Can a Boiler Explode? The Real Risks and Warning Signs

Why modern boilers are designed not to explode

A boiler heats water, and water under pressure that gets too hot is the thing that makes any heating appliance dangerous. So every modern gas boiler sold in the UK has layers of protection that each work on their own, so that one failing does not leave the boiler unprotected.

• Flame supervision. A sensor watches the flame. If it does not see one when the gas valve opens, or the flame goes out, the gas is shut off within seconds and the boiler locks out.
• Overheat thermostat. A high limit stat cuts the burner if the water in the heat exchanger gets hotter than it should. On most boilers it needs a manual reset, so the fault cannot be ignored.
• Pressure relief valve. On sealed systems, including every combi and system boiler, a valve opens at around 3 bar and lets water out through a pipe, usually to outside. That drops the pressure long before anything could rupture.
• Fan and flue checks. The boiler confirms the fan is running and the flue is clear before it lights, so it does not fire into a blocked flue.
• Low pressure cut out. If the system pressure falls too low, the boiler will not fire at all.

What people usually mean by a boiler explosion

When you hear about a boiler exploding, it is worth asking what actually happened. It is almost always one of these.

• A gas leak that found a spark. Gas from a leaking pipe, fitting or appliance builds up and ignites. The boiler may be the source of the leak or the spark, but the explosion is the gas, and it can happen with any gas appliance. That is why a smell of gas always comes first.
• A loud bang when the boiler lights. Called delayed ignition, it happens when gas builds up for a moment before it lights, often because of a worn electrode, a partly blocked burner or a flue problem. It is startling, it can damage the boiler, and it needs an engineer, but it is not the boiler blowing up.
• Water or steam from a pipe outside. That is the pressure relief valve doing its job. The pressure got too high, and the valve released it.
• Old or solid fuel boilers. Very old open vented boilers, and solid fuel boilers such as some back boilers, did not have the same electronic protection. Where an open vent pipe was blocked or a system was altered badly, overheating was possible. These are now rare in London, but if you have one, it needs checking.

Warning signs to take seriously

Your boiler will usually tell you something is wrong well before anything dangerous happens. Switch it off and get it checked if you notice any of these.

• A smell of gas anywhere near the boiler or pipework. Treat this as an emergency; see what to do if you smell gas.
• Your carbon monoxide alarm sounding, or symptoms like headaches, dizziness or nausea that ease when you leave the house.
• Black or brown sooty staining on or around the boiler, or a lazy yellow or orange flame on an older boiler where you can see the flame. Both suggest poor combustion.
• Pressure on the gauge climbing to 3 bar or beyond, or water regularly coming from the relief pipe outside. See boiler pressure too high.
• Loud bangs when the boiler lights, or a kettling, rumbling noise while it runs. Kettling is usually scale or sludge in the heat exchanger making the water boil locally.
• Repeated lockouts. One reset is reasonable. If it keeps locking out, the boiler is protecting itself from something. Our guide on how to reset a boiler explains when to stop.

Never try to bypass, disconnect or wedge a safety device because it keeps tripping, and never cap or block a relief valve pipe because it drips. Those devices are the reason boilers do not explode.

The bigger risk: carbon monoxide

Carbon monoxide is a far more likely danger from a faulty boiler than an explosion. It is produced when gas does not burn properly, and you cannot see, smell or taste it. A boiler with a blocked flue, a failed seal or poor combustion can leak it into the room.

Every home with a gas boiler should have a carbon monoxide alarm that meets BS EN 50291, fitted in the room with the boiler and ideally in bedrooms too. Test it monthly. If it goes off, open the windows, turn off the boiler if you can do it safely, get everyone outside and call 0800 111 999. Our guide on carbon monoxide alarm going off explains what each sound means.

What about the hot water cylinder?

If you have a system boiler or a heat only boiler, there is a hot water cylinder as well, and people often worry about that too. An unvented cylinder, the sealed mains pressure type, holds a lot of hot water under pressure and has its own set of safety devices: a control thermostat, an overheat cut out and a temperature and pressure relief valve that discharges through a tundish. With those intact it is very safe. Our guide on can an unvented cylinder explode covers the risks and warning signs in detail.

The warning sign to know is hot water running from the tundish, the small open funnel on the pipe near the cylinder, or every tap in the house suddenly running scalding. Switch the boiler and immersion off and call an engineer. The guide on hot water cylinder overheating explains what to do next.

How to keep a boiler safe

The chance of a boiler becoming dangerous is lowest when it is looked after. A few habits make a real difference.

• Have it serviced once a year by a Gas Safe registered engineer. You can check any engineer on the Gas Safe Register before they start.
• Keep the area around the boiler and the flue clear. Do not box in a boiler tighter than the manufacturer allows.
• Fit and test a carbon monoxide alarm.
• Keep an eye on the pressure gauge on a sealed system, usually 1 to 1.5 bar cold.
• Deal with repeat lockouts, bangs and noises early rather than resetting and hoping.
• Never let anyone who is not qualified work on the gas side, and never let anyone disable a safety device to get the heating back on.

Worried about your boiler or cylinder?

If you smell gas, call 0800 111 999 first. For anything else, including a boiler that keeps locking out, pressure that will not stay put, water from the relief pipe or a cylinder that is running too hot, call us on 020 4652 5111 or send an enquiry. We have 20 engineers across London and work on boilers and unvented cylinders every day. Tell us what you are hearing or seeing, and we will tell you honestly whether it needs someone today. Our rate is on the pricing page and agreed before we attend.