What the E110 code means
On most Baxi combi and system boilers, including the Baxi 100 and 600 series and older ranges such as the Duo-tec and Platinum, E110 is listed as "safety thermostat operated". The safety thermostat is a separate overheat cut-out fitted near the heat exchanger. The boiler's normal temperature sensors control the burner day to day. The safety thermostat only steps in if the water temperature goes past a fixed limit, and when it does, it shuts the burner off regardless of what the controls are asking for.
Some EcoBlue manuals add "pump fault" to the E110 description, which tells you where Baxi expects the problem to lie. In most cases the boiler overheats because heat is not being carried away fast enough, and that nearly always comes back to circulation.
The lockout itself is a safety feature working as designed. A single E110 after an unusual event, such as a power cut part way through a heating cycle, is not always a worry. Repeated E110 lockouts are a sign that something needs fixing, and running a boiler that keeps overheating can shorten the life of the heat exchanger and pump.
Common causes of E110
Pump not circulating
The pump pushes water through the boiler and round the radiators. If it has seized, is wearing out or has lost power, hot water sits in the heat exchanger and the temperature climbs quickly. Pumps often stick after a long idle spell, so E110 is common on the first cold days of autumn.
Air in the system
Air trapped in the pump or pipework stops water flowing properly. This often follows work on the heating system or a large loss of pressure.
Sludge and blockages
Older systems collect magnetite sludge, a black iron oxide deposit. It can restrict flow through the heat exchanger, the pump and any filters, which leads to hot spots and overheating.
Low pressure or closed valves
Very low pressure can starve the pump. Isolating valves under the boiler left closed after work, or most radiator valves being shut at once, can also restrict flow on some systems.
Faulty thermostat or sensor
Less often, the safety thermostat itself or a temperature sensor (NTC) fails and reports a problem that is not really there.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Low system pressure | Gauge below 1 bar when cold | You, via the filling loop |
| Air in the radiators | Cold at the top, gurgling pipes | You, by bleeding radiators |
| Seized or failing pump | Humming, radiators stay cold, E110 on start-up | Gas Safe engineer |
| Sludge or blocked heat exchanger | Cold spots on radiators, noisy boiler | Gas Safe engineer |
| Faulty safety thermostat or NTC | Circulation fine but E110 keeps returning | Gas Safe engineer |
Safe checks you can do yourself
- Switch the heating off at the controls and give the boiler 15 to 20 minutes to cool down.
- Check the pressure gauge. If it reads below 1 bar with the system cool, top up to 1 to 1.5 bar using the filling loop. Our guide on how to repressurise a boiler explains each step.
- If radiators are cold at the top, bleed them with the heating off, then check the pressure again and top up if it has dropped.
- Make sure at least a few radiators are turned on, so the heating has somewhere to send the water.
- Check your thermostat and timer settings are normal.
- Press reset as your manual describes. Stop after two or three attempts if E110 returns.
Do not remove the casing, tap the pump or try to release air from it yourself. The pump and safety thermostat sit inside the boiler next to live electrical parts and gas components, so only a Gas Safe registered engineer should work on them.
When to call a Gas Safe engineer
Call an engineer if E110 comes back after two or three resets, if the pressure is correct and the radiators are bled but the fault continues, or if you can hear the pump humming or grinding. The engineer will usually check that the pump is running and moving water, release any trapped air, test the safety thermostat and temperature sensors, and look for signs of sludge or a blocked heat exchanger.
If E110 appears alongside E125, the Baxi circulation fault, mention both codes. They often share a cause, and knowing both helps the engineer bring the right parts.
What to tell the engineer
Give the exact model, the code and how often it has appeared. Say whether it happens on heating, on hot water or both, whether you topped up or bled radiators beforehand, and whether any work has been done on the system recently. Mention any new noises too. Details like these help the engineer tell a pump problem from a sensor fault, which can save a second visit.
Typical repair costs in the UK
These ranges are typical only. Prices vary by region, model and the engineer.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Safety thermostat replacement | Typically £100 to £180 |
| Temperature sensor (NTC) replacement | Typically £90 to £160 |
| Pump replacement | Typically £250 to £450 fitted |
| Magnetic filter fitted | Typically £150 to £250 |
| System clean or powerflush | Typically £400 to £700 |
How to stop E110 coming back
Keep the pressure at 1 to 1.5 bar when cold and check it monthly. Run the heating for a short while once a month over the summer so the pump does not stick. If your system has no magnetic filter, ask your engineer whether one is worth fitting, particularly on an older system with cold spots on the radiators. An annual service gives an engineer the chance to check the pump and look for early signs of sludge. Our boiler service cost guide explains what a service includes.