Display shows E4 and thermostat stops heating, often after long operation at high load

Safety first: Turn OFF power at the breaker. The thermostat may be hot; allow it to cool before handling.

Steps to troubleshoot:

• 1. Check installation conditions:
  • Remove the thermostat from the wall and inspect the wall box.
  • Ensure there is enough space behind the thermostat for air circulation.
  • Confirm that insulation or foam has not been packed tightly behind the unit.
• 2. Inspect wiring and terminals:
  • Check the L, N, and load (OUT) terminals for discoloration or melted insulation.
  • Tighten all terminal screws firmly; loose connections generate heat.
• 3. Verify load current:
  • Check the total wattage of the connected floor heating or panel heaters.
  • Divide watts by voltage (e.g. 230 V) to get current. Compare with the thermostat's max rating (e.g. 16 A for many Heatit models).
  • If the load exceeds the rating, move part of the load to another circuit or use an external Heatit relay or contactor controlled by the thermostat.
• 4. Improve ventilation:
  • If the thermostat is in a very warm location (e.g. above a radiator), consider relocating it.
  • Ensure the wall box is not sealed with vapor barrier tape directly behind the thermostat.
• 5. Reset and test:
  • After cooling and correcting issues, restore power.
  • The E4 error should clear automatically. If it reappears quickly, the internal overheat sensor or triac may be damaged and the thermostat should be replaced.