Boiler
E8
The E8 error code on a Glow-worm boiler indicates a fault with the domestic hot water (DHW) temperature sensor or an overheating condition on the hot water circuit — it’s a safety lockout that needs prompt attention, but not necessarily an emergency.
What does this error mean?
Glow-worm boilers display E8 when the control board detects that the hot water temperature sensor (NTC thermistor) is reporting an abnormal reading — either too high, too low, or no signal at all. The boiler locks out as a precaution to prevent overheating or scalding. Your central heating may still work, but hot water from taps will be unavailable until the fault is resolved.
Est. repair cost
£100–£200 for DHW sensor replacement (parts + Gas Safe labour); new boiler £1,800–£3,500 installed
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button (usually marked with a flame or ‘R’) for 3–5 seconds. Wait 2 minutes and check if the boiler fires up normally. If the E8 returns immediately or within a short time, proceed to the next steps.
Step 2 — Check your water pressure gauge: If the pressure is below 1 bar, top up via the filling loop until it reads 1–1.5 bar. Low pressure can trigger secondary faults. Instructions are in your Glow-worm user manual.
Step 3 — Check isolating valves: Ensure the DHW isolation valves on the boiler are fully open. A partially closed valve restricts flow and can cause overheating on the hot water circuit.
Step 4 — Bleed any radiators and check for airlocks: Air in the system can cause erratic temperature readings. Bleed each radiator with a radiator key until water flows steadily, then re-check boiler pressure.
Step 5 — Do NOT attempt to replace the NTC sensor, inspect the PCB, or open the boiler casing yourself. Gas boiler internal components must only be worked on by a Gas Safe registered engineer. This is a legal requirement in the UK.
Step 6 — If the reset doesn’t hold and the basic checks above change nothing, turn the boiler off at the programmer or room thermostat and call a Gas Safe registered engineer.
📞 When to call a professional
Call a Gas Safe registered engineer if the E8 code returns after a reset, or if you notice any signs of overheating such as a pressure relief valve discharging water outside. Tell the engineer: ‘My Glow-worm boiler is showing an E8 fault code — I believe it’s a DHW temperature sensor or overheating issue on the hot water circuit.’ This helps them bring the right parts. Do not attempt to open the boiler casing or interfere with gas pipework yourself — it is illegal and dangerous.
⚖️ Repair or replace?
A DHW NTC sensor replacement by a Gas Safe engineer typically costs £100–£200 including parts and labour, and is well worth doing on a boiler under 10 years old. If your Glow-worm boiler is over 10–12 years old and this is one of several faults, a new boiler (typically £1,800–£3,500 installed) may be more cost-effective long-term. Get a like-for-like repair quote first, then compare it against a new boiler quote before deciding.