The 10 Most Common Boiler Faults and How to Fix Them
- 5 hours ago
- 13 min read
Most Common Boiler Faults and How to Fix Them
Boilers fail in roughly the same ten ways. After 27 years of installing and repairing boilers across Liverpool, Wirral and the wider Northwest, we have found the patterns are consistent. This guide covers the ten faults that account for most of the call-outs the DD Wilson team attends: pressure loss, F22 lockouts, hot water failures, ignition problems, banging radiators, dripping pressure relief valves, condensate pipe blockages, diverter valve leaks, silent lockouts, and the general "no heating, no idea why" call. In Most Common Boiler Faults and How to Fix Them, you will find what it means, what you can safely check yourself, and when it must be taken to a Gas Safe Registered engineer.
A note before you start. Anything inside the sealed boiler casing or anything involving the gas supply is gas-side work. Under the Gas Safety (Installation and Use) Regulations 1998, only an engineer on the Gas Safe Register can legally carry that out. The checks in this guide are limited to what is safe and lawful for a homeowner to do.
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Why is my boiler showing F22?
Direct answer. F22 on a Vaillant boiler means low water pressure. The pressure has dropped below the level the boiler needs to fire safely, and the appliance has locked itself out to protect the system.
Most common cause. A small leak somewhere in the heating system, an expansion vessel that has lost its air charge, or a gradual pressure drop over weeks that has finally taken the system below Vaillant's normal operating range of 1.0 to 1.5 bar. The exact lockout point varies by model. Check the manual for your specific appliance.
What you can check
Look at the pressure gauge on the front of the boiler. A cold reading should sit between 1.0 and 1.5 bar.
Check visible pipework, radiator valves and ceilings under upstairs bathrooms for damp patches or staining.
If pressure is below 1 bar and you can find no visible leak, the appliance manual will show you how to repressurise via the filling loop. We also walk through the steps in our guide to re-pressurising a boiler.
When to call us. If pressure keeps dropping after you repressurise, water is escaping somewhere you cannot see. Repeated repressurisation masks a leak that will cost more to fix the longer it runs. Anything inside the boiler casing, including the expansion vessel, is Gas Safe work.
What changes the cost. A simple repressurise plus a system check is the cheapest outcome. A faulty filling loop or expansion vessel recharge is the middle band. A hidden leak that needs tracing under floors or behind walls is the largest variable: pressure-testing and access drive the bill more than the part itself. Request a quick quote for your specific boiler and symptoms.
DD Wilson is a Vaillant Advance Master Tech accredited installer, which means we hold the manufacturer training and parts access to diagnose Vaillant lockouts to the component level.
Why has my boiler lost pressure?
Direct answer. A boiler loses pressure either because water is escaping the heating system or because the expansion vessel can no longer cushion the pressure changes as the system heats up and cools down.
Most common cause. From 27 years of Liverpool call-outs, the running order is: a weeping radiator valve, a slow leak at a pipe joint, a failed expansion vessel, a dripping pressure relief valve (covered below), or a poorly installed magnetic filter.
What you can check
Inspect every radiator valve and visible pipe joint. Run a dry tissue around the base of each.
Check the floor near the boiler and behind it where you safely can.
Note whether pressure falls more when the system is cold (often a vessel issue) or hot (often a leak).
Photograph the gauge once a day for a week so you can see the rate of loss.
When to call us. If you find damp anywhere, do not try to repair it. Even on the water side of the system, a poor repair causes more damage than the original leak. If you cannot find one but pressure keeps dropping, the leak is hidden under floorboards or in a wall and needs professional tracing. For more on the symptoms, see our piece on low water pressure in the home.
What changes the cost. Cost depends on whether the leak is visible or concealed, whether the expansion vessel or pressure relief valve needs replacing, and how much pipework has to come up to reach a hidden leak. Location of the leak (under floor, in wall, behind units) drives the largest cost variance. Visible pipe-joint leaks are the cheapest fix. Concealed leaks under fitted flooring are the most expensive.
Why has my boiler stopped giving hot water?
Direct answer. If the central heating runs but hot water has failed (or the other way round), the diverter valve is the prime suspect on a combi boiler. If both have stopped, the cause is usually pressure, power supply, or ignition.
Most common cause. On combi boilers: a worn or stuck diverter valve. On system boilers with a separate cylinder: a failed motorised valve, a stuck cylinder thermostat, or an immersion that has been left on without a working timer.
