Error E110

Safety precautions:

1) If you smell gas, evacuate the property, call your gas emergency number and do not operate the boiler. 2) Switch the boiler off at the mains and allow it to cool before touching pipework or the boiler casing. 3) Do not open the boiler casing or attempt any gas/electrical component repairs unless you are a Gas Safe registered engineer. 4) Wear gloves/eye protection when bleeding radiators to avoid scalding; have a cloth/container ready.

Initial homeowner checks (safe, simple steps):

1) Note the exact display and any preceding events. Try a single reset following the boiler manual and see if the code returns. If it resets and stays off, monitor the boiler closely. 2) Check the system pressure gauge. If it is below ~1.0 bar this is a common cause of overheating; the correct range is typically 1.0–1.5 bar. If pressure is low and you are comfortable doing so, top up using the filling loop: open the filling loop valves slowly until the gauge reaches about 1.2 bar, then close both valves and remove/secure the filling loop per the manual. 3) Bleed radiators to remove trapped air: start with the radiator lowest in the system or those that are cold at the top. Turn the bleed valve until air stops and water flows, then check pressure again. 4) Check that external controls/thermostats are calling correctly and that no radiator or system valves are accidentally closed (in particular any system isolation or bulkhead valves). 5) Listen for the pump running (a light vibration/noise through the casing when heating is on) — if the pump appears dead or very noisy this may indicate a pump problem.

Further diagnostic and fix steps (for homeowners to observe or for engineers):

1) If initial checks don’t clear the fault, do not keep repeatedly resetting the boiler. Allow it to cool and document conditions (pressure, noises, whether there was a heating or hot water demand when it tripped). 2) An engineer will check continuity of the overheat (safety) thermostat and confirm whether it has operated (it is often a non-resetting or auto-reset device that may need replacement if opened). 3) Check temperature sensor (NTC) readings and connections — a faulty sensor can falsely report overheat. Replace or re-seat sensors only by a qualified engineer. 4) Check circulation: confirm pump operation, unblock or replace if seized; inspect diverter valve operation if combi, as a stuck diverter can cause poor circulation. 5) Inspect the primary heat exchanger and system pipework for blockages, debris or scale restricting flow; flush or chemically clean the system as needed (engineer task). 6) Check for airlocks and ensure the system has been correctly bled and repressurised. 7) Verify gas supply and gas valve operation only by a Gas Safe engineer if combustion irregularities are suspected. 8) After any corrective action, the engineer will reset the boiler and monitor temperatures and flow to ensure the overheat condition does not recur.

When to call a professional:

1) Call a Gas Safe registered engineer if the E110 returns after simple homeowner checks (reset, bleeding, repressurising). 2) Call immediately for gas smells, leaks, loud banging/gurgling that you cannot attribute to trapped air, repeated trips, suspected pump/diverter/valve failures, faulty sensors, or if you’re unsure about topping up pressure or bleeding. 3) Any internal work, replacement of thermostats/sensors, pump, diverter, gas valve, or heat-exchanger inspection/cleaning must be carried out by a qualified engineer.

After any repair or correction, monitor the boiler for a few heating cycles to ensure the fault does not recur. If E110 appears again, record the conditions (system pressure, which demand was on, any noises) and provide that information to the engineer to help with diagnosis.