An air conditioning system and a method of heating air flowing through a heater core of a HVAC module is disclosed. The method includes flowing essentially all of the air flowing through an evaporator through a heater core for all HVAC operating conditions. A flow control valve is opened long enough to allow a flow of hot coolant from an engine into the heater core. The valve is then closed to stop the flow of the hot coolant. When the temperature of the hot coolant is determined to have cooled to a predetermined temperature threshold, the flow control valve is then opened again to allow hot coolant to flow into the heater core.