What you can check
Check the boiler display for fault codes. Note any flashing lights or sequence patterns.
Test both functions in turn. Run a hot tap. Then check a radiator for heat response.
On a system or regular boiler, check the cylinder thermostat setting.
Listen for the diverter valve clicking when you switch between hot water and heating demand. Silence usually means it is stuck.
When to call us. Diverter valve replacement is inside the appliance and is therefore Gas Safe work. The same applies to motorised valves and any cylinder-side work that involves the boiler controls.
What changes the cost. On a combi, a diverter valve replacement is the most likely repair. On a system or regular boiler with a cylinder, costs vary by whether it is a motorised valve, a cylinder thermostat, or a control wiring fault. Brand drives part pricing: Vaillant and Worcester Bosch parts cost more than budget brands but are stocked locally and fit on the first visit. Older models with discontinued parts may not be worth repairing.
Why is my pilot light out?
Direct answer. Most modern UK combi boilers do not have a pilot light. They use electronic ignition. If your boiler has a standing pilot, it is an older floor-standing model, an older heat-only boiler, or a back boiler unit, and pilot failure usually means a faulty thermocouple, a dirty pilot assembly, or a gas supply problem.
Most common cause. A worn thermocouple is the usual cause on older boilers. The safety device cannot detect the pilot flame and shuts the gas valve as a precaution. Dirt or carbon around the pilot jet can also create a weak or yellow flame that the thermocouple will not register.
What you can check
Confirm the gas supply is on across the property. Try a gas hob if you have one.
If you can safely see the pilot assembly, check whether the flame is a clean blue colour. Yellow or weak flames indicate dirt or incomplete combustion.
Try the manual ignition sequence per the appliance manual. Do not repeat it more than the manual allows.
When to call us. If the pilot will not light, will not stay lit, or burns yellow, stop trying. A weak pilot is an early warning sign of incomplete combustion, which produces carbon monoxide. Book a same-day Gas Safe call-out. For the symptoms of carbon monoxide poisoning and emergency steps, see our 5 essential tips for carbon monoxide safety in UK homes.
What changes the cost. Thermocouple replacement is a relatively quick repair. A pilot assembly clean sits in the middle. Gas valve issues are more involved and may require a longer visit. On older floor-standing or back-boiler units, the engineer's diagnosis may reveal that the appliance is at the end of its life and a boiler replacement is more economical than continued repair.
Why are my radiators making a banging noise?
Direct answer. Banging or knocking from radiators is usually air in the system, mineral or sludge build-up restricting flow, or a circulation pump struggling because the system needs cleaning.
Most common cause. Across Liverpool's older terraced and Victorian housing stock, internal sludge is the usual answer. Decades of corrosion debris and mineral deposit collect at the base of radiators, restricting flow and forcing the boiler to overheat in short bursts. Banging often follows.
What you can check
Feel each radiator from top to bottom while the heating is on. Cold at the bottom but hot at the top suggests sludge. Cold at the top but hot at the bottom suggests air.
If it is air, bleed the radiator with a radiator key. A quarter turn anticlockwise is usually enough. Catch the drip with a cloth.
Re-check the system pressure after bleeding. You will usually need to repressurise via the filling loop.
When to call us. If the noise continues after bleeding, the system needs a power flush. A proper power flush, often with MagnaCleanse and an inhibitor refresh, transforms older Liverpool systems and extends boiler life. We run them regularly and can quote on a same-visit assessment.
What changes the cost. Power flush cost depends on the number of radiators, the severity of the sludge build-up, whether a magnetic filter is being fitted at the same time, and the inhibitor product used. A standard flush sits at one price; a flush combined with a new filter and full inhibitor refresh is a bigger job but extends boiler life significantly. We always quote both options.
Why is my pressure relief valve dripping outside?
Direct answer. A pressure relief valve dripping outside the property usually means the boiler pressure is too high, the pressure relief valve itself has failed, or the expansion vessel has lost its air charge.
Most common cause. Over-pressurisation after someone topped the system up too far. The second most common cause is a sticky or failed pressure relief valve that no longer holds against normal operating pressure.
What you can check
Read the pressure gauge while the boiler is cold. Anything above 1.5 bar indicates over-filling.
If pressure is high, you can release a small amount by bleeding a radiator slowly while watching the gauge. Stop when it drops back to around 1.2 bar.
If the gauge reads normal but the valve continues to drip, the valve itself has failed and needs to be replaced.
