The invention relates to a system for heating a storage container (1) for an aqueous operating liquid in a motor vehicle having a combustion engine cooled by means of a liquid heat transfer medium, using the waste heat from the combustion engine, which is transferred to the heat transfer medium. The system comprises a first heat exchanger (9), which is provided in or on the storage container (1) and can be heated by means of the heat transfer medium in a cooling circuit (3) for the combustion engine, wherein the first heat exchanger (9) is part of a storage container heating circuit (10), which comprises a heat transfer medium feed (11) and a heat transfer medium return (12), wherein the storage container heating circuit (10) is connected hydraulically in parallel with a second heating circuit (7b) with a second heat exchanger (7) for heating the passenger cell (8) of the motor vehicle. The storage container heating circuit (10) comprises a valve with a controllable volume flow and further comprises means for measuring the M volume flow of heat transfer medium in the storage container heating circuit (10) and means for controlling the valve (13) in accordance with one or more temperature signals from various temperature sensors (T1 to T5). The invention furthermore relates to a method for heating a storage container (1) using the system.