A volute assembly, and an induced draft fan and a gas furnace using same. The volute assembly comprises a volute (10) and a cover plate (11) mounted at the bottom of the volute (10). The volute (10) and the cover plate (11) enclose to form an inner cavity (12) in which a fan wheel can be mounted. The cover plate (11) is provided with an air inlet (111). An air exhaust pipeline (13) extends out from the side surface of the volute (10). An air outlet (130) is formed at an end portion of the air exhaust pipeline (13). The air exhaust pipeline (13) comprises a connecting segment (131) and a diverging segment (132). The connecting segment (131) is connected to the side wall of the volute (10). The inner diameter of the diverging segment (132) is greater than the outer diameter of the connecting segment (131). A step plate (133) is formed between the connecting segment (131) and the diverging segment (132). An air guide pipe (2) is mounted on the step plate (133). The step plate (133), the diverging segment (132), and the air guide pipe (2) enclose to form a water storage cavity (100). The tail portion of the diverging segment (132) is provided with a water discharge pipe (134) communicated with the water storage cavity (100). The volute assembly features strong universality, a simple structure and simple manufacturing, and low manufacturing costs, can improve a product qualification rate, and can improve the performance of an induced draft fan by matching between air guide pipes of different air exhaust diameters and a volute.