F18
Ideal Frozen or Blocked Flue / Intake

Ideal furnace shows F18, unit may lock out after several failed attempts, often in very cold weather

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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

Ice buildup at flue terminal, snow blocking air intake, condensate freezing in external pipe, undersized or incorrectly sloped flue on Ideal condensing furnace

How to Fix / Troubleshooting

Safety: Turn off the furnace before working on the flue. Use caution on icy or snowy surfaces.

Steps:

  • Inspect outside terminations: Locate the flue and intake terminals. Clear any snow, ice, or debris. Do not alter the terminal configuration.
  • Check condensate discharge: Ensure the condensate pipe is not frozen where it exits the building. Thaw gently with warm (not boiling) water if safe to do so.
  • Restart furnace: After clearing obstructions, restore power and call for heat. Monitor operation for several cycles.
  • Prevent recurrence: Discuss with a technician about insulating or rerouting external condensate and ensuring the flue is installed to Ideal’s cold-weather guidelines.
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Repair Difficulty

Medium 3/5

Required Part

Flue Terminal Kit
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