F18
Saunier Duval Modulation / power control fault

F18 displayed, unit may start and stop repeatedly, temperature fluctuates

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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

Faulty gas valve modulation coil, Defective fan speed control, PCB modulation output failure, Incorrect combustion calibration

How to Fix / Troubleshooting

Safety first: Turn off gas and power. Only qualified personnel should adjust combustion or gas valve settings.

Steps:

  • Visual inspection: Check gas valve and fan connectors for secure fit and no signs of overheating.
  • Check flue and air intake: Ensure there are no restrictions that could affect combustion.
  • Professional diagnostics: A technician should check gas valve modulation signals, fan speed control, and perform a combustion analysis. Replace faulty components as indicated.

Warning: Incorrect modulation can cause overheating or poor combustion. Do not continue using the heater with persistent F18 errors.

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Repair Difficulty

Professional Required 5/5

Required Part

Gas Valve Assembly
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