The Baxi error code 118 is one of the most common faults reported by Baxi boiler owners across the UK, and the good news is that it is also one of the more straightforward to understand. It is a low water pressure warning: your boiler has detected that pressure has dropped below the safe operating threshold and has shut itself down to avoid running dry and damaging internal components.
You will most likely see it displayed as E118 on the front panel, often alongside a pressure gauge reading well below 1.0 bar. Cold radiators, lukewarm taps, and a boiler that refuses to ignite are the usual signs that this fault is active.
This guide explains what causes the pressure drop, what you can safely fix yourself, and when to call a Gas Safe registered engineer. If you want context on how the 118 fits into the wider Baxi fault code range, the baxi boiler troubleshooting tips guide covers related faults in detail.
For anyone considering whether the fault signals something more serious about the boiler’s age or efficiency, the Baxi boiler energy efficiency ratings comparison and the Baxi boiler pricing and models guide are both useful references when weighing up repair versus replacement.
Watch Our Latest Video Guide About It Here:
What Does the Baxi Boiler Error Code 118 Mean?
Error code 118 (also shown as E118) means the water pressure in your sealed heating circuit has dropped below approximately 0.5 to 1.0 bar, depending on the model. At that level, the boiler cannot circulate water safely and triggers a protective lockout to prevent damage to the heat exchanger and pump.
A healthy Baxi boiler should maintain pressure between 1.0 and 1.5 bar when the system is cold. Once it drops below that range, performance suffers. Below 0.5 bar, most Baxi models including the 600 and 800 series will lock out entirely and display the 118 or E118 code.
The boiler will not restart until pressure is restored to a safe level. Simply pressing reset without topping up the system first will not clear the fault.
Signs You Are Dealing with Error Code 118
- The display panel shows 118 or E118, sometimes flashing
- The pressure gauge reads below 1.0 bar, often in the red zone
- Radiators are cold or only partially warming
- Taps are producing lukewarm water or no hot water at all
- The boiler fires briefly then cuts out, or will not ignite at all
- Some models show an additional low pressure warning light on the panel
What Causes the Baxi Boiler Error Code 118?
The 118 fault is always triggered by insufficient water in the sealed heating circuit. The table below summarises the main causes at a glance before each one is covered in detail.
| Cause | How Pressure Drops | DIY or Engineer? |
|---|---|---|
| System leak | Water escapes slowly from joints or valves | Engineer if hidden; DIY visible minor drips |
| Radiator bleeding | Air released takes water with it | DIY: repressurise via filling loop |
| Faulty pressure relief valve | Valve discharges water unnecessarily | Engineer: replace valve |
| Expansion vessel fault | Vessel cannot absorb pressure changes | Engineer: recharge or replace vessel |
| Natural evaporation/loss | Gradual micro-losses over months | DIY: top up; engineer if recurring |
| Faulty pressure sensor | False low reading despite adequate water | Engineer: clean or replace sensor |
The baxi boiler 117 error code solutions guide is worth reading alongside this one, as the 117 and 118 faults are closely related and can sometimes appear together or in sequence when pressure issues are intermittent.
System Leaks
A slow leak somewhere in the heating circuit is the most common cause of the 118 fault. Water escapes gradually from corroded pipework, loose radiator valve connections, or failing seals at boiler joints. Because the loss is slow, many homeowners do not notice it until the pressure has already dropped far enough to trigger a lockout.
Older properties with copper pipework are more susceptible to pinhole leaks and corrosion at soldered joints. Some leaks occur inside walls or under floors where they are not visible without specialist equipment.
How to Fix System Leaks
- Check all visible pipework, radiator valves, and boiler connections for damp patches, rust staining, or drips. A dry piece of tissue pressed against joints can help reveal very slow seeps.
- Gently tighten any loose connections with a spanner. Do not over-tighten, particularly on older copper fittings, as this can crack the joint.
- Repressurise the system to 1.2 bar via the filling loop and reset the boiler.
- Monitor the pressure gauge over the next few days. If it drops again quickly (within 24 to 48 hours), there is an active leak that needs tracing. Do not keep topping up: call a Gas Safe engineer to locate and repair it.
Professional leak detection typically costs between £100 and £300 including parts such as replacement seals, depending on how accessible the leak is.
Recent Radiator Bleeding
Bleeding a radiator releases trapped air from the system, but some water comes out with it. If you have bled several radiators without topping up the pressure afterwards, a 118 fault is the likely result. This is one of the easiest causes to fix yourself.
