HVACopediaFlowMax · Combi boiler
E03 Flue gas thermostat / exhaust thermofuse tripped
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
- Warning: high flue temperature can damage plastic (PVC/CPVC) venting; inspect the vent before returning the unit to service.
- Check the electrical connection at the flue thermostat / thermofuse.
- Restart the unit and check whether the thermostat resets.
- If it is open with normal flue temperature, replace the part.
- Look for the cause of high flue temperature: dirty or scaled heat exchanger, over-firing, poor water flow.
Settings for these units
Installation clearances · FlowMax condensing combi (FLOWMAX-90/120)
| Location | Service clearance (in) | To combustibles |
|---|---|---|
| X (side) | 0 | 0 in |
| Y (side) | 0 | 0 in |
| L | 16 | - |
| H | 24 | - |
| A (above) | 12 | 0 in from jacket top |
| B (below) | 12 | - |
| Front | - | 0 in |
| Rear | - | 0 in |
Wall-mount on a flat, solid vertical wall that can carry the weight.
Letters follow fig. 1 of the manual; L and H are the clearance zone in front/around the unit as drawn.
Hot water scald risk by temperature · FlowMax condensing combi (FLOWMAX-90/120)
| Temperature setting | Time to 2nd/3rd degree burn (adult skin) |
|---|---|
| Over 170°F | Nearly instantaneous |
| 160°F | About 1/2 second |
| 150°F | About 1-1/2 seconds |
| 140°F | Less than 5 seconds |
| 130°F | About 30 seconds |
| 120°F or less | More than 5 minutes |
120°F lowers scald risk; children, elderly and disabled people burn faster.
Fit a tempering (mixing) valve if water is stored or set hotter, e.g. with parameter 1 = 03 (up to 160°F).
Models that show this code
2 models in FlowMax condensing combi (FLOWMAX-90/120)
Other codes on the same units
- 31 Incompatible remote control
- E01 Ignition / ionization lockout (no flame)
- E02 Safety (overheat) thermostat or air pressure switch tripped
- E05 Heating (supply) sensor fault
- E06 Domestic hot water (DHW) sensor fault
- E12 DHW storage tank (cylinder) sensor fault
- E14 Air pressure switch tripped
- E16 Fan (blower) fault
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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.