The invention relates to a control device (12) for an engine-independent heater (3) that heats a liquid heat transfer medium of a heat transfer medium circuit (1) especially for motor vehicles. Said control device (12) reduces a heater (3) capacity when an actual temperature gradient ((dT/dtIst) of the heat transfer medium is equal to or exceeds a temperature gradient threshold value ((dT/dt)Schwell). The control device (12) determines the temperature gradient threshold value ((dT/dt)Schwell) dynamically in accordance with a difference in temperature (Δφ) between a target temperature (φSoll) of the heat transfer medium and an actual temperature (φIst) of the heat transfer medium. The invention further relates to a heater (3), a heating system (1), and a method for controlling the inventive heater (3).