When to call us. Pressure relief valve replacement is gas-side work. A continuously dripping valve that has been ignored often also indicates a failed expansion vessel, which only a Gas Safe engineer can diagnose and repair properly.
What changes the cost. A straight PRV swap is the cheapest scenario. An expansion vessel recharge is the middle band. An expansion vessel replacement is a bigger job, particularly on combi boilers where the vessel is housed inside the appliance casing and requires partial strip-down. The engineer can usually tell which is needed within the first ten minutes of the visit.
Why is my condensate pipe blocked or frozen?
Direct answer. A blocked or frozen condensate pipe stops the boiler from firing because the appliance cannot expel the acidic condensate water produced during combustion. In winter, this is one of the most common UK boiler call-outs. The external section of the pipe freezes and the boiler locks out.
Most common cause. A condensate pipe installed externally without adequate insulation, or with too shallow a fall, will freeze in cold weather. Internal blockages are less common but happen, usually from scale or debris build-up.
What you can check
Locate the condensate pipe. It is normally a white plastic pipe exiting the wall and ending at an outside drain.
If frozen, pour warm (not boiling) water along the external section to thaw it. Boiling water can crack the plastic.
Once thawed, reset the boiler per the manufacturer's instructions.
When to call us. If the pipe keeps freezing every winter, the installation is not fit for purpose. Lagging, an internal re-route, or both will fix it permanently. Building Regulations Part J covers condensate routing for new installations and gives the right standard to work to.
What changes the cost. Fitting lagging to existing external pipework is the quickest, cheapest fix. Re-routing the condensate pipe internally is the most permanent solution, but it involves more labour and may need new drainage. Replacing a damaged or cracked pipe sits between the two. External vs internal routing is the main cost driver. A site survey gives the only accurate quote.
Why is my diverter valve leaking?
Direct answer. A leaking diverter valve usually shows up as water inside the boiler casing, or as hot water that runs lukewarm despite the boiler firing.
Most common cause. Age. Diverter valves wear over time and the internal seals fail. On boilers more than seven or eight years old it becomes a routine repair. Sludge and limescale shorten the life noticeably.
What you can check
Note the symptom pattern. Hot water sometimes works, sometimes runs lukewarm or cold, with no fault code.
Listen for the diverter clicking when you switch between hot water demand and central heating demand. Silence often means it is jammed.
When to call us. Diverter replacement is inside the appliance and therefore Gas Safe only. Our service vans carry the common Vaillant and Worcester Bosch diverter parts, so most of our diverter jobs are completed on the first visit.
What changes the cost. Brand drives most of the variance: Vaillant and Worcester Bosch parts cost more than budget brands but tend to be in stock and fit on the first visit. Age of the boiler matters too: some older models have discontinued diverters and the right answer may be a boiler replacement rather than a repair. If sludge has damaged adjoining components, the heat exchanger or pump may also need attention.
Why has my boiler locked out with no fault code?
Direct answer. A lockout with no displayed code usually means the appliance has hit a safety threshold and shut itself down, but the diagnostic display either does not show codes, has failed, or has cycled through the code too quickly to read.
Most common cause. On older boilers, a failing flame sensor or thermocouple. On newer boilers, an intermittent fault on the printed circuit board, a flue sensor issue, or a fan fault that only triggers under load.
What you can check
Try a single reset following the manual. Do not repeatedly reset the appliance.
Note the time of day and what was happening at the moment of the lockout. Cold start, hot water demand, heating call.
If your boiler has an accessible error history in the user menu, check it.
When to call us. Two consecutive lockouts on the same day, or any lockout you cannot clear with a single reset, needs a Gas Safe engineer. Repeated reset attempts can damage the appliance and override the safety logic that the lockout enforces.
What changes the cost. A silent lockout is the hardest fault to price without seeing the boiler, because the cause could be anything from a flame sensor (cheaper) through a fan replacement or flue sensor (mid-range) to a full printed circuit board replacement (most expensive). The diagnostic visit identifies the actual fault and produces a fixed quote for the repair. We never start work without your sign-off on the price.
When should I stop checking and call a Gas Safe engineer?
Stop and call a Gas Safe Registered engineer in any of these situations:
You can smell gas
You hear gas hissing from a pipe, joint or appliance
You suspect carbon monoxide
You have reset the boiler twice without success
You see water inside the boiler casing
You have been topping up the pressure repeatedly to keep the system running
Smell gas?
