Boiler displays E05 and does not respond to thermostat calls

Safety: Do not operate a boiler with low or no water; this can severely damage the heat exchanger. Allow the system to cool before adding water.

• Check pressure gauge: On most Dettson boilers, normal cold pressure is around 12–20 psi (consult your manual). If the gauge reads very low, the system needs to be refilled.
• Inspect for leaks: Look around the boiler, relief valve discharge pipe, and visible piping for signs of water leakage or corrosion. Any active leak must be repaired before refilling.
• Verify automatic fill valve: Ensure the water supply to the boiler is open. The pressure-reducing fill valve should automatically bring the system up to pressure. If it does not, it may be clogged or failed and should be serviced by a technician.
• Reset low water fault: Once proper pressure is restored and leaks are addressed, reset the boiler. If E05 persists despite correct pressure, the Dettson pressure sensor or wiring may be faulty and require replacement.
• Professional help: If you are not familiar with hydronic system filling and bleeding, contact a heating professional to avoid introducing excessive air or over-pressurizing the system.