How to Fix It After Radiator Bleeding
- Confirm the pressure gauge is reading below 1.0 bar.
- Locate the filling loop, usually a flexible braided hose beneath the boiler with two valves. Open both valves slowly and allow water to enter the system.
- Watch the gauge and close both valves once it reaches 1.2 bar. Do not overfill: going above 2.0 bar can trigger the pressure relief valve to discharge.
- Reset the boiler and run a full heating cycle to confirm even warmth across all radiators.
- Consider adding a corrosion inhibitor to the system water if this has not been done recently. This reduces future air formation and debris build-up.
Faulty Pressure Relief Valve
The pressure relief valve (PRV) is a safety device that opens to release water if pressure climbs too high, typically above 3 bar. When the valve wears out or debris holds it slightly open, it can discharge water continuously even at normal operating pressures, causing a slow but steady pressure drop.
A tell-tale sign of a weeping PRV is a damp patch or steady drip from the external discharge pipe, which exits through an outside wall or terminates above a drain.
How to Fix a Faulty Pressure Relief Valve
- Check the discharge pipe outside the property. A drip when the boiler is running and has not been overpressurised usually points to a failing valve.
- Repressurise and reset to see whether the dripping stops. If it continues, the valve is not seating correctly.
- Isolate the boiler and call a Gas Safe engineer to replace the valve. Replacement typically costs £150 to £250 including parts and labour.
- After replacement, monitor pressure for several days to confirm the loss has stopped.
Expansion Vessel Fault
The expansion vessel absorbs the increase in water volume that occurs as the system heats up. If the vessel loses its charge pressure or the internal diaphragm fails, it cannot perform this function. The result is pressure swings: the PRV may discharge at high temperatures, leaving the system low on water when it cools.
This fault is often overlooked because the system appears to hold pressure when cold but loses it repeatedly after a heating cycle. If that pattern applies to your boiler, the expansion vessel is worth checking.
How to Fix an Expansion Vessel Fault
- This is not a DIY repair. A Gas Safe engineer can test the vessel’s charge pressure using a simple gauge. If it has lost pressure, re-gassing the vessel may be possible.
- If the internal diaphragm has failed, the vessel will need replacing. This is a common repair on boilers over 8 to 10 years old.
- Typical cost for expansion vessel repair or replacement is £150 to £300 including labour.
Evaporation and Natural Pressure Loss
Over many months, sealed systems can lose very small amounts of water through micro-pores in seals and fittings. This gradual loss is normal to a degree, but if your boiler needs topping up more than once or twice a year, it suggests there is a contributing fault beyond natural evaporation.
How to Fix Natural Pressure Loss
- Top up to 1.2 bar via the filling loop and reset the boiler.
- Note the date and monitor weekly. A healthy sealed system should hold pressure for months between top-ups.
- If pressure drops monthly or more frequently, add a system sealant such as Fernox F4 to plug micro-leaks. This should be done alongside, not instead of, investigating any underlying fault.
- At the next annual service, ask the engineer to check seal condition and expansion vessel charge.
Faulty Pressure Sensor
Occasionally the 118 fault is triggered not by an actual pressure drop, but by a failing pressure sensor that is sending an inaccurate reading to the PCB. The boiler acts on the false reading and locks out, even though the system actually contains adequate water.
This is less common than a genuine pressure loss, but it is worth suspecting if the gauge appears normal and you cannot find any source of water loss. Scale build-up around the sensor or a loose wiring connection are the usual culprits.
How to Fix a Faulty Pressure Sensor
- Repressurise to 1.2 bar and reset. If the code clears and holds, a momentary sensor glitch may have been the cause.
- If the 118 code returns despite the gauge showing an adequate reading, contact a Gas Safe engineer. They can verify actual pressure using an independent gauge and test the sensor’s output.
- Sensor cleaning or replacement typically costs £150 to £300 including parts. A system flush may also be recommended to remove scale that could be affecting the sensor.
How to Repressurise a Baxi Boiler: Quick Steps
Most 118 faults can be cleared with a straightforward pressure top-up. Here is the process that applies to most Baxi combi and system boiler models.
- Turn the boiler off at the controls and allow it to cool for around 10 minutes.
- Locate the filling loop. On most Baxi models this is a flexible braided hose beneath the boiler with a valve at each end, or sometimes a keyless filling link. Refer to your user manual if you cannot find it.
- Open both filling loop valves slowly. You should hear water entering the system.
- Watch the pressure gauge and close both valves as soon as it reaches 1.2 bar. Do not exceed 1.5 bar.