Call the National Gas Emergency Service immediately on 0800 111 999. Open windows, leave the property, and do not switch on electrics or use a phone inside the building until the emergency service has cleared the building.
Suspect carbon monoxide?
Symptoms include headache, dizziness, nausea and tiredness, and they are commonly mistaken for the flu. If your carbon monoxide alarm sounds, treat it as an emergency. The NHS guidance on carbon monoxide poisoning is at nhs.uk/conditions/carbon-monoxide-poisoning. For symptoms, alarm placement and prevention, see our 5 essential tips for carbon monoxide safety in UK homes and our advice on how to know if your boiler is unsafe.
Verify your engineer
Before any gas work, check the engineer's registration on the Gas Safe Register at gassaferegister.co.uk and ask to see their ID card. DD Wilson's registration number is 583586. The Gas Safety (Installation and Use) Regulations 1998 make it a legal requirement for anyone working on gas in a UK home to be on the register. The Health and Safety Executive enforces the regulations and publishes guidance on what constitutes unregistered gas work and why it is dangerous. For DD Wilson's own position on gas safety, see our commitment to gas safety.
Still not sure what is wrong? Talk to a Gas Safe engineer.
Darren Wilson and the DD Wilson team have been diagnosing boiler faults across Liverpool, Wirral and Northwest England for 27 years. We are Gas Safe Registered (583586), Vaillant Advance Master Tech accredited, and an ATAG-approved installer. If anything on this list sounds like your fault, book a diagnostic call-out, and we will have an engineer with you the same day, where possible. Spreading the cost of a repair or upgrade is straightforward through our finance options, and you can read verified customer reviews before deciding.
Book online | Call 0151 739 8945 | Request a quick quote
Frequently Asked Questions
Why does my boiler keep losing pressure? Repeated pressure loss means water is leaving the system somewhere, or the expansion vessel has failed. Check for visible leaks at radiator valves and pipe joints. If you cannot find one and pressure keeps dropping after repressurisation, the leak is hidden and needs a Gas Safe engineer to trace.
What does F22 on a Vaillant boiler mean? F22 means low water pressure. The boiler has detected system pressure below the normal operating range of 1.0 to 1.5 bar and has locked out for safety. The specific lockout figure varies by model. Repressurise via the filling loop if you are comfortable with it. If pressure keeps dropping, call an engineer.
Can I fix a boiler fault myself in the UK? You can carry out non-gas checks, such as reading the pressure gauge, bleeding radiators, and thawing a frozen condensate pipe with warm water. Anything inside the sealed boiler casing, anything involving gas pipework, and any work on the gas supply is unlawful for an unregistered person to carry out under the Gas Safety (Installation and Use) Regulations 1998. We cover this in more detail in our piece on why DIY gas boiler installation is never a good idea.
How much does a boiler diagnostic call-out cost in Liverpool? DD Wilson charges a fixed, transparent fee for a diagnostic call-out across Liverpool, Wirral and Northwest England. The current fee is £96 and is the same whether the visit results in a quick fix on the day or a quote for a larger repair. There are no surprise charges. For the current fee or to book a visit, request a quick quote or call 0151 739 8945.
Is it safe to keep using my boiler if it has a fault code? It depends on the code. Pressure faults and condensate freezes are often safe to manage briefly while you book a Gas Safe engineer. Anything involving ignition failure, flame loss, gas valve operation or a carbon monoxide sensor means you should stop using the boiler immediately and call a Gas Safe Registered engineer the same day.
About the owner
Darren Wilson founded DD Wilson Gas & Heating Engineers in 1998. He is Gas Safe Registered (583586), a Vaillant Advance Master Tech, and ATAG accredited. DD Wilson holds 2,365+ customer reviews at an average of 4.9 stars, covering Liverpool, Wirral, Southport, St Helens, Warrington and the wider Northwest.
Sources and further reading
Gas Safety (Installation and Use) Regulations 1998: legislation.gov.uk
Gas Safe Register: gassaferegister.co.uk
Health and Safety Executive on gas safety: hse.gov.uk
NHS on carbon monoxide poisoning: nhs.uk/conditions/carbon-monoxide-poisoning
National Gas Emergency Service: 0800 111 999 (verified current at nationalgas.com/emergency-contacts, May 2026)
Manufacturer fault code documentation: Vaillant, Worcester Bosch, Ideal Heating, Baxi (publicly available technical literature)
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