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FlowMax · Combi boiler

E03 Flue gas thermostat / exhaust thermofuse tripped

High severity FlowMax condensing combi (FLOWMAX-90/120)

What the control is telling you

The flue (exhaust) temperature safety device has opened, usually from high flue temperature. The 90 user manual calls it the exhaust vent thermofuse.

Applies to: FLOWMAX-90, FLOWMAX-120

Likely causes

  • Flue thermostat broken
  • Flue thermostat wire disconnected
  • Flue thermostat locked out on high flue gas temperature

Test and repair steps

  1. Warning: high flue temperature can damage plastic (PVC/CPVC) venting; inspect the vent before returning the unit to service.
  2. Check the electrical connection at the flue thermostat / thermofuse.
  3. Restart the unit and check whether the thermostat resets.
  4. If it is open with normal flue temperature, replace the part.
  5. Look for the cause of high flue temperature: dirty or scaled heat exchanger, over-firing, poor water flow.

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Source: FlowMax FLOWMAX-120 Instruction Manual, 6.10 Troubleshooting table, p.66; FlowMax FLOWMAX-120 Instruction Manual, 6.11 Diagnostics error code list, p.67; FlowMax FLOWMAX-90 User Manual, Diagnostics - Error codes, p.8-9. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.