Can a boiler explode? The real risks and warning signs
A modern gas boiler that is installed properly and serviced is extremely unlikely to explode. It has several safety devices that shut it down or release pressure long before anything dangerous happens: a pressure relief valve, an overheat cut-out and a flame safety system that stops the gas if the flame goes out. The real dangers are different. A gas leak can ignite, a badly burning or blocked appliance can produce carbon monoxide, and safety devices that have been tampered with or never serviced can fail. If you smell gas, turn off the supply at the meter if you can do so safely, open windows, don't touch electrical switches, leave the property and call the National Gas Emergency Service on 0800 111 999.
Loyal Plumbers · Updated
Discuss your plumbing workWhy a modern boiler is designed not to explode
The word explode makes people picture the boiler bursting like a pressure cooker. A combi or system boiler on a sealed heating system runs at low pressure, typically around 1 to 1.5 bar when cold and a little higher when hot. Several separate devices stand between that and a dangerous situation, and each one works on its own.
| Device | What it guards against | What you might notice |
|---|---|---|
| Pressure relief valve | System pressure going too high, usually set to open at 3 bar | Water dripping or running from a copper pipe outside |
| Overheat cut-out | Water in the boiler getting too hot | Boiler locks out and shows a fault code |
| Flame supervision | Gas flowing with no flame to burn it | Boiler fails to light and locks out |
| Fan and flue checks | Burning gas without the fumes being carried away | Boiler refuses to fire or shows a flue or fan fault |
| Expansion vessel | Pressure rising as the water heats | Pressure swinging high when hot and low when cold if it fails |
What can actually go wrong
Serious boiler incidents in homes are rare, and when they happen they almost always come from one of a handful of causes. A gas leak from the boiler, the pipework or another appliance can build up and be ignited by a spark, which is why the advice on smelling gas is so strict. An appliance that is not burning properly, or whose flue is blocked, damaged or disconnected, can fill the room with carbon monoxide, which has no colour or smell and is a far more common danger than an explosion.
The other risk is safety devices that no longer work. A pressure relief valve that has been capped off to stop a drip, an overheat thermostat bypassed to keep a failing boiler running, or an old boiler that has not been serviced in years takes away the protection the manufacturer built in. Work on gas appliances by anyone who is not on the Gas Safe Register is illegal, so always ask to see the engineer's Gas Safe ID card.
Hot water cylinders are a separate case
If your home has a hot water cylinder, particularly an unvented one fed straight from the mains, it holds a large volume of water under pressure. It has its own safety devices: a thermostat, a separate overheat cut-out and a temperature and pressure relief valve that opens if the water gets too hot, discharging through the tundish, the open funnel on the pipe nearby.
Hot water pouring through the tundish, steam, or a cylinder that is too hot to touch with water scalding at the taps means those safety devices are working, and something upstream has failed. Switch off the boiler and any immersion heater, leave the cold supply on, and call an engineer. Unvented cylinders should be serviced regularly by someone qualified to work on them.
Warning signs to take seriously
None of these means your boiler is about to explode. They do mean something is wrong, and that the boiler should be looked at rather than reset repeatedly and left.
- A smell of gas anywhere near the boiler or the gas meter
- A carbon monoxide alarm sounding, or symptoms such as headaches that ease when you leave the house
- Black or brown soot marks around the boiler case or the flue
- A lazy yellow or orange flame on an older boiler with a visible flame, instead of a crisp blue one
- The pressure gauge climbing towards 3 bar, or water dripping from the pressure relief pipe outside
- Loud banging, rumbling or kettling from the boiler
- The boiler repeatedly locking out, or tripping the electrics
- Water leaking from inside the case
What to do straight away
If you smell gas, act first and investigate later. Turn off the gas at the emergency control valve next to the meter if you can reach it safely, open doors and windows, do not use light switches, doorbells or anything that could spark, put out any naked flames, leave the property and call the National Gas Emergency Service on 0800 111 999. The service is free and runs 24 hours a day.
If a carbon monoxide alarm goes off, open windows, switch off gas appliances if it is safe, get everyone outside and call the same number. Get medical help if anyone feels unwell.
For banging, high pressure, water from the relief pipe or repeated lockouts, turn the boiler off at its controls and let it cool. If the gauge is near 3 bar, do not keep topping it up or resetting the boiler. Book a Gas Safe registered engineer to find out why it is happening. It helps to know in advance where the lever is; our guide on how to turn off the gas at the meter explains where it is usually found and which position is off.
Never cap off a dripping pressure relief pipe, block a flue, or remove a boiler case to look inside. Those are the actions that turn a nuisance fault into a dangerous one.
Keeping the risk low
An annual boiler service is the single best protection. The engineer checks that the gas burns correctly, that the flue is sound and sealed and that the safety devices still work. Fit a carbon monoxide alarm in rooms with a gas appliance and test it regularly, and do not ignore a boiler that keeps needing its pressure topped up or keeps resetting itself.
If you rent the property out, the law requires a gas safety check on every gas appliance and flue each year by a Gas Safe registered engineer, with a copy of the record given to your tenants.
Getting your boiler checked
If your boiler is banging, losing or gaining pressure, sooting up or locking out, have it looked at before the weather turns cold and it is working hardest. Loyal Plumbers' gas work is carried out by Gas Safe registered engineers, with 10 vans covering London and everywhere inside the M25, 24 hours a day. Our call-out charge is £60.
When you call or send an enquiry, tell us the boiler make, what you have noticed, the pressure reading and when it was last serviced. If you smell gas, call the National Gas Emergency Service first; once they have made the supply safe, we can repair or replace the appliance.
Common questions
- Can a combi boiler explode?
- It is extremely unlikely. A combi runs at low pressure and has a relief valve, an overheat cut-out and flame supervision. The real risks are a gas leak or carbon monoxide from a faulty or unserviced appliance.
- Can a boiler explode if the pressure is too high?
- The pressure relief valve opens at around 3 bar and lets water out through the pipe outside, so the pressure cannot keep rising. High pressure still needs fixing, because it points to a fault.
- Is a banging boiler dangerous?
- Banging usually means overheating water in the heat exchanger, often from limescale, sludge or poor circulation. It is not a sign of an imminent explosion, but it should be checked by an engineer.
- Should I turn my boiler off if it is making strange noises?
- Yes, if the noise is new, loud or getting worse. Turn it off at the controls, note the pressure and any fault code, and book an engineer.
- Who do I call if I smell gas?
- The National Gas Emergency Service on 0800 111 999, free and open 24 hours. Leave the property first if you can.
Sources and further reading
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