Why Is My Boiler Not Working? A Step-by-Step Checklist

Why Is My Boiler Not Working? Start With the Obvious

Before assuming the worst, run through the basics. A surprising number of engineer call-outs in the UK turn out to be caused by something simple — a tripped circuit breaker, a blank timer display after a power cut, or even a boiler that has simply been switched off at the mains by someone in the household.

Check the following first: - Is the boiler switched on at the mains and at its own fused spur (a small switch usually found near the boiler)? - Has a circuit breaker tripped in your consumer unit? Reset it if so. - If you are on a prepayment meter, has your credit run out? - Is the display completely blank? If yes, the problem is likely electrical rather than a boiler fault.

If your boiler has power but still will not fire up, move on to the checks below.

Boiler Stopped Working? Check the System Pressure

Low system pressure is one of the single most common reasons a boiler stops working in the UK. Most combi and system boilers need a pressure of between 1 and 1.5 bar when the system is cold. If it drops below around 0.5 bar, the boiler will typically lock out as a safety measure.

Find the pressure gauge on the front of your boiler — it is usually a dial or a digital readout. If the needle is in the red zone or reads below 1 bar, the system needs repressurising.

How to repressurise safely: - Locate the filling loop — a flexible braided hose or a built-in valve under the boiler. - Open the valve(s) slowly and watch the gauge rise. Stop at around 1.2–1.5 bar. - Close the valve(s) securely. - Press the reset button and check whether the boiler fires up.

A small drop in pressure over winter is normal as the system cools. However, if you are topping up more than once or twice a year, there is likely a slow leak somewhere in the pipework, radiators, or boiler itself. In that case, contact a Gas Safe registered engineer to trace and repair the leak.

No Heating or Hot Water? Check Thermostat and Timer Settings

If your boiler appears to have power and correct pressure but is still not firing, the thermostat or programmer is the next place to look. This is a more common cause than most people realise.

• Is your room thermostat set high enough? Try turning it up by several degrees above the current room temperature to see if the boiler kicks in.
• Check that the boiler timer or programmer is set to the correct day and time. Power cuts and the clocks changing twice a year can knock these settings out completely.
• If you have a smart thermostat, check the app and the boiler connection — a lost Wi-Fi link can prevent it from sending a call for heat.
• Old or battery-powered thermostats may have failing batteries. Replace them and see if that resolves the issue.

Thermostats placed near a heat source such as a sunny window or a lamp can also give false readings, causing the boiler to think the room is warmer than it is and switching off prematurely.

Boiler Not Working in Winter? Check for a Frozen Condensate Pipe

Modern high-efficiency condensing boilers — which account for the vast majority of boilers fitted in UK homes — produce acidic wastewater called condensate. This drains away through a plastic pipe, usually 20–22mm in diameter, which often runs outside to a drain or gulley.

In cold UK winters, the water in the external section of this pipe can freeze solid. When it does, condensate backs up inside the boiler, triggering a safety shutdown. You may notice a gurgling noise before the lockout, and your boiler display will often show an error code (such as L2 on an Ideal boiler or F75 on a Vaillant).

How to safely thaw a frozen condensate pipe: - Locate the plastic pipe running outside your home, particularly where it bends or exits the wall — this is where freezing usually occurs. - Pour warm (not boiling) water over the frozen section. A kettle cooled for a few minutes works well. Do not use a naked flame. - Repeat until you hear the condensate start to flow. - Once clear, reset the boiler and allow it to restart.

To reduce the risk of this happening again, you can insulate the exposed section of pipe with foam lagging, available cheaply from any DIY store. If the pipe is very exposed or freezes every winter, a Gas Safe engineer can advise on rerouting it.

Boiler No Heating but Hot Water (or Vice Versa)?

If your boiler is producing hot water but your radiators are cold, or you have heating but no hot water, this points to a more specific fault rather than a total boiler failure.

Hot water working but no heating: - The most likely culprit in a combi boiler is a faulty diverter valve. This valve switches the boiler between heating and hot water modes. If it sticks or fails, the boiler may default to hot water only, leaving your radiators cold. Diverter valve failure is one of the most common combi boiler faults in the UK, particularly in boilers that are more than eight to ten years old. - Airlocks in the central heating circuit can also prevent hot water reaching the radiators. Try bleeding your radiators using a radiator bleed key — open the valve at the top of each radiator slightly, let the air hiss out, then close it once water starts to trickle out. Check the boiler pressure afterwards and top up if needed.

Heating working but no hot water: - On a combi boiler, this can also point to the diverter valve, a faulty flow sensor, or a plate heat exchanger that has become scaled up or blocked. - Low system pressure can also cause hot water to drop out before heating is affected — check the gauge as a first step.

Both of these situations require a Gas Safe registered engineer to investigate and repair properly.

Boiler Making Noises? Understanding Kettling

If your boiler is making a loud banging, rumbling, or whistling noise — similar to a kettle coming to the boil — it is likely suffering from a problem known as kettling.

Kettling is caused by a build-up of limescale or sludge inside the heat exchanger, which restricts the flow of water. The water trapped in the narrowed passages overheats and partially turns to steam, creating pressure fluctuations and that characteristic noise. It is particularly common in hard-water areas of England — such as the South East and Midlands — but can affect any system that has not been properly maintained.

