Boiler Not Working? Check These Things First
When a boiler stops working, most people reach for the phone to call an engineer straight away. And in some cases, that is absolutely the right call. But more often than not, the cause is something straightforward that you can sort out yourself in a matter of minutes. This guide walks you through the most common reasons a boiler stops working, in order of priority, so you can identify the problem quickly without wasting time or money. It also covers the warning signs that mean you should not touch the boiler at all and need a Gas Safe registered engineer on-site as soon as possible.
1. Check the Power Supply and Fuse Box
It sounds obvious, but the most commonly overlooked cause of a boiler stopping dead is simply a loss of power. Start by checking whether the boiler’s isolation switch is turned on. This is usually a single switch on the wall nearby, often above or beside the unit. If that is fine, head to your fuse box and look for a tripped breaker. Reset it once. If it trips again immediately, do not reset it a second time. A breaker that keeps tripping is telling you there is a fault somewhere in the circuit, and that needs a qualified electrician or engineer to investigate. While you are at it, check your thermostat too. The temperature may have been turned down accidentally, or the batteries could be dead. Both are surprisingly common culprits.
2. Check the Boiler Pressure
Low water pressure is one of the single most common reasons a boiler locks out. On the front or underside of your boiler, there will be a pressure gauge showing the current pressure in the system. Normal operating pressure sits between 1 and 1.5 bar. If the needle has dropped below 0.5 bar, the boiler has almost certainly cut out as a safety measure. To fix this, you need to repressurise the system using the filling loop, which is usually a short flexible hose with valves at each end, located underneath the boiler. Open the valves slowly and watch the gauge. The moment it reaches 1 to 1.5 bar, close both valves fully. If you find yourself topping up the pressure every few days, there is a leak somewhere in the system that needs to be tracked down and repaired properly.
3. Read the Fault Code on the Display
Most modern boilers have a digital display that shows a fault code when something goes wrong. These codes are your first real clue about what is happening inside the unit, and they are worth taking seriously rather than ignoring. You can look up the specific code in your boiler’s manual or on the manufacturer’s website. Some faults, particularly ignition lockouts, can be cleared by pressing and holding the reset button on the boiler for a few seconds. However, fault codes that relate to flue gas, gas supply, or heat exchanger issues are a different matter entirely. If the code points to anything involving combustion, gas flow, or sensors, do not attempt to fix it yourself. These require a Gas Safe registered engineer.
4. Check the Pilot Light
If you have an older boiler, it will have a small permanent pilot flame that needs to stay lit for the boiler to fire up. If this has gone out, the boiler simply will not ignite. Most models have a specific relighting procedure printed on a label inside the casing, or detailed in the manual. Follow the steps carefully. On newer boilers, there is no standing pilot flame. Ignition is electronic. If you can hear the boiler clicking as it tries to ignite but the burner is not catching, the problem is likely a faulty ignition electrode or a gas valve issue. Neither of these is something you should attempt to fix at home.
5. Check the Flue and Air Intake
Wall-mounted boilers need a clear path to draw in fresh air for combustion and to expel exhaust gases safely outside. The flue pipe, which exits through an external wall, can sometimes become blocked by debris, leaves, ice, or even a bird nesting nearby. When this happens, the boiler’s safety sensors detect the problem and shut the unit down, which is exactly what they are designed to do. A frozen flue is a particularly common cause of sudden boiler failure during cold snaps in the UK. Go outside and take a look at the terminal on the external wall. If there is a visible obstruction you can safely remove, do so. However, never disconnect or attempt to repair the flue pipe yourself. Any work on the flue must be carried out by a Gas Safe registered engineer.
6. Check the Gas Supply Valve
If the power is on, the pressure is correct, and there are no fault codes showing, the next thing to check is whether the gas supply valve to the boiler is actually open. This valve sits on the gas pipe feeding into the boiler, usually at the bottom. When the handle is perpendicular to the pipe, the valve is closed. When it runs parallel to the pipe, gas is flowing through. Before touching the valve, check whether other gas appliances in the property are working. If your hob or gas fire is also out, the issue is likely upstream of the boiler entirely. In that case, contact your gas supplier or call the National Gas Emergency Service on 0800 111 999.
