A degassing device (2; 29) which can be at least connected to a cooling or heating installation with a circuit and medium circulating in the circuit during operation, which degassing device (2; 29) comprises: - a reservoir for connection via at least a supply conduit and a discharge conduit to the cooling or heating installation and having a vent for degassing the medium; - a pump (15; 32) in the discharge conduit between the reservoir and the cooling or heating installation; and - a controllable valve (9, 10; 30, 31) in the supply conduit, characterized by - a pressure sensor connected to the reservoir; - a temperature sensor connected to the reservoir; and - a control which is connected at least to the pressure sensor and the temperature sensor and which is configured to drive at least one of the pump (15; 32) and the controllable valve (9, 10; 30, 31) during the degassing subject to a pressure in the reservoir sensed by the pressure sensor and to a temperature in the reservoir sensed by the temperature sensor and to hold the temperature of the medium in the reservoir below a boiling point associated with the pressure and the temperature of the medium and to prevent decrease of pressure in the reservoir to too low a value by admitting medium from the system into the reservoir during the degassing, wherein the control is configured to either or both of: at least partially open the controllable valve (9, 10) in the supply conduit to the reservoir during the degassing and to provide a throughfeed capacity through the controllable valve (9, 10), which is lower than a pump capacity of the pump, and drive the pump in the discharge conduit to the cooling or heating installation during the degassing with a throughfeed capacity higher than a throughfeed capacity of the controllable valve (30, 31). - a reservoir for connection via at least a supply conduit and a discharge conduit to the cooling or heating installation and having a vent for degassing the medium; - a pump (15; 32) in the discharge conduit between the reservoir and the cooling or heating installation; and - a controllable valve (9, 10; 30, 31) in the supply conduit, - a pressure sensor connected to the reservoir; - a temperature sensor connected to the reservoir; and - a control which is connected at least to the pressure sensor and the temperature sensor and which is configured to drive at least one of the pump (15; 32) and the controllable valve (9, 10; 30, 31) during the degassing subject to a pressure in the reservoir sensed by the pressure sensor and to a temperature in the reservoir sensed by the temperature sensor and to hold the temperature of the medium in the reservoir below a boiling point associated with the pressure and the temperature of the medium and to prevent decrease of pressure in the reservoir to too low a value by admitting medium from the system into the reservoir during the degassing, at least partially open the controllable valve (9, 10) in the supply conduit to the reservoir during the degassing and to provide a throughfeed capacity through the controllable valve (9, 10), which is lower than a pump capacity of the pump, and drive the pump in the discharge conduit to the cooling or heating installation during the degassing with a throughfeed capacity higher than a throughfeed capacity of the controllable valve (30, 31).