Left untreated, kettling reduces boiler efficiency (pushing up your energy bills), accelerates wear on key components, and can eventually cause the heat exchanger to fail — one of the more expensive repairs on a boiler.

A Gas Safe registered engineer will typically address kettling by: - Power flushing the system to remove sludge and loosened scale from the radiators and pipework. - Descaling or chemically treating the heat exchanger. - Adding a corrosion inhibitor to protect the system going forward. - Fitting a magnetic filter to catch debris before it can cause further damage.

Power flushing is not a DIY job, and resolving kettling requires professional diagnosis to confirm the exact cause.

Check Your Boiler's Fault Code Before Calling an Engineer

Most modern boilers — whether Baxi, Worcester Bosch, Vaillant, Ideal, Glow-worm, or Alpha — display a fault code on their screen when something goes wrong. These codes are your boiler's way of telling you (and the engineer) what it thinks the problem is.

Before calling anyone out, note down the fault code and look it up. BOYLA has dedicated pages for hundreds of fault codes across all major UK brands — searching for your specific code can tell you exactly what is wrong, whether there is a safe fix you can try at home, and what an engineer will need to do.

Common fault code categories include: - Pressure-related faults (often the first thing to check) - Ignition failures (the boiler tried to light but could not) - Sensor faults (temperature or flow sensors giving incorrect readings) - Fan or flue faults (the boiler cannot safely vent exhaust gases)

Always look up your fault code before spending money on a call-out — you may save yourself a fee simply by being informed.

When to Reset Your Boiler (and When Not To)

Most boilers have a reset button on the front panel, and pressing it after a one-off lockout is perfectly reasonable. The boiler will attempt to restart its ignition sequence, and in many cases — particularly after a pressure top-up or a condensate thaw — it will fire straight back up.

However, there are some important points to bear in mind: - Limit yourself to two or three reset attempts. If the boiler locks out again immediately or repeatedly, there is an underlying fault that a reset will not cure. - Repeatedly resetting can mask a more serious problem and, in rare cases, cause additional wear on components trying to ignite against a fault. - Never reset a boiler if you can smell gas, suspect a carbon monoxide issue, or have any concern about safety. Leave the property and call the National Gas Emergency Service on 0800 111 999.

If your boiler will not stay on after a reset, make a note of any fault code and call a Gas Safe registered engineer.

What a Gas Safe Engineer Will Do — and What Repairs Typically Cost

If your safe checks do not resolve the problem, the next step is to book a Gas Safe registered engineer. Always ask to see their Gas Safe ID card and verify their registration number on the Gas Safe Register website before allowing any work to begin.

During a visit, the engineer will typically: - Read any stored fault codes from the boiler's control board. - Carry out a full visual inspection of the flue, heat exchanger, and components. - Check gas pressure and combustion readings. - Test and replace any faulty parts.

Prices vary by region — expect the top end (or 20–30% more) in London and the South East, and generally less in the North and Scotland. The figures below are typical UK ranges for 2025/26.

For a straightforward repair involving a single component and an hour's labour, most people pay in the range of £120–£300. More complex jobs involving the PCB, heat exchanger, or a power flush will sit at the upper end of the scale or beyond. If your boiler is more than ten to fifteen years old and facing a major repair, it is worth getting a replacement quote at the same time — a new boiler typically costs £1,500–£5,000 supply and fitted, with most homeowners paying around £3,500–£4,000 for a quality combi.

An annual boiler service (£70–£140 at most engineers) is the single best way to catch faults early and keep breakdown risk low. Around 36% of UK households skip their annual service — a false economy that often results in avoidable breakdowns.

Step by step

1. Check the power supply

   Make sure the boiler is switched on at the mains and at the fused spur. Check your consumer unit for tripped breakers and reset any that have tripped. If you are on a prepayment meter, check your credit has not run out.

2. Check the system pressure

   Look at the boiler pressure gauge. It should read between 1 and 1.5 bar when cold. If it is below 1 bar, repressurise using the filling loop following your boiler manufacturer's instructions, then attempt a reset.

3. Check the thermostat and timer

   Confirm the room thermostat is set above the current room temperature. Check the programmer or timer shows the correct day and time — reset it if a power cut may have affected it. Replace batteries in wireless or battery-powered thermostats.

4. Check for a frozen condensate pipe

   In cold weather, locate the external plastic pipe (usually 20–22mm, white or grey) and feel for a frozen blockage. Pour warm — not boiling — water over the frozen section until it thaws, then reset the boiler.

5. Check for a fault code

   Note any code on the boiler display and look it up on BOYLA or in your boiler manual. The code will tell you what the boiler thinks is wrong and whether there is a safe homeowner action to take.

6. Reset the boiler

   Press and hold the reset button as described in your manual. Allow the boiler to attempt ignition. If it locks out again within a short time, do not keep resetting — call a Gas Safe registered engineer.

7. Call a Gas Safe registered engineer

   If none of the above resolves the problem — or if you can smell gas, notice yellow or orange flames, or suspect carbon monoxide — stop using the boiler and call a professional immediately. For a gas emergency, call 0800 111 999.

Typical costs

Typical UK ranges as a guide only — prices vary by region (expect the top end, or 20–30% more, in London and the South East) and by how accessible your system is. Always get a written quote.

Frequently asked questions