99
Rinnai
Severe Combustion or Overheat Fault
Boiler locks out with error 99; may be accompanied by signs of overheating or poor combustion.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Blocked heat exchanger, Severe scale buildup, Incorrect gas/air ratio, Failed safety sensor not detecting overheat until critical
How to Fix / Troubleshooting
Safety first: Turn off power and gas immediately. Do not attempt to restart the boiler until it has been inspected.
- Visual inspection: After cooling, inspect the heat exchanger and burner area for heavy soot, discoloration, or signs of overheating.
- Check venting and air supply: Confirm that intake and exhaust are clear and correctly installed.
- Do not bypass safeties: Never bypass thermal cutoffs or sensors to clear this error.
- Professional service required: A Rinnai-qualified technician must inspect the heat exchanger, perform descaling or replacement, and verify combustion safety before the boiler is returned to service.
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Repair Difficulty
Professional Required
5/5
Required Part
Primary Heat Exchanger
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