Retrofitting R-1234yf mobile air conditioning systems is prohibited maximum cooling performance (pressure or temperature control), different settings may be required when changing the refrigerant. Refrigerant pressure control of the compressor with either cycling compressors or variable displacement compressors is affected by the refrigerant used in the system. Systems that use air temperature sensing for controlling the evaporator can also be affected by changing from R1234yf to R-134a. R-1234yf systems that control freeze protection by pressure may encounter loss of cooling performance if charged with 100% R-134a. This is due to the requirement of R-134a to have a lower setting of 3.3 psig to achieve freeze protection at 32 F. This may result in a 4-degree F. (2.2C.) warmer outlet air temperature. Figure 1 The mixing of R-134a with R-1234yf will change refrigerant pressure and may result in evaporator freeze-up on pressure control systems, reducing system airflow. ❆ Figure 1 Spring 2017 * ACTION/NAPA 17