A rack oven having an oven interior, an access door, a baking chamber, a passive steam generator, and a heat exchanger. The heat exchanger includes a chamber containing burners, an undivided exhaust collection plenum positioned above the burner chamber, and a plurality of straight heat exchange tubes extending upwardly between and communicating with the burner chamber and the collection plenum. A blower circulates air within the oven interior among the heat exchange tubes, the steam generator and the baking chamber, and the steam generator produces steam using the circulating air heated by the heat exchange tubes.