest of the six to put right.
- The condensate pipe or its trap. Modern boilers produce acidic condensate that drains through a small plastic pipe, and the trap inside can crack or the connection can come loose. It is a slow, persistent drip rather than a flow, and the water is faintly acidic so it marks surfaces over time.
- The pump seal. Water appearing from inside the casing around the pump. A pump seal is a known wear item and this is a component replacement rather than a boiler failure.
- An internal valve or automatic air vent. Similar picture: water inside the case, from an identifiable component, replaceable.
- The heat exchanger. The most serious of the six, because on many boilers it is the single most expensive component. Water from the heat exchanger area on an older appliance is frequently the point at which repair against replacement becomes a real conversation rather than an obvious repair.
How Serious Is It?
Honest answer: mostly not very, in the sense that a leaking boiler is rarely a safety emergency, and mostly quite serious in the sense that water damage compounds while you wait.
The appliance is not going to explode and it is not going to catch fire because it is wet, and most leaks are a component doing something predictable at the end of its life. What makes them urgent is what water does to floors, ceilings and, in a flat, to the property below.
Two things do change the picture and are worth taking seriously. Water reaching the electrical side of the appliance, which shows as tripping, buzzing or a burning smell and means isolate now. And any smell of gas at any point, which is not a boiler leak question at all: get everyone out and ring 0800 111 999, the National Gas Emergency Service, who hold the statutory duty to make a gas escape safe and attend free.
Beyond those, the sensible framing is that it wants dealing with promptly rather than immediately, and that turning the system off in the meantime is usually enough to reduce it to a drip.
The Pressure Relief Case, Because It Is Different
This one deserves separating because it is common, it looks alarming, and it is not a leak in the sense the others are.
The pressure relief valve is a safety device set to open at around 3 bar. When it opens, water leaves through a discharge pipe to the outside. That is the system working correctly, not failing.
The question is why the pressure got that high, and there are two usual answers. The filling loop has been left slightly open, so mains water is continuously feeding the system, which is easy to check and free to fix. Or the expansion vessel has lost its air charge, so the water expanding as it heats has nowhere to go and the pressure climbs every heating cycle.
The signature of the second is distinctive: normal pressure when cold, very high when hot, and back to normal once it cools, with water discharging outside each time. If you are also topping the system up regularly, that is the same fault seen from the other end, since the water you add is going out of the discharge pipe.
A relief valve that has lifted repeatedly often will not seal afterwards, because debris gets under the seat. So a system that reached 3 bar once may then weep permanently, and the valve needs replacing along with fixing whatever caused the pressure.
If You Have a Combi Specifically
The sources above apply to every type, and a combi adds two of its own worth knowing about.
The diverter valve, which switches the boiler between heating and hot water thousands of times a year. Its seals are a wear item and a weeping diverter shows as water inside the casing, often accompanied by the classic diverter symptoms of hot water but no heating, or heating firing whenever a tap runs.
And the plate heat exchanger, the second exchanger a combi uses for the taps. It can leak at its seals or, less often, internally, which produces a rising pressure rather than a visible leak because mains water crosses into the sealed heating circuit.
That second one is worth recognising because it presents backwards. If your pressure climbs on its own with the filling loop definitely closed and the heating off, particularly after someone has run a hot tap, that is a plate heat exchanger rather than anything you will see dripping.
What Not to Do
Three things, all of which make it worse.
Do not take the boiler casing off. It is a sealed appliance, the casing is part of how it works safely, and there is live electricity and gas inside it. Looking at the pipework underneath is fine; opening the case is not a homeowner job.
Do not keep topping the pressure up to compensate for water going out of the relief pipe. Every top up introduces fresh oxygenated mains water into a sealed circuit, which drives corrosion and sludge, and it is hiding the actual fault while it gets worse.
And do not tape or seal a leaking joint under the boiler and leave it. Unlike a leak on ordinary pipework, anything under a boiler is close to gas connections and electrics, and a temporary fix there is a different risk from a temporary fix on a pipe under a sink.
What a Boiler Leak Costs in a Tenement
The reason to deal with this promptly in Glasgow is rarely the boiler and almost always the building. A slow leak from a boiler on an internal wall in a flat has a short journey to somebody else's ceiling, and the first anyone often knows is a stain appearing downstairs rather than a puddle appearing upstairs.
Boiler positions make it worse. A great many flats here have the boiler in a kitchen wall unit or a hall cupboard, boxed in during a refit, and a slow weep inside that boxing is invisible until it has soaked into the floor. Original tongue and groove boards with gaps and no membrane underneath pass water to the plaster below far quicker than a modern floor would, and there is nothing in the way to slow it.
That gives a practical rule that differs from advice written for a house: if you find water under a boiler in a flat, look at the floor around and under the boxing rather than only at the appliance, and check whether the boards are damp or lifting. Water that has been tracking for a fortnight is a different conversation with a neighbour from water found the same day.
The shared-building dimension is worth being ahead of too. Where damage has reached another property, that is a buildings insurance and factor conversation as well as a repair, and having noticed the date, the rate and where the water was coming from makes that conversation considerably easier than reconstructing it afterwards.
When to Call Someone Out
Worth calling promptly for any boiler leak, and immediately if there is tripping, buzzing or a burning smell, which means isolate the appliance and stop. The useful things to have ready are where the water is emerging, how fast, and what the pressure gauge reads cold and hot, because between them those usually identify the component before anyone arrives. Mo and Muhammad are Gas Safe registered for domestic work and cover boiler repairs. Several of the six sources above are inexpensive component replacements rather than reasons to replace an appliance, and we will tell you which one you have rather than quoting a new boiler on sight. If you smell gas at any point, that is not this: get out and ring 0800 111 999.
Boiler Repair in GlasgowCommon Questions
Most often a pipe joint below the appliance, the condensate trap, or water running down from a component inside the casing such as the pump seal. Water travels down and along before it drips, so the wet patch is rarely under the source. Find the highest wet point with a torch before concluding anything.
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