A control system for a heat transfer system, the control system comprising: an input for receiving one or more signals indicating a flow rate of heat transfer fluid within the heat transfer system and an amount of heat transferred between the heat transfer fluid and a medium being heated or cooled, the one or more signals comprising signals indicating an input temperature of the heat transfer fluid into the heat transfer system, an output temperature of the heat transfer fluid out of the heat transfer system and a temperature of the medium; an output for issuing control signals to change the flow rate within the heat transfer system; and a controller configured to: determine a target output temperature for the heat transfer fluid for achieving a predefined amount of usable heat transferred between the heat transfer system and the medium based on the one or more received signals; determine a target flow rate for achieving the target output temperature, including: determining the power transferred between the heat transfer system and the medium based on the flow rate, the input temperature, the output temperature and a specific heat capacity of the heat transfer fluid; determining a temperature difference between the fluid and the medium; determining the thermal resistance of the medium based on the temperature difference and the power; and determining the target flow rate based on the power and the thermal resistance; and issue a control signal via the output terminal to adjust the flow rate within the heat transfer system to set the flow rate to the target flow rate to achieve the target output temperature. an input for receiving one or more signals indicating a flow rate of heat transfer fluid within the heat transfer system and an amount of heat transferred between the heat transfer fluid and a medium being heated or cooled, the one or more signals comprising signals indicating an input temperature of the heat transfer fluid into the heat transfer system, an output temperature of the heat transfer fluid out of the heat transfer system and a temperature of the medium; an output for issuing control signals to change the flow rate within the heat transfer system; and a controller configured to: determine a target output temperature for the heat transfer fluid for achieving a predefined amount of usable heat transferred between the heat transfer system and the medium based on the one or more received signals; determine a target flow rate for achieving the target output temperature, including: determining the power transferred between the heat transfer system and the medium based on the flow rate, the input temperature, the output temperature and a specific heat capacity of the heat transfer fluid; determining a temperature difference between the fluid and the medium; determining the thermal resistance of the medium based on the temperature difference and the power; and determining the target flow rate based on the power and the thermal resistance; and issue a control signal via the output terminal to adjust the flow rate within the heat transfer system to set the flow rate to the target flow rate to achieve the target output temperature. determine a target output temperature for the heat transfer fluid for achieving a predefined amount of usable heat transferred between the heat transfer system and the medium based on the one or more received signals; determine a target flow rate for achieving the target output temperature, including: determining the power transferred between the heat transfer system and the medium based on the flow rate, the input temperature, the output temperature and a specific heat capacity of the heat transfer fluid; determining a temperature difference between the fluid and the medium; determining the thermal resistance of the medium based on the temperature difference and the power; and determining the target flow rate based on the power and the thermal resistance; and determining the power transferred between the heat transfer system and the medium based on the flow rate, the input temperature, the output temperature and a specific heat capacity of the heat transfer fluid; determining a temperature difference between the fluid and the medium; determining the thermal resistance of the medium based on the temperature difference and the power; and determining the target flow rate based on the power and the thermal resistance; and issue a control signal via the output terminal to adjust the flow rate within the heat transfer system to set the flow rate to the target flow rate to achieve the target output temperature.