- Turn the boiler back on. The 118 code should clear automatically once pressure is within range.
- If the code does not clear, press the reset button once and wait for the boiler to complete its start-up sequence.
If pressure drops again within a day or two, stop topping up and call a Gas Safe engineer. Repeated repressurising masks the underlying fault and can allow a slow leak to cause greater damage over time.
When Should You Get a New Boiler Quote?
Most 118 faults are a repair job, not a reason to replace the boiler. That said, if your Baxi is over 10 to 12 years old and the 118 fault is appearing alongside other recurring issues, the economics of ongoing repairs can shift in favour of a replacement.
A modern condensing combi boiler is considerably more energy efficient than a unit from a decade ago. The Baxi boiler efficiency ratings and reviews guide covers how current Baxi models compare against other leading brands on efficiency and running costs.
Get in touch with the Boilers 2 Go team for a new boiler quote. We cover installations across the UK, often available the next day.
FAQs
How does seasonal maintenance prevent Baxi error code 118?
A pre-winter check by a Gas Safe engineer includes a full pressure test, visual inspection for leaks, and a check of the expansion vessel charge. Catching a small pressure drop or a weeping valve before the heating season starts means you are less likely to face a 118 lockout on a cold January morning when demand on the boiler is at its highest.
What accessories help monitor Baxi boiler pressure remotely?
Smart thermostats with boiler monitoring functionality, such as Hive and Nest, can alert you to heating system irregularities that may indicate pressure loss. Some third-party smart pressure gauges can also be added to the system to send notifications if pressure drops below a set threshold. This is particularly useful for landlords who cannot check the boiler in person regularly.
Are there UK incentives for upgrading from a faulty Baxi boiler?
The UK government’s ECO4 scheme (Energy Company Obligation) provides funding for boiler replacements and heating improvements for eligible low-income households. If your Baxi is old, inefficient, and generating frequent fault codes, it is worth checking eligibility via your energy supplier. Some local authorities also offer additional support through the Great British Insulation Scheme.
How do pipe materials affect error code 118 frequency?
Older copper pipework is more prone to pinhole corrosion and joint failure over time, particularly in areas with aggressive water chemistry or where the system has never been treated with inhibitor. Modern plastic (PEX or polybutylene) pipework is more resistant to corrosion, though it is not immune to joint failures at compression fittings. If your property has a mix of old copper and newer plastic pipework, the joints between the two materials are worth checking first.
Can adding chemicals stop recurring 118 codes?
System sealants such as Fernox F4 or Sentinel X100 can plug very small leaks and reduce micro-losses, which may extend the time between top-ups. They are not a substitute for fixing a genuine leak: if your system is losing pressure quickly, a sealant will not hold it. Used as a preventive measure during annual servicing, alongside a corrosion inhibitor, they do help reduce the frequency of pressure-related faults in well-maintained systems.
Is the 118 fault the same as the 117 fault on Baxi boilers?
They are related but not identical. Error 117 typically indicates that pressure is low but has not yet hit the lockout threshold, acting as an early warning. Error 118 means pressure has dropped to the point where the boiler has shut down entirely. The baxi boiler 117 error code solutions guide covers the 117 fault in detail and explains the steps to take before it escalates to a full 118 lockout.
Watch Our Latest Video Guide About It Here:
What Does the Baxi Boiler Error Code 118 Mean?
The Baxi boiler error code 118 means your system’s water pressure has fallen too low, typically below 0.5-1 bar, prompting a safety lockout to avoid running dry and damaging parts. To ensure optimal performance and prevent issues like the error code 118, it’s important to consider Baxi boiler efficiency ratings and reviews when selecting a model. Higher efficiency ratings can lead to significant energy savings and a more reliable heating system. Homeowners should also pay attention to user feedback on various Baxi models to make the most informed choice.
In my experience with Baxi models like the 600 or 800 series, this code—sometimes shown as E118—protects the heat exchanger and pump by halting operation until pressure is restored, often resulting in no heating or hot water.
Signs You’re Dealing with Error Code 118
You’ll notice cold radiators, lukewarm taps, or the boiler refusing to ignite, with the display flashing 118 alongside a low pressure warning—common in UK properties where minor drips go unnoticed.
What Causes the Baxi Boiler Error Code 118?
