A jet pump of special design, having a variable-geometry nozzle, is applied to the external recirculation of flue gas in a furnace system. This new concept improves reliability by eliminating the need for a flue-gas fan, and enables the recycle of high-temperature gas to be effected. Recycle is induced by simply increasing the main air supply pressure by a reasonably modest amount, while the nozzle area of the jet pump is reduced by the progressive insertion of a co-axial spear. Details are given of the jet pump design. A model of its operating characteristics is given, which modifies standard incompressible-flow jet-pump theory such that it can be applied to the large nozzle : mixing-tube area ratio designs required for this application. The model is used to predict the jet pump＇s operating characteristics and the potential performance envelope of a flue-gas recirculation system. It is demonstrated that the amount of hot flue gas that can be recycled rises with available air-supply pressure but varies inversely with the recycle gas temperature. Results are given for experiments conducted with two jet pumps, including the use bf hot air and helium to obtain data at various density ratios. The results agree well with the proposed model, and confirm the proposed operation of the flue-gas recycle system.