Boiler
E5
The E5 error code on a Glow-worm boiler indicates an overheat fault — specifically that the boiler’s heat exchanger or primary circuit has reached an unsafe temperature. This is a safety lockout and should not be ignored.
What does this error mean?
When your Glow-worm boiler displays E5, it means the boiler’s internal thermistor or high-limit thermostat has detected temperatures above the safe operating threshold, typically above 95°C. The boiler shuts down automatically to prevent damage to the heat exchanger or a potentially dangerous situation. It will not fire again until the underlying cause is identified and resolved.
Est. repair cost
£80–£350 for most repairs (pump, thermistor, or minor work); £300–£600 for a full power flush if sludge is the cause
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Let the boiler cool: Do not attempt a reset immediately. Wait 15–20 minutes for the boiler to cool down fully before doing anything.
Step 2 — Press reset: Once cool, press and hold the reset button (usually marked with a flame or ‘R’ symbol) for 3–5 seconds. If the boiler fires up and runs normally, monitor it closely for recurrence.
Step 3 — Check all radiator valves: Ensure every thermostatic radiator valve (TRV) and manual valve in your home is fully open. Closed valves restrict flow and cause overheating.
Step 4 — Bleed your radiators: Air locks reduce circulation efficiency. Use a radiator bleed key to release trapped air from each radiator, starting upstairs and working down.
Step 5 — Check the system pressure: Look at the pressure gauge on the boiler front panel. It should read between 1–1.5 bar when cold. If low, top up via the filling loop following your boiler manual.
Step 6 — Stop here and call a Gas Safe engineer: If the E5 fault returns after a reset, or if you suspect pump failure, heat exchanger scaling, or a faulty thermistor, do not attempt further investigation yourself. This is a gas appliance and internal work must be carried out by a qualified professional.
📞 When to call a professional
Call a Gas Safe registered engineer if the E5 error returns after a reset, if your radiators are cold at the bottom (indicating sludge), or if your pump is making unusual noises. Tell the engineer: ‘My Glow-worm boiler is showing an E5 overheat fault — I’ve reset it but the fault keeps returning.’ They will check the pump, heat exchanger, thermistor, and system water quality.
⚖️ Repair or replace?
A pump replacement typically costs £150–£350 including labour; a heat exchanger clean (power flush) runs £300–£600 depending on system size; a thermistor replacement is usually £80–£150. If your Glow-worm boiler is over 10–12 years old and facing multiple component failures, replacement with a modern A-rated combi boiler (£1,800–£3,000 installed) is likely more cost-effective in the long run. For boilers under 8 years old in otherwise good condition, repair is almost always worth it.