Why Is My Boiler Not Working?

What has to be true before it fires

A boiler is a chain of permissions. Every link has to say yes.

No demand — the boiler thinks it's finished

Start here, because it costs nothing and it is embarrassingly common.

The boiler only fires when something asks it to. If nothing is asking, it will sit there looking dead while being in perfect health.

• Room thermostat batteries. Wireless thermostats run on batteries and do not always announce that they are flat. A dead thermostat sends no demand, so the boiler never starts. This is the single cheapest fix on this page and it catches people every winter.
• The thermostat is set below room temperature. Turn it up ten degrees and listen. If the boiler fires within a minute, that was it.
• The programmer is in the wrong mode. Off, holiday mode, or a schedule that ended. Clocks that lost time in a power cut are a classic — the heating now runs at three in the morning instead of six in the evening.
• The clock is on the wrong day. Many programmers have separate weekday and weekend schedules. A programmer that thinks Tuesday is Sunday behaves very strangely.
• Summer setting. Some systems have a hot-water-only mode that suppresses heating entirely.
• A smart thermostat that has lost its connection to the receiver wired at the boiler. The app may look fine while the radio link is down.

Test it directly: set the programmer to constant, push the room thermostat well above the current room temperature, and turn a hot tap on for thirty seconds. If none of those three produces any sound from the boiler, the demand chain is not the problem and you can move on.

Power, fuses and the switch nobody looks at

A boiler with a blank display is not necessarily broken; it may simply have nothing to run on.

• The fused spur. Nearly every boiler is fed from a small switched fuse unit on the wall nearby, often behind a cupboard door or above the boiler where nobody looks. It can be switched off by accident during cleaning or decorating, and its fuse can blow.
• The consumer unit. Check whether a breaker or RCD has tripped. If it trips again the moment you reset it, stop — that is a fault, not an inconvenience, and it needs an electrician or a gas engineer depending on what tripped it.
• A recent power cut. Some boilers and most programmers need resetting afterwards; a few will hold a lockout until reset manually.
• The isolating switch on the boiler itself, where fitted.

If the display is lit but nothing happens, power is fine and the problem is further down the chain.

Pressure, which is the single commonest cause

Sealed heating systems — which is most modern systems, and effectively all combi boilers — hold water under a modest pressure. There is a gauge on or near the boiler, either a dial or a number on the display.

Cold, it should normally read somewhere around 1 to 1.5 bar. Your manual gives the exact figure for your model and it takes precedence. When the system is hot the reading rises, which is normal.

If the needle is sitting near zero, the boiler will refuse to fire. It does this deliberately: firing a burner into a system that may be short of water risks damaging the heat exchanger, so low pressure triggers a lockout.

Why the pressure dropped matters as much as the number:

• It drifts down slowly over months. Normal. Small amounts of air are released and vented over time. Topping up once or twice a year is routine maintenance, not a fault.
• It dropped right after bleeding radiators. Expected — you let water out. Top it back up.
• It drops within days of every top-up. That is a leak, and it needs finding rather than feeding. Look for damp patches under radiators, at pipe joints, and on the ceiling below the boiler. Repeatedly refilling a leaking system pushes fresh oxygenated water through it and accelerates internal corrosion.
• It swings wildly between cold and hot — near zero cold, near the red hot — which usually points at the expansion vessel having lost its charge. That is an engineer's job.

Topping up is done with the filling loop: a braided silver hose or a pair of built-in valves underneath the boiler. Open it slowly, watch the gauge, close it as soon as you reach the figure in the manual, and close both ends firmly afterwards. Leaving a filling loop open lets mains pressure into the system, which is its own problem.

Overshooting is common and fixable — bleeding a small amount of water from a radiator brings it back down.

The frozen condensate pipe

This one is seasonal, extremely common, and completely invisible unless you know it exists.

Modern condensing boilers extract so much heat from the exhaust that water vapour condenses back into liquid. That liquid — mildly acidic — has to drain away, through a plastic pipe usually around 20 to 30 millimetres across. When that pipe runs outside the building, a hard frost can freeze it solid.

The boiler detects that the condensate cannot escape and shuts down to avoid flooding itself internally. The signature is unmistakable once you know it:

• It happened during or just after a cold night, typically the first proper freeze of the season.
• There is a gurgling or bubbling sound from the boiler when it tries to start.
• The display shows a fault code, and the manual attributes that code to condensate or drainage.
• It worked fine yesterday and there is nothing else wrong.

The remedy is to thaw the external section: warm water — not boiling, which can crack cold plastic — poured slowly along the exposed pipe, or a hot water bottle held against the most likely blockage point. Blockages form where the pipe bends, where it meets a drain, and at any exposed open end. Once it drains, reset the boiler and it will normally start.

If it freezes every winter, the pipe itself is the problem rather than the weather. Insulating it, re-routing it internally, or increasing its diameter are all standard remedies an engineer can carry out — and worth arranging before next winter rather than during it.

Gas supply and prepayment meters

You are not going to inspect anything on the gas side, but you can establish in thirty seconds whether gas is reaching the property at all.

