Boiler
E16
The E16 error code on a Glow-worm boiler indicates a flow temperature limiter lockout — the boiler has detected that water inside the heat exchanger has overheated and shut itself down as a safety measure. This is a fault you should take seriously and not repeatedly reset without investigating the cause.
What does this error mean?
Your Glow-worm boiler continuously monitors the temperature of water flowing through the heat exchanger. When that temperature exceeds its safe upper limit — typically around 95–100°C — the overheat thermostat (also called the high-limit stat) trips and triggers the E16 lockout. The boiler locks out to prevent damage to components and, more importantly, to keep your home safe. A single trip can be a one-off blip, but repeat lockouts always indicate an underlying problem.
Est. repair cost
£150–£300 for pump replacement; £300–£500 for a system power flush; £80–£150 for an overheat thermostat replacement
🔍 Likely causes
🛠️ Step-by-step fix
Check the pressure gauge on the boiler’s front panel. If it reads below 1 bar, repressurise the system using the filling loop until it reaches 1.5 bar, then press the reset button. Instructions for repressurising are in your Glow-worm manual.
Press the reset button (usually marked with a flame or reset symbol) once and wait 5–10 minutes for the boiler to attempt a restart. If it fires up and stays on, monitor it over the next hour.
Check that all radiator valves (thermostatic and lockshield) around your home are open and not accidentally closed — a fully closed valve on even one radiator can restrict system flow.
Bleed your radiators to release any trapped air. Start upstairs and work downwards. If a lot of air escapes, recheck and top up boiler pressure to 1.5 bar afterwards.
If the error returns after one reset, do not keep resetting the boiler — repeated resets on an overheating boiler can damage the heat exchanger. At this point, call a Gas Safe registered engineer.
Do NOT attempt to inspect, replace, or bypass the overheat thermostat, pump, or heat exchanger yourself. These tasks involve gas components and require a Gas Safe registered engineer by law.
📞 When to call a professional
If the E16 error returns within a few hours of resetting, or if the boiler locks out repeatedly over several days, stop resetting it and call a Gas Safe registered engineer immediately. Tell them: ‘My Glow-worm boiler is showing an E16 overheat lockout and it keeps returning after reset.’ They will check the pump, overheat stat, heat exchanger flow rate, and system pressure to diagnose the root cause.
⚖️ Repair or replace?
A pump replacement typically costs £150–£300 including parts and labour; a power flush to clear a scaled heat exchanger costs £300–£500 depending on system size. If your Glow-worm boiler is over 10–12 years old and requires a major component repair, replacement with a modern A-rated combi boiler (£1,800–£3,000 installed) is often more cost-effective long term. For boilers under 8 years old, repair is almost always the right call.