An air conditioning system, comprising a controller for controlling the air conditioning system, a heat source or a cold source (10, 20) for a first heat medium; a first heat exchanger configured to exchange heat between a second heat medium and indoor air; a fan configured to deliver the indoor air to the first heat exchanger; a second heat exchanger configured to exchange heat between the first heat medium and the second heat medium; a pump configured to circulate the second heat medium between the first heat exchanger and the second heat exchanger; and a temperature sensor (25, 26, 34, 35) configured to detect a temperature of the second heat medium, the controller being configured to start operation of the heat source or the cold source (10, 20) before a set operation start time of the fan by a preliminary operation time period, wherein the controller is configured to, before the operation start time of the fan, calculate a heat capacity of the second heat medium, calculate a heat storage amount of the second heat medium from the temperature detected by the temperature sensor (25, 26, 34, 35) and the heat capacity, and determine the preliminary operation time period from the heat storage amount; wherein calculating the heat capacity of the second heat medium includes one of the following: the controller is configured to calculate the heat capacity based on an amount of heating by the heat source (10, 20), and a temperature variation detected by the temperature sensor (25, 26, 34, 35); the controller is configured to calculate the heat capacity based on a volume of the second heat medium stored in advance; the controller is configured to calculate a length of a pipe for circulating the second heat medium based on the differential pressure before and after the pump, said differential pressure being measured by a pressure sensor comprised by the air conditioning system, based on a flow rate-head characteristic of the pump stored in advance, and based on a flow path resistance characteristics of the first heat exchanger and the second heat exchanger stored in advance. the controller being configured to start operation of the heat source or the cold source (10, 20) before a set operation start time of the fan by a preliminary operation time period, wherein the controller is configured to, before the operation start time of the fan, calculate a heat capacity of the second heat medium, calculate a heat storage amount of the second heat medium from the temperature detected by the temperature sensor (25, 26, 34, 35) and the heat capacity, and determine the preliminary operation time period from the heat storage amount; wherein calculating the heat capacity of the second heat medium includes one of the following: the controller is configured to calculate the heat capacity based on an amount of heating by the heat source (10, 20), and a temperature variation detected by the temperature sensor (25, 26, 34, 35); the controller is configured to calculate the heat capacity based on a volume of the second heat medium stored in advance; the controller is configured to calculate a length of a pipe for circulating the second heat medium based on the differential pressure before and after the pump, said differential pressure being measured by a pressure sensor comprised by the air conditioning system, based on a flow rate-head characteristic of the pump stored in advance, and based on a flow path resistance characteristics of the first heat exchanger and the second heat exchanger stored in advance.