Heat exchanger having flat tubes

An automotive exhaust system has a heat exchanger for recovery of effluent heat. The heat exchanger has an array of flat tubes (3) forming ducts (10) for hot exhaust gases an separated by a further array of ducts for a coolant. The flat tubes have a by-pass allowing for the passage of hot exhaust gases. Some of the flat tubes (3) are sub-divided into a cooled zone (11) and a non-cooled zone (12). The bypass is located in the non-cooled zone. Each pair of flat tubes (3) is separated by a duct (10).