Baxi Boiler Error Code 118: Causes & Fixes

As a seasoned boiler expert who’s tackled countless Baxi systems in homes and rentals across the UK, I’ve seen the Baxi boiler error code 118 pop up frequently—it’s that low pressure alert that can leave your heating lifeless, especially when the weather turns. 

If your Baxi combi or system boiler is displaying error code 118 and shutting down, this guide shares my practical knowledge to explain what it signals, the usual reasons, and step-by-step ways to sort it for homeowners and landlords alike.

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. 

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.

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

1. Inspect visible pipes, radiator valves, and joints for drips or damp spots—mark any suspects with tissue.
2. Tighten loose connections gently with a spanner if safe, avoiding over-tightening that could crack fittings.
3. Repressurise the system via the filling loop to 1.2 bar and reset the boiler to test.
4. 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

1. Confirm pressure is low on the gauge post-bleeding.
2. Use the filling loop to add water slowly until 1.2 bar, listening for flow.
3. Close valves securely and reset the boiler.
4. 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

1. Locate the external discharge pipe and check for dripping even when the boiler is off.
2. Repressurise and reset to see if the valve stops leaking.
3. If dripping continues, isolate the boiler and call a Gas Safe engineer to replace the valve, around £150-£250.
4. 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

1. Top up pressure via the loop to 1.2 bar and reset.
2. Monitor weekly, noting any slow decline.
3. Add system sealant chemicals to plug micro-escapes.
4. 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

1. Repressurise and reset multiple times to check if code clears.
2. Verify pressure manually if possible.
3. Gas Safe engineer cleans or replaces the sensor, £150-£300.
4. 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.