Baxi E110 Fault Code: Boiler Overheat Safety Lockout
The E110 code means your Baxi boiler's safety thermostat has detected that the primary water temperature has climbed beyond its safe upper limit — typically somewhere in the 90–100°C range — and has shut the boiler down as a protective measure. The boiler will not fire again until the underlying cause is identified and resolved. This is a hard lockout, meaning a simple reset alone is unlikely to provide a lasting fix unless the root cause is addressed first.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Baxi E110 fault code mean?
The E110 code means your Baxi boiler's safety thermostat has detected that the primary water temperature has climbed beyond its safe upper limit — typically somewhere in the 90–100°C range — and has shut the boiler down as a protective measure. The boiler will not fire again until the underlying cause is identified and resolved. This is a hard lockout, meaning a simple reset alone is unlikely to provide a lasting fix unless the root cause is addressed first.
Common causes
- Low system pressure Common
When system pressure drops below around 1.0 bar, there simply isn't enough water circulating through the heat exchanger to carry heat away efficiently. Hot spots develop and the safety thermostat trips. Pressure can drop gradually due to small leaks, natural system evaporation over time, or pressure loss following radiator work.
- Faulty or seized circulation pump Common
The pump is responsible for keeping water moving constantly through the system. If it has seized, is running at reduced speed, or a stuck diverter valve is blocking flow, heat cannot be distributed away from the heat exchanger quickly enough. Overheating follows within minutes.
- Blocked or scaled heat exchanger Common
In hard-water areas, limescale accumulates inside the heat exchanger over time, narrowing the waterways. Sludge from an unfiltered system has the same effect. The restricted flow means heat cannot transfer properly, causing localised overheating even at normal boiler settings.
- Air locks in the system Sometimes
Pockets of trapped air interrupt the smooth circulation of water, creating areas where flow stalls. The boiler continues to fire but heat cannot escape those stagnant sections, raising temperatures enough to trigger the safety thermostat.
- Faulty NTC temperature sensor Sometimes
If the flow or return NTC sensor has drifted out of calibration or developed an intermittent fault, it may send readings to the PCB that indicate overheating even when water temperatures are actually within normal range. The result is a nuisance E110 trip with no genuine overheat present.
- Faulty safety thermostat Sometimes
The overheat thermostat itself can wear and begin to trip prematurely, particularly on older boilers. Poor flow conditions elsewhere in the system can accelerate its degradation. When this component fails it will log E110 regardless of actual water temperature.
- Gas supply or combustion issue Rare
A disrupted or restricted gas supply can cause irregular combustion, leading to uneven heat release inside the burner. In some cases this produces localised temperature spikes that register as an overheat condition, even though the broader system pressure and flow appear normal.
How to fix it
- Let the boiler cool down completely before doing anything else DIY safe
Do not attempt to reset an E110 straight away. The safety thermostat needs time to cool and reset itself naturally. Switch the boiler off at the programmer or room thermostat and wait at least 20–30 minutes before proceeding.
- Check your system pressure on the front gauge DIY safe
The pressure gauge on your Baxi boiler should read between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, use the filling loop (usually a silver or grey braided flexible hose beneath the boiler) to top up to around 1.2–1.3 bar. Open the valve slowly, watch the gauge, and close it once you reach the target. If you're not sure how to locate the filling loop, check your boiler's user guide.
- Bleed your radiators to release trapped air DIY safe
If you have cold spots at the top of radiators, or the system has been recently drained, air may be locked in the circuit. Use a radiator bleed key to open each radiator bleed valve in turn (starting upstairs), release the air until water appears, then close the valve. Re-check system pressure afterwards and top up again if it has dropped.
- Attempt a single boiler reset DIY safe
Once the boiler has cooled and you have corrected any pressure issue, press and hold the reset button on the control panel for approximately 3–5 seconds, then release. Allow the boiler a few minutes to complete its ignition sequence. If the E110 returns immediately or comes back within a short period, do not reset again — repeated resets on an overheating boiler risk damaging internal components.
- If the fault persists, contact a Gas Safe registered engineer Gas Safe engineer
A returning E110 after a cool-down and reset points to a mechanical or component fault that requires professional diagnosis. An engineer will carry out flow-rate testing, check the circulation pump and diverter valve, inspect the heat exchanger for scale or sludge, and test the NTC sensors and safety thermostat with appropriate tools. Do not open the boiler casing yourself — all internal work must be carried out by a Gas Safe registered engineer.
Parts you may need
- Circulation pump · from £85
- NTC flow/return temperature sensor · from £25
- Safety overheat thermostat · from £20
- Diverter valve motor/cartridge · from £45
- Heat exchanger (primary) · from £200
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£450, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler to clear the E110?
We'd strongly advise against it. A single reset after a cool-down is reasonable, but if E110 comes back, resetting again only masks the problem. The boiler is shutting down because water inside it is getting dangerously hot. Repeatedly forcing it back on without fixing the cause risks damaging the heat exchanger — a repair that can cost £400–£700 or more — and could create a genuine safety hazard.
How much does it cost to fix a Baxi E110 in the UK?
Most homeowners pay somewhere between £150 and £450 depending on what has failed. A pressure top-up or bleed carried out during a call-out might cost £80–£150 just for the engineer's time. Replacing a pump, sensor, or diverter valve typically falls in the £200–£350 range including parts and labour. A heavily scaled heat exchanger requiring a power flush or replacement can push costs toward £450 or beyond — and in rare cases a full heat exchanger swap can exceed £600, though this is uncommon on boilers that have been regularly serviced.
Why does my Baxi keep showing E110 even though the pressure looks fine?
Low pressure is the most common cause, but not the only one. If pressure is correct and the fault keeps returning, the circulation pump may be failing to move water quickly enough, there could be sludge restricting flow through the heat exchanger, or the NTC temperature sensor may be sending false readings to the PCB. An engineer can run flow-rate checks and sensor tests to pinpoint which component is at fault.
Will an annual boiler service help prevent E110?
Yes — it's one of the most cost-effective ways to avoid this fault. During a service, an engineer will check water flow rates, inspect the pump, clean or flag a scaled heat exchanger, verify sensor accuracy, and check inhibitor levels in the system water. Fitting a magnetic system filter (if you don't already have one) is also highly recommended, as it catches sludge before it can restrict flow and cause overheating.