Rifled Air Conditioning Systems listed by BTU/HR Ratings: If you do not know what type of system is suitable for your particular bus configuration or environmental location, use the A/C Wizard. RAC In-Dash Systems Available for Larger Medium Duty Type Buses Most often dash a/c systems are used in combination with other RAC Components. A good "Rule of Thumb," is to substitute dash a/c evaporator for front mount RE-10 or RE-29 evaporator.