Thermostat shows E04 and heating output is locked out

Safety first: Switch off power to the heating system and thermostat before working on wiring or sensors.

Steps:

• Allow the floor and room to cool for at least 30–60 minutes. Check if the E04 code clears automatically once temperature drops.
• Verify the maximum floor temperature limit in the installer settings (often default 27–35°C). If it is set too low for your floor type, adjust within manufacturer recommendations.
• Check the floor sensor resistance with a multimeter and compare to the Heatmiser temperature/resistance chart. A sensor reading much lower resistance than expected may falsely indicate high temperature and should be replaced.
• Ensure the thermostat is not installed above a heat source (radiator, towel rail, or direct sun). If it is, consider relocating the thermostat or using a remote room sensor.
• If the floor genuinely overheats, have an installer check the underfloor heating output, pipe spacing, and flow temperature from the manifold or boiler.