• Does another gas appliance work? A gas hob is the ideal test. If the hob lights normally, gas is arriving and the problem is the boiler. If nothing gas-powered works, the problem is the supply.
• Prepayment meter credit. Running out stops the boiler dead and, on some meters, the supply stays off until the credit is applied and the meter is reset by hand.
• The gas valve at the meter may have been closed — by works nearby, by a previous engineer, or by somebody in the household.
• A supply interruption in the street. Your gas supplier or network operator will confirm.

If gas has been off and has now come back, the boiler may need a reset to clear a lockout, and the first ignition may take a few attempts as air clears from the line. That is normal.

Fault codes and the reset button

If there is a code on the display, that code is the most valuable piece of information you have. Write it down before doing anything else, because resetting clears it.

Codes are manufacturer-specific — the same number means entirely different things on different boilers — so there is no universal table worth trusting. Your manual, or the manufacturer's website with your exact model number, is the only reliable source.

About the reset button:

• One reset is reasonable. Lockouts do happen for transient reasons, and a single reset often clears them for good.
• Do not keep resetting. Manufacturers commonly advise no more than two or three attempts. A boiler that locks out repeatedly is protecting itself from a real fault, and forcing it to keep trying can turn a repair into a replacement.
• Note what happens between resets. Does it fire and then cut out after a few seconds? Does it not attempt ignition at all? Does the fan run? An engineer arriving to a description that precise will diagnose it far faster than one arriving to "it doesn't work".

Keep a note of the model number and the code when you call. It genuinely shortens the visit, and some faults can be identified over the phone.

Heating but no hot water, or the reverse

A boiler that does one job and not the other is telling you something quite specific, and it is a different problem from one that does nothing at all.

On a combi boiler, a component called the diverter valve decides whether heat goes to the radiators or to the hot water. It gives hot water priority — open a tap and the radiators stop for the duration. When that valve sticks, you get one function and not the other. Hot water but cold radiators is the classic presentation.

On a system or conventional boiler with a hot water cylinder, motorised valves do the same job, and the immersion heater, cylinder thermostat and timer add their own possible causes.

Two related symptoms worth separating out:

• Radiators cold at the top, warm at the bottom — air in the system. Bleeding fixes it, and expect the pressure to drop afterwards, so top it back up.
• Radiators cold at the bottom, warm at the top — sludge settling inside them. That is a system cleaning job, not a boiler fault, and it makes the whole system expensive to run in the meantime.

Neither is a reason for the boiler to refuse to start, but both are commonly reported as "the boiler isn't working".

If you rent

Heating and hot water are not optional extras, and in most jurisdictions a landlord has a legal duty to keep the heating system in working order and to have gas appliances safety-checked by a qualified engineer at defined intervals.

Practically, that means:

• Report it in writing — a message or email, not a phone call — so there is a dated record of when you told them.
• Do the free checks first (thermostat batteries, programmer settings, fuse) so you are not waiting a week for somebody to press a button you could have pressed.
• Do not arrange gas work yourself and expect to deduct it, unless your local law specifically allows it and you have followed the required steps. Getting that wrong leaves you out of pocket.
• Ask for the gas safety record if you have not been given one. You are normally entitled to a copy.

What to check, in order

1. Smell gas or a CO alarm sounding? Stop. Ventilate, leave, call the emergency number from outside.
2. Write down any fault code on the display, plus the model number.
3. Is there demand? Programmer to constant, room thermostat well up, hot tap on. Listen.
4. Thermostat batteries. Replace them if they are wireless and you cannot remember the last change.
5. Is the display lit? If not, check the fused spur near the boiler and the consumer unit.
6. Check the pressure gauge cold. Near zero means top up via the filling loop to the figure in your manual.
7. Cold night, gurgling noise? Thaw the external condensate pipe with warm water.
8. Does the gas hob light? If not, the problem is the supply or the meter, not the boiler.
9. Prepayment meter — check credit and whether the supply needs manually reinstating.
10. Reset once. If it locks out again, stop and book an engineer.
11. Describe the behaviour precisely when you call: the code, whether the fan runs, whether it ignites and cuts out, and what changed just before it stopped.

Stopping it happening again

• Change wireless thermostat batteries every autumn, before the heating season rather than during it.
• Check the pressure gauge once a month. Ten seconds. It turns an emergency into a scheduled two-minute top-up.
• Insulate or re-route an external condensate pipe if it has ever frozen. It will freeze again.
• Run the heating for a few minutes in summer, once a month. It keeps pumps and diverter valves from seizing after months of standing still — which is why so many boilers fail on the first cold day rather than gradually.
• Have it serviced annually, ideally in late summer when engineers are not fully booked and a fault found is an inconvenience rather than a cold house.
• Keep the manual somewhere findable, or save a PDF of it with the model number in the filename. When a code appears at ten at night, that is what you will want.
• Fit a carbon monoxide alarm if there isn't one, and test it. It is the cheapest safety device in the house.