Bryant Evolution +95s Plus – Error 33, Hi Limit Trip (2024)

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I own a Bryant Model 355CAV060100 natural gas furnace that was installed in my house in 2010 and worked well for 2-3 years and then occasionally/rarely indicated a failure on the Evolution thermostat. The LED on the circuit board would indicate Error 33. The filter was changed regularly and the registers were all open and unobstructed. The system would reset automatically and run fine. Over then next couple winters the problem would occur more often especially when it would get bitter cold outside. I was primarily using a wood stove along with the gas furnace as an auxiliary unit, therefore the repair was not urgent to maintain comfort.

The inducer would spin, the igniter would glow, the solenoid would switch on the gas and the burner would ignite, once the heat exchanger warmed up the blower would start, after a few minutes the unit would trip off for a few minutes and then the cycle would repeat.

I attempted troubleshooting the unit including: replace air filter, verify registers open/unobstructed, verified combustion air intake/exhaust piping is free of obstruction, cleaned condensate trap and drain lines, verified blower spins freely with firmly attached blower squirrel cage blades and nothing seemed abnormal. I purchased a manometer and checked the three-stage gas valve pressure and monitored the voltage to the solenoids with a digital volt meter. The input gas pressure and each Low/Med/High gas pressure settings were within specification. The solenoid 24V control signals controlled the valves and the gas pressure followed as expected (ie. not a stuck solenoid issue).

I found the limit switch was opening due to high heat condition in the heat exchanger. The flame roll-out switch was not open, and the manual reset was in a normal position (not requiring manual reset). I replaced both of these sensors with OEM parts because they were inexpensive and possibly the source of the fault. The problem was not resolved. I tried one cycle with the filter removed and the filter door open to provide plenty of air intake, with no change to the problem.

I modified the original hi limit switch to accommodate a thermometer and allow me to jumper/bypass the limit circuit. I first measured the temperature at which the limit switch opened, then with the limit circuit bypassed I measured the temperature and it continued to rise. I shut the furnace off manually once it reached maximum operating temperature to avoid a fire safety issue. This confirmed for me that the unit is not just running a little hot or the limit switch is a little out of spec, the system is not balancing the heat generated with the airflow around the heat exchanger.

I measured temperature rise to be 62 deg F when the unit shuts down on medium heat, this value is in spec however I don’t know if it reached a steady state temperature before shut down.

I opened the evaporator and inspected the coil for restricted airflow. The coil looked fairly clean and a flashlight easily shined thru. I used CRC coil cleaner to clean the evaporator while I had it open for inspection. This did not fix the issue. I measured the static pressure with a pressure probe and found it less than the maximum 0.5 inches H2O (med=0.1 and high fan speed=0.2 inches H2O).

I determined the unit uses an electronic control module to run the DC motor and not start/run capacitors that could affect the RPM of the blower motor. So a simple capacitor replacement is not an option for my unit.

I concluded that the issue is with either the thermostat, main circuit board, motor control module or the blower motor. These parts are too expensive to randomly replace. I then called the company who installed the system, they checked many of the same items that I verified/replaced. On the third service call the circuit board was swapped and the furnace now runs properly. I don’t know the exact circuit board failure mode, but assume the blower speed directed by the board was an incorrect slower speed than required.

I don’t have a question, I just wanted to share this overall experience as it may help others with similar issues.