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Troubleshooting a High Efficiency Condensing Furnace

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No Ignition - Clogged Condensate Drain
Troubleshooting a High Efficiency Condensing Furnace

Furnace condensate lines are small diameter tubes running from the near top of the furnace cabinet to the floor drain.

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In addition to the possible reasons for failed ignition with a conventional furnace, furnace ignition problems with a condensing furnace may happen for other reasons such as a clogged flue vent condensate line.
  • A partially clogged flue vent condensate line or clog at the condensate collector box near the inducer fan will often trip the furnace’s pressure switch.
  • If the condensate drain is blocked by debris, improper draining or by frozen condensate, then the pressure switch will not allow normal operation. It will open the switch contacts and keep them open, preventing unit operation.
  • Because the pressure switch senses the accumulation of condensate in the furnace drain pan, the furnace will not operate until the condensate drain has been cleared and the condensate flows freely.
  • You may also notice excessive water coming out of the drainage hole of the smaller secondary heat exchanger when the drain line is disconnected.
  • Ignition problems caused by a condensate drain clog may be intermittent. They can start and stop as the restricted water flow drains away over time and allows the furnace to cycle back on, but reappear once the condensate backs up if the problem is not fixed.

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