The Baxi boiler error code 118 is caused by insufficient water in the sealed heating circuit, disrupting circulation and safety checks. From my fixes in various setups, this stems from gradual loss rather than sudden failure; here’s the breakdown of main reasons with targeted steps. When troubleshooting issues, understanding the baxi boiler 117 error code solutions can help in quickly identifying the problem. Ensuring that the pressure levels are adequate and that there are no blockages in the system can prevent the recurrence of this error. Regular maintenance checks are also essential to keep the boiler running efficiently and minimize future disruptions. In addition to the baxi boiler troubleshooting tips, familiarizing yourself with how to bleed radiators can enhance overall system efficiency. Furthermore, checking the thermostat settings and ensuring that they correspond with your desired temperatures will help in optimizing performance. Keeping an eye on the age of the boiler is also wise, as older models may require more frequent checks and potential upgrades.
System Leaks
System leaks cause error code 118 by slowly draining water from pipes, radiators, or joints, dropping pressure over time. Why it’s caused includes corrosion in older UK plumbing, loose fittings from vibrations, or faulty valves in high-use homes.
How to Fix System Leaks
- Inspect visible pipes, radiator valves, and joints for drips or damp spots—mark any suspects with tissue.
- Tighten loose connections gently with a spanner if safe, avoiding over-tightening that could crack fittings.
- Repressurise the system via the filling loop to 1.2 bar and reset the boiler to test.
- If leaks persist or are hidden, call a Gas Safe engineer to trace and repair, usually £100-£300 including parts like seals.
Recent Radiator Bleeding
Recent radiator bleeding causes error code 118 by releasing water along with air, unintentionally lowering overall pressure if not topped up afterwards. This happens after DIY maintenance to remove airlocks, common when prepping for winter in UK households.
How to Fix Recent Radiator Bleeding
- Confirm pressure is low on the gauge post-bleeding.
- Use the filling loop to add water slowly until 1.2 bar, listening for flow.
- Close valves securely and reset the boiler.
- Run a heating cycle to check even warmth; add inhibitors to prevent future air buildup.
Faulty Pressure Relief Valve
A faulty pressure relief valve causes error code 118 by discharging water unnecessarily when pressure rises slightly, mimicking a leak. Why it’s caused is wear from age or debris jamming the valve in systems exposed to hard water minerals.
How to Fix Faulty Pressure Relief Valve
- Locate the external discharge pipe and check for dripping even when the boiler is off.
- Repressurise and reset to see if the valve stops leaking.
- If dripping continues, isolate the boiler and call a Gas Safe engineer to replace the valve, around £150-£250.
- Test post-replacement for stable pressure over days.
Evaporation or Natural Loss
Evaporation or natural loss causes error code 118 in unsealed or older systems where small amounts of water escape through vents or seals over months. This gradual drop is why it’s caused without obvious leaks, common in unmaintained UK boilers.
How to Fix Evaporation or Natural Loss
- Top up pressure via the loop to 1.2 bar and reset.
- Monitor weekly, noting any slow decline.
- Add system sealant chemicals to plug micro-escapes.
- Upgrade to a sealed system or add inhibitors during service for long-term stability.
Faulty Pressure Sensor
A faulty pressure sensor causes error code 118 by misreading actual levels, falsely signalling low pressure despite adequate water. Why it’s caused includes scale buildup on the sensor or electrical faults in humid environments.
How to Fix Faulty Pressure Sensor
- Repressurise and reset multiple times to check if code clears.
- Verify pressure manually if possible.
- Gas Safe engineer cleans or replaces the sensor, £150-£300.
- Flush system to remove scale affecting readings.
When Should You Get a New Boiler Quote?
If error code 118 returns frequently despite topping up, or your Baxi is ageing with other faults emerging, it’s time to consider a replacement for consistent performance and lower running costs.
Request a new boiler quote from Boiler 2 Go today!
FAQs
How does seasonal maintenance prevent Baxi error code 118?
Pre-winter checks top up pressure and spot leaks early, cutting 118 occurrences in cold months.
What accessories help monitor Baxi pressure remotely?
Smart gauges or apps alert to drops, ideal for absent landlords.
Are there UK incentives for upgrading from faulty Baxi models?
ECO grants aid efficient swaps if codes like 118 highlight waste.
How do pipe materials affect error code 118 frequency?
Older copper prone to pinholes spikes leaks—modern plastic reduces risks.
Can adding chemicals stop recurring 118 codes?
Sealants plug micro-leaks, extending time between top-ups in treated systems.
I’m Penny North, a home energy heating expert. My mission is to demystify new boilers and complex heating systems to help you achieve a warm, cosy home with lower energy bills and a smaller environmental footprint.