Furnace display shows E15, unit may short-cycle or shut down with a low temperature or freeze protection alarm (often on Thermia hybrid or hydronic-integrated furnaces).

Safety first: Turn off power. Hydronic components may be hot and under pressure. Allow system to cool before opening any valves.

• 1. Check system pressure (if applicable): On Thermia furnaces integrated with hydronic coils, verify that the boiler or hydronic loop pressure is within the recommended range.
• 2. Bleed air from system: If radiators or coils are gurgling, bleed air from the highest points using bleed valves. Follow Thermia and boiler manufacturer procedures.
• 3. Inspect circulation pump: Confirm that the hydronic circulation pump is running when there is a call for heat. Listen for motor operation and feel for vibration. Replace the pump if seized or silent.
• 4. Check water temperature sensor: Locate the Thermia water temperature sensor on the hydronic supply line. Inspect wiring and test resistance against Thermia temperature/resistance charts. Replace if out of specification.
• 5. If unsure: Hydronic systems are complex; contact a qualified technician to diagnose E15 conditions.