How to Fix Carrier Error Code 33 or 13 After Upgrading to a Higher MERV Rated Filter

Upgraded to a Higher MERV-Rated Filter and Now Your Carrier Furnace is Throwing 33 or 13 Error Codes?

Due to COVID-19, many people are taking precautions to better their in-home air quality. The big problem however, is a higher MERV rated filter can restrict the air flow going over the furnace's heat exchanger and cause the furnace to go off on high limit.

On Carrier, Bryant and Payne furnaces, the lack of airflow will cause an error 33, and sometimes lead to error 13 due to safety lockout. Error 33 is a Limit Circuit Fault. It means that the high limit or flame rollout switch is open. This can lead to error code 13, Limit Circuit Lockout. Error 13 means that the limit or flame rollout has been open for more than 3 minutes.

To address this issue, we need to ensure the furnace has proper air flow, and increase it if necessary. Below are some troubleshooting tips you can run through before you spend your money on parts.

Check for proper switch operation

  1. Check for continuity in the high limit switch after furnace has cooled down (15 minutes). If the high limit switch is still open, then the switch is bad.
  2. If flame rollout switch is tripped, look for back drafts, blocked flue and check heat exchanger to determine the cause of flame rollout.
  3. Reset the flame rollout switch(es) by pressing the button behind the disc. If switch is still open after pressing the button, then the flame rollout switch is bad.

Check for air flow issues

  1. Check condition of filter. Make sure it is not dirty and blocking air flow.
  2. Open all vents and return registers.
  3. Make sure the ducts and register grilles are clean.
  4. Clean the A/C evaporator coil.
  5. Check condition of the blower wheel. Make sure it is clean, tight and not missing any blades.
  6. Check blower motor and capacitor to make sure the blower motor is running up to speed.
  7. Make sure there is proper combustion air. on 90%+ furnaces, check the incoming PVC pipe. On 80% furnaces, make sure the furnace isn't locked up in a closet and not receiving any combustion air. 80% furnaces pull combustion air from the ambient air around it.
  8. If all those check out, increase the speed of the blower motor during heat mode by switching to a higher speed on the control board.
  9. Check for air flow issues

Check the flame rollout switches and observe the burner flames

  1. Reset the flame rollout switches near the burner by pushing the switch on the back of it
  2. Check for continuity on the switch. If there is no continuity after the switch has cooled down and reset, then the flame rollout switch, HOWEVER, it is important to replace it and OBSERVE THE FLAME.
  3. With the burner door open, have the thermostat do a call for heat.
  4. Observe the flames. The flames should be nice and blue and shooting directly into the heat exchanger tubes.
  5. If the flames are dancing and "rolling out" and tripping the flame rollout switches, TURN THE FURNACE OFF AND KEEP IT OFF UNTIL THE HEAT EXCHANGER IS REPLACED. Your heat exchanger is plugged or has holes in it, and is presenting an extreme safety hazard.

What Flame Rollout Looks Like on a Furnace

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About the Author

James Clark

James Clark is the Ecommerce Manager of and HVAC Controls Specialist (certified in Tridium and Carrier i-Vu controls) with a decade of experience at Technical Hot & Cold. His extensive background includes service calls, installations, and providing technical support to parts customers. James has been featured in the Washington Post and continues to share his knowledge, helping homeowners navigate HVAC repairs. In his spare time, he's playing music with his children and spending too much time working on his lawn.

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