Boiler
E18
The E18 error code on a Glow-worm boiler indicates a flow temperature sensor fault — specifically that the boiler is overheating or detecting an excessively high water temperature. This is a safety lockout, and your boiler will not fire until the underlying cause is resolved.
What does this error mean?
Your Glow-worm boiler constantly monitors the temperature of water flowing through the heat exchanger. When that temperature exceeds a safe threshold — or when the sensor reading the temperature behaves erratically — the boiler triggers an E18 lockout to prevent damage or danger. In plain terms: the boiler thinks it’s getting too hot and has shut itself down as a precaution.
Est. repair cost
£80–£300 for most common repairs (thermistor or pump); up to £600 for heat exchanger work
🔍 Likely causes
🛠️ Step-by-step fix
Check your boiler’s pressure gauge — if it reads below 1 bar, repressurise the system using the filling loop until it reaches 1.0–1.5 bar, then attempt a reset by pressing and holding the reset button for 3 seconds.
Check that all thermostatic radiator valves (TRVs) and lockshield valves throughout your home are fully open — closed valves restrict flow and trigger overheating lockouts.
Bleed your radiators to remove any trapped air, which can impede circulation and cause the boiler to overheat.
After completing the above checks, attempt a boiler reset. If the E18 clears and the boiler fires normally, monitor it over the next 24 hours to confirm it doesn’t return.
If the error returns within a short time or immediately after reset, do not repeatedly attempt to reset the boiler — this is a gas appliance safety lockout and further investigation requires a Gas Safe registered engineer.
Do not attempt to inspect, replace, or adjust the thermistor, pump, heat exchanger, or PCB yourself — these tasks are either unsafe or legally require a qualified professional on a gas appliance.
📞 When to call a professional
If the E18 error returns after a reset, or if pressure and valve checks don’t resolve it, call a Gas Safe registered engineer immediately. Tell them you have a Glow-worm boiler showing an E18 lockout and that you suspect a flow temperature sensor fault or circulation issue — this helps them arrive prepared. Do not attempt more than two resets; repeated overheating can cause serious damage to the heat exchanger.
⚖️ Repair or replace?
A thermistor replacement typically costs £80–£150 including labour; a pump replacement runs £150–£300; heat exchanger descaling or replacement can reach £300–£600. If your Glow-worm boiler is over 10–12 years old and repair costs exceed £400, replacement with a modern A-rated combi boiler (typically £1,800–£2,800 fully installed) is the more cost-effective long-term choice. For boilers under 8 years old, repair is almost always the right call.