Glow-worm Domestic Hot Water Sensor GLOWWORMS5704200

The Glow-worm S5704200 is a domestic hot water sensor — a sensing switch that monitors the temperature of the domestic hot water circuit inside a Glow-worm appliance. It reports that reading back to the boiler's control board so the appliance can regulate hot water delivery and shut down safely if the water overheats. It is a direct replacement component supplied under the Glow-worm part number S5704200.

• Domestic hot water runs cold or only lukewarm while the heating circuit behaves normally
• Hot water temperature swings between scalding and cold instead of holding steady
• The appliance locks out or throws a hot water or sensor-related fault code
• Hot water demand is not recognised, so the boiler does not fire for a tap or shower
• Intermittent hot water that returns after the appliance is reset, then fails again

• Confirm the appliance model and GC number against the manufacturer's literature — the GC number is the reliable identifier, not the model name alone
• Check the part number printed on the existing sensor matches S5704200 before removing it
• Inspect the sealing washer or O-ring on the sensor port and replace it if it is compressed, hardened or damaged