Overview
E110 on a Potterton Assure combi or system boiler indicates the overheat protection has tripped — the boiler has detected temperatures or a temperature rise (gradient) beyond its safe limit. The boiler will shut down to protect itself and the system. Common underlying causes are poor primary circulation (a jammed or seized pump), air trapped in the heating circuit causing localised overheating, blockages or sludge in the heat exchanger or pipework, or a safety thermostat/temperature sensor that has tripped or failed. Low system water pressure and flow-switch problems can also contribute to conditions that cause E110. Severity: this is a safety shutdown and should be treated seriously. It is designed to prevent damage and hazards, so the boiler should not be repeatedly forced to run while the fault remains. Some basic checks and simple corrective actions can be done by a competent homeowner, but because the causes often involve the pump, sensors, or gas and electrical systems, a Gas Safe registered engineer will usually be required if the fault is not cleared by simple steps or if it recurs. DIY vs professional: Carry out only safe, non-invasive checks (reset, check pressure, bleed radiators, ensure valves are open). Do not remove covers, work on gas, electrical parts or internal pump components — these require a qualified engineer. If the code persists after the basic checks, or if there are signs of leaks, overheating, flame or gas smells, or if you are unsure, turn the boiler off and call a Gas Safe engineer immediately.
Possible Cause: Boiler Max temperature exceeded for gradient (or overtemperature) Probable pump jammed or air in the circuit or Safety thermostat sensor tripped
Troubleshooting Steps
Safety precautions:
1) If you smell gas, or see flame problems or carbon monoxide alarm activation, evacuate the property and call the emergency gas number and/or Gas Safe immediately. Do not attempt repairs.
2) Before any checks, turn off the boiler and allow it to cool for at least 20–30 minutes. Avoid touching hot components. If you need to isolate the boiler, switch off the electrical supply at the isolator and do not tamper with gas connections.
3) Do not remove boiler covers or attempt internal electrical or gas work. Those tasks require a qualified engineer.
Initial homeowner checks (safe to perform):
1) Note the fault: write down the code, when it appeared and any recent symptoms (loud noises, no heating, repeated lockouts).
2) Attempt a single reset: follow the boiler’s reset procedure (usually hold the reset button for 3–5 seconds) and see if the boiler restarts and runs normally. Do not keep resetting if the code returns immediately or repeatedly.
3) Check system pressure on the boiler gauge. Normal cold pressure is typically about 1.0–1.5 bar. If the pressure is significantly low, top up using the filling loop to around 1.0–1.5 bar per the user manual. If pressure is very low or keeps dropping, do not continue — call an engineer.
4) Bleed radiators to remove trapped air: working from the lowest to highest radiator, use a radiator key to release air until water flows, then recheck system pressure and repressurise if needed.
5) Ensure all valves are open: check the boiler’s flow and return isolating valves (if accessible externally) and thermostatic radiator valves are open. Check the external pump isolation valves are not closed by accident.
Specific diagnostic and fix steps (homeowner-friendly):
1) After bleeding and repressurising, try restarting once. If E110 cleared and boiler runs normally, monitor for recurrence over the next 24–48 hours. If it returns, stop further DIY attempts and call a pro.
2) Listen and feel for pump operation: with the boiler on, place your hand (carefully, when cool enough) on the pipes either side of the pump or listen for a soft hum. A seized/jammed pump may be very quiet or make grinding noises. Do not try to dismantle the pump. If the pump appears not to be operating or makes unusual noises, call an engineer.
3) Check for obvious blockages/leaks: inspect visible pipework and the condensate/drain area for blockages or frozen condensate pipe in cold weather. Clear external condensate pipe obstructions (if safely accessible) but do not force internal parts.
4) Avoid repeated resets: if E110 reappears after one reset and the safe checks above, do not keep resetting the boiler. Continued resets can mask an underlying fault and may cause further damage.
When to call a professional and what the engineer will likely do:
1) Call a Gas Safe registered engineer if E110 persists after the basic checks, if the pump seems seized, if system pressure cannot be maintained, if there are signs of leaks, or if you detect any gas or combustion issues.
2) The engineer will perform safe live diagnostics: check flow and return temperatures, measure pump current and rotation, test NTC sensors and safety thermostat, inspect the heat exchanger for scale/sludge, check the flow switch and pressure sensors, and review PCB fault history. They may replace or service the pump, purge or powerflush the system, replace faulty sensors or the safety thermostat, or carry out repairs to the primary circulation circuit.
3) If your boiler is under warranty or service contract, quote that to the engineer. Keep a record of error occurrences and the steps you have already taken to help diagnosis.
Final note: E110 is a protection fault that indicates the boiler has experienced unsafe temperature conditions. Simple maintenance like repressurising and bleeding radiators will clear many transient causes, but persistent or repeated E110 faults must be investigated and repaired by a Gas Safe qualified heating engineer to ensure safe, reliable operation.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Assure Combi and system.