Abstract: A furnace that includes a heater configured to heat a fluid and discharge a heated fluid flow, a first heat exchanger at least partially positioned in a air duct and in fluid communication with the heater to receive the heated fluid flow, and a second heat exchanger at least partially positioned in the air duct and in fluid communication with the first heat exchanger so that the heated fluid flow flows through the second heat exchanger after flowing through the first heat exchanger. The first heat exchanger and the second heat exchanger are arranged in the air duct so that the flow of air travels through the first heat exchanger to transfer heat between the heated fluid flow and the flow of air, and then through the second heat exchanger after the first heat exchanger to transfer heat between the heated fluid flow and the flow of air.

Abstract: A gas-fired unit heater (10) is provided and includes a burner (20), a heat exchanger tube (18) located to receive a flow of the combustion products from the burner (20), the tube (18) including at least one linear tube run (30) extending along a longitudinal axis (18), and a corrugated baffle (40) received in the tube run (30).