I have a rheem gas/ac rtu. When the unit calls for heat it fies up fine but later kicks out due to limit switch in burner compartment tripping, while watching the burners I notice that 1 of the five burner tubes is not burning smoothly and allows the flame to escape into the compartment rather than going into the tube, this creates excess heat in the compartment and trips the limit. Why is only one burner firing this way? The tube is clear.

The unit needs a clean and check. During that time it is possible that either the burner tube(S) or the heat exchanger will be found defective by a HVAC professional.