Overview
E110 on a Baxi Duo-tec / Megaflo means the boiler has detected an overheat condition in the primary water circuit and has shut down automatically. This is a safety trip: the boiler’s overheat thermostat or control electronics have decided temperatures have risen above safe limits, typically around the overheat thermostat opening point (often near 100–105°C on many units). The display forces a lockout to prevent damage to the heat exchanger, pump, seals and to reduce any safety risk to occupants. There are several common reasons the overheat condition is reported: low system pressure or airlocks that prevent circulation, a seized or failing circulation pump, a stuck or faulty diverter valve, blockages or scale in the heat exchanger, faulty temperature sensors or the overheat thermostat itself, or problems with the gas/combustion causing abnormal behaviour. If the code appears once and clears after a safe reset it may be a transient fault; if it repeats, the cause is likely mechanical or electrical and needs investigation. Some basic checks are safe for a competent homeowner (reset, check pressure, bleed radiators), but most internal repairs or anything involving gas work, electrical testing, or replacing overheat thermostats, sensors, pump or diverter must be done by a Gas Safe registered engineer.
Possible Cause: Boiler overheat error
Troubleshooting Steps
Safety precautions:
1. If you smell gas, leave the property immediately, do not operate electrical switches or the boiler, ventilate and call the gas emergency service and a Gas Safe engineer.
2. Before any checks, allow the boiler to cool fully; components and plumbing will be hot and can cause scalding.
3. Do not remove the boiler casing, attempt internal repairs, or bypass any safety device. Any work on gas or sealed components must be carried out by a Gas Safe registered engineer.
Initial homeowner checks (safe, simple steps):
1. Note the exact display and any other symbols. Try a single reset using the method in the boiler manual. Wait and see if the boiler runs; if the code returns immediately or soon after, do not keep resetting.
2. Check the system pressure on the gauge when the boiler is cold. If pressure is below about 1.0 bar, top up using the filling loop until the gauge reads approximately 1.0–1.5 bar, then close the filling loop valves. Do this only if you know how and are confident; do not over-pressurise the system.
3. Look and feel for visible water leaks around the boiler, pipework and radiators. Persistent pressure loss indicates a leak and requires a professional.
4. Bleed radiators to remove trapped air: with the heating off and boiler cool, use a radiator key to open bleed valves until a steady flow of water appears, then close. This can remove airlocks that impede circulation.
5. Listen to the boiler when a heat demand is made (turn up thermostat or open a hot tap): can you hear the pump running? Strong knocking/gurgling or silence when it should be running can indicate pump or circulation problems.
Diagnostic and next-step actions (what the homeowner can check/prepare, and what a pro will do):
1. If low pressure or air was the issue, after topping up and bleeding radiators, reset the boiler and observe. If pressure rapidly falls again or the E110 returns, do not attempt internal repairs—call a Gas Safe engineer.
2. Check that all external valves are open (e.g., system isolation valves, filling loop closed after topping up). Ensure the boiler programmer/controls are set correctly and transport/lockshield valves on radiators are not shut.
3. If there is evidence the pump is not running (no noise when heating demand exists) or it runs continuously and cannot control temperature, the pump may be faulty or the PCB/pump wiring is at fault; this requires an engineer to test, free or replace the pump.
4. If you have regular scale or hard-water issues, or if radiators are cold at the bottom, the heat exchanger may be partially blocked; only an engineer should inspect and, if needed, chemically flush or replace the exchanger.
5. Recurrent E110 with no obvious circulation fault often means the overheat thermostat or an NTC temperature sensor has failed. An engineer will isolate power, allow the boiler to cool, test continuity/resistance of the safety thermostat and sensors, and replace defective parts.
6. If there are unusual noises (banging, gurgling) or the boiler runs but will not fire (flame symbol with a cross), the engineer will also check the diverter valve, condensate drains and the gas supply/combustion components.
When to call a professional:
1. Call a Gas Safe registered engineer immediately if the fault returns after basic checks, if you find a leak, if the pump appears faulty, if you suspect a blocked heat exchanger, or if any internal component, sensor or thermostat needs testing or replacement.
2. Do not attempt any gas valve, burner, combustion or wiring work yourself. Repeated overheat trips indicate an underlying fault that, if ignored, can lead to more serious damage.
Summary reminder: E110 is the boiler protecting itself from overheating. Safe homeowner actions are limited to cooling the appliance, resetting once, checking and topping up system pressure, bleeding radiators and checking for obvious leaks or closed valves. Persistent, repeating or unclear causes always need a Gas Safe engineer to diagnose and repair safely.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi Duo-tec/Megaflo.