:Recent trends towards engines with lower heat rejection rates in production vehicles has progressively reduced the amount of available ‘waste’ heat for compartment heating. In an effort to reduce compartment heat up time while increasing the compartment temperature, a new PTC (positive temperature coefficient) electric heater system has been developed to supplement the performance of current hot water based vehicle heaters. The PTC supplemental heater system features automatic power adjustment in response to ambient temperature conditions and HVAC fan speed, along with self regulation of the heater temperature. In addition, the power draw of the PTC heater can be matched automatically to the available electrical power level for all vehicle operating conditions. This paper describes the construction, performance characteristics, and applications of the PTC supplemental heater system.