7. When to Call a Gas Safe Engineer Immediately
Some situations are not for troubleshooting. They require you to act fast and get out. If you smell gas anywhere in the property, turn off the gas supply at the meter, open windows and doors, do not operate any light switches or electrical appliances, and leave the building. Once outside, call the National Gas Emergency Service on 0800 111 999. Beyond a gas smell, call a Gas Safe registered engineer without delay if you notice black or sooty marks around the boiler casing, if the flame is burning yellow or orange rather than a steady blue, if there is any water leaking from the unit, or if the boiler repeatedly locks out after being reset. Attempting to repair a gas boiler without Gas Safe registration is illegal in the UK, dangerous, and will invalidate your home insurance.
8. Annual Servicing: The Cheapest Insurance You Can Buy
Most boiler breakdowns that happen on the coldest day of January are not bad luck. They are the result of a boiler that has been sitting unused through spring and summer, without anyone checking whether it is still in good shape. An annual service by a Gas Safe registered engineer typically involves cleaning the heat exchanger, inspecting the ignition electrodes, testing the pressure relief valve, checking the flue integrity, and confirming that the gas combustion is running efficiently and safely. In London and the surrounding area, you can expect to pay somewhere between 60 and 120 pounds for a standard service. Set against the cost of an emergency call-out in the middle of winter, or a heat exchanger replacement running into several hundred pounds, that is money very well spent.
Frequently Asked Questions
My boiler fires up but there is no hot water. What is wrong?
If your central heating is working but you are getting no hot water from the taps, the most likely cause is a faulty diverter valve. This component switches the boiler’s output between the heating circuit and the hot water circuit. When it sticks in one position, you lose one or the other. A Gas Safe engineer will need to replace it.
My boiler is making a banging or rumbling noise. Should I be worried?
That sound, often called kettling, usually means there is limescale or sludge build-up on the heat exchanger, or air trapped in the system. It is not an immediate safety risk, but it puts extra strain on the boiler over time and can lead to a cracked heat exchanger if left unchecked. Try bleeding your radiators first. If the noise continues, book a service.
I top up the pressure but it keeps dropping. Why?
A boiler that repeatedly loses pressure almost always has a leak somewhere in the system. That could be at a radiator valve, a pipe joint, or within the boiler itself. Continually topping up the pressure without finding and fixing the source of the leak will eventually cause damage to the system and is not a long-term solution.
Is it safe to reset the boiler myself?
Yes, pressing the reset button is perfectly safe for a homeowner to do. Hold it for a few seconds and see whether the boiler restarts normally. If the same fault code comes back, stop resetting it. Repeated resets without addressing the underlying problem can cause further damage and in some cases mask a more serious issue.
Should I leave the boiler running in winter when I am away from home?
It is generally a good idea to leave the boiler set to a low temperature, around 10 to 15 degrees Celsius, rather than switching it off completely during cold weather. This protects the pipework from freezing. Most modern boilers also have a built-in frost protection mode that kicks in automatically as long as the boiler has power.
In most cases, a boiler that has stopped working has a simple explanation. Low pressure, a tripped fuse, a turned-down thermostat, or a blocked flue terminal are behind the majority of call-outs we see. A few minutes of methodical checking will often get you back up and running without needing an engineer at all. That said, anything involving the gas supply, combustion, or repeated lockouts is not something to push through on your own. When in doubt, the safe move is always to call a professional.
Still Not Working? We Can Help
If your boiler is still not working after these checks, the fault may sit in the parts of your heating system we are able to work on: radiators, system pressure, heating pipework, the condensate pipe or the controls. We are not Gas Safe registered and we do not carry out gas work ourselves. Anything involving the gas supply, the combustion side or the inside of the boiler casing must be carried out by a Gas Safe registered engineer, and we can arrange one for that part of the job.