Air Mix versus Coolant Flow to Control Discharge Air Temperature in Vehicle Heating and Air Conditioning Systems

The purpose of heating, ventilation and air conditioning (HVAC) systems in passenger vehicles is to maintain comfortable interior conditions within the vehicle's passenger compartment. In order to maintain precise control of the interior temperature, the discharge air temperature of the HVAC system of most passenger vehicles is commonly controlled via one of two methods. North American vehicle manufacturers have generally relied on using an air mix, partial series reheat system. Alternatively, a water valve controlled, full reheat method has been favored by many of the European vehicle manufacturers. Either method can be used to provide a HVAC system which results in the precise control of the interior cabin temperature but the concerns and trade-offs involved in producing each of these systems are uniquely different in terms of development time, effort and several performance characteristics.