Utica Dunkirk E35 Error Code

What does Utica Dunkirk E35 mean?

E35 on the Utica-Dunkirk DMG-240 condensing boiler indicates that the supply and return water temperatures appear to be reversed — the return sensor is reading higher than the supply sensor. This condition is physically impossible in a correctly piped and functioning system, so the boiler infers either that the two sensors are swapped, one sensor is faulty, or water circulation direction is reversed through the heat exchanger.

Symptoms

• Boiler display shows E35 and burner shuts down
• Return pipe feels hotter than the supply pipe
• Fault appears from initial startup on new installations
• Boiler may operate briefly then lock out as temperatures diverge

Common causes

• Supply and return NTC sensor wiring connected to the wrong pipes
• Heating system piped with supply and return connections reversed at the boiler
• Water circulation pump running in reverse direction (flow backwards through boiler)
• One NTC sensor failed in a way that produces a falsely high reading

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Diagnostic steps

1. Verify sensor placement on pipes

   Confirm the supply NTC sensor is on the boiler outlet (flow) pipe and the return NTC sensor is on the inlet (return) pipe. On new installations, misplacement is the most common cause.

2. Check sensor wiring at the PCB

   Trace each sensor cable back to its PCB terminal. Swap the connectors if the sensors are physically correct on the pipes but their wiring is reversed at the board.

3. Verify pump rotation direction

   Check the circulator pump arrow indicating correct flow direction. A pump installed backwards will push water in the wrong direction, making return temperature exceed supply.

4. Check piping connections at the boiler

   Confirm that the system supply pipe connects to the boiler's CH outlet and the system return connects to the boiler's CH inlet. Reversed piping causes E35 immediately.

5. Measure and compare sensor resistance

   Measure resistance on both sensors at the same ambient temperature. Identical resistance values confirm both are functional; a large discrepancy indicates a failed sensor.

When to call a professional

If the piping or pump requires correction to resolve E35, call a licensed technician. Reversing piping connections on a pressurized hydronic system requires draining, cutting, and re-soldering or re-fitting pipe connections — work that must be performed by a qualified plumber or HVAC technician with the correct tools and isolation procedures.