Condensate Trap Diagnostic

Check and clean your furnace condensate trap to ensure it's working normally.

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All high efficiency furnaces have a condensate trap. Moisture that condenses inside the secondary heat exchanger drains into the condensate trap and then into the floor drain.

Refer to the furnace glossary if you need help identifying furnace parts or terms.

Condensate trap blockages

Condensate trap blockage can be a difficult issue to diagnose because it may not appear until colder weather requires the furnace to run continuously for 10-30 minutes.

How the condensate trap gets blocked

Impurities in combustion air and natural gas collect inside the secondary heat exchanger, and condensed moisture carries them into the condensate trap. Over time, these impurities can build up to partially or fully block the condensate trap.

Problems caused by a blocked condensate trap

When the trap is blocked, condensate produced during normal furnace operation backs up in the secondary heat exchanger, reducing the flow of air and combustion gas.

Reduced flow prevents pressure-proving switches from closing when the inducer motor ramps up, stopping normal furnace operation and causing “pressure switch failed to close” error codes.

How to tell if the condensate trap is blocked

If you can hear water sloshing inside the collector box or inducer motor when it starts to run, the condensate trap may be partially or fully blocked.

Cleaning the condensate trap

Cleaning out the condensate trap to remove any blockages can restore proper condensate drainage and allow the furnace to operate normally again.

Tools needed

  • basic hand tools and pliers

Cleaning procedure

  1. Turn off the power to the furnace.
  2. Remove the lower furnace cover and locate the condensate trap.
  3. Most furnaces have multiple rubber hoses connected to the condensate trap with spring steel clamps. Make note of how and where they connect.
  4. Remove the hoses and uninstall the condensate trap. Each furnace is different — consult the furnace service manual if you are unsure.
  5. Once removed, thoroughly rinse out the trap. You may need to blow air through the ports to flush out built-up debris.
  6. Once you are satisfied the condensate trap is clean and unobstructed, reinstall it and reconnect the hoses.
  7. Reinstall the lower furnace cover.
  8. Turn on the power to the furnace and set the thermostat to call for heat. Condensate should now drain as designed.

If after cleaning the condensate trap you continue to have pressure switch errors, early furnace shut down, or hear water inside the furnace, check for proper furnace grading (as described below) and intake and exhaust blockages.

Proper furnace grading

High-efficiency furnaces should be installed with a grade — roughly 1/2″ from back to front, and 1/2″ to the side where the condensate trap is located.

This front-grading of the furnace is essential to properly drain condensate from the secondary heat exchanger into the condensate trap and out of the furnace.

Problems caused by back-grading

Back-graded furnaces often work OK for a few years, only developing pressure switch errors and condensate drainage problems after five years or more. This happens as impurities build up over time within the secondary heat exchanger.

1st and 2nd generation Carrier (Bryant) furnaces are particularly sensitive to to improper furnace grading, which can cause early failure of the secondary heat exchanger.

Checking the grade of your furnace

You can check the grade of your furnace with a handheld level.

If you find the furnace is back-graded (the back of the furnace is lower than the front) you should book a visit with a qualified technician to properly grade the furnace. Otherwise, the life expectancy of your furnace can be greatly reduced.

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Furnace repairs should only be performed by a certified technician.

This website is for informational purposes only. Advanpro Ltd. is not liable for property damage, personal injury, or death caused directly or indirectly by actions taken as a result of the information provided here.

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Our technician will identify the problem and tell you how much it will cost to repair. Once you know the problem and the cost, you can decide if you want to proceed.

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