Boiler
E9
The E9 error code on a Glow-worm boiler indicates an overheating (high limit thermostat) fault — the boiler has detected dangerously high water temperatures and has shut itself down as a safety measure. This is a serious fault that requires prompt attention and should not be ignored.
What does this error mean?
Your Glow-worm boiler has a built-in safety thermostat (the overheat thermostat or ‘STL’) that trips when the water temperature inside the heat exchanger exceeds a safe threshold — typically around 95–100°C. When this limit is breached, the boiler locks out and displays E9 to prevent damage to components or, in a worst case, a dangerous pressure event. The boiler will not restart normally until the underlying cause is resolved.
Est. repair cost
£150–£350 for most repairs (pump, thermostat, powerflush); £400–£600+ for heat exchanger replacement
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Let the boiler cool down: Do not attempt to restart the boiler immediately. Wait at least 20–30 minutes for it to cool fully before touching any controls.
Step 2 — Attempt a manual reset: Once cool, press and hold the reset button on the boiler fascia for 3–5 seconds (refer to your Glow-worm manual for exact reset procedure for your model). If the E9 returns within minutes, do not reset again — stop here.
Step 3 — Check all radiator and system valves are fully open: Locate your lockshield and thermostatic radiator valves (TRVs) and ensure none are fully closed, as restricted flow contributes to overheating.
Step 4 — Bleed your radiators: Use a radiator bleed key to release any trapped air from each radiator, starting from the ground floor upward. Check the system pressure afterwards and top up to 1–1.5 bar if needed.
Step 5 — Do not attempt anything further: Any investigation beyond these steps — including accessing the boiler’s internal components, checking the pump, or inspecting the heat exchanger — must only be carried out by a Gas Safe registered engineer. Opening the boiler casing yourself is unsafe and illegal for gas work.
📞 When to call a professional
Call a Gas Safe registered engineer immediately if the E9 error returns after a single reset, or if the boiler is making unusual sounds (banging or kettling) before shutting down. Tell the engineer: ‘My Glow-worm boiler is showing an E9 overheating lockout fault’ — this helps them arrive prepared with the right diagnostic tools and likely replacement parts such as a pump or overheat thermostat.
⚖️ Repair or replace?
A professional repair for an E9 fault typically costs £150–£350 depending on the root cause — a pump replacement sits at the lower end, while a scaled-up heat exchanger replacement can push toward £400–£600 or more. If your Glow-worm boiler is over 10–12 years old and requires a major component like a heat exchanger, replacement with a new A-rated boiler (£1,800–£2,800 installed) is almost always the more cost-effective long-term decision. For boilers under 8 years old in otherwise good condition, repair is usually the right call.