Abstract: Air enters each pass of a two-pass air heater in the same direction with respect to gas flows. Bypass air conduits connect each tube bundle of a first tier to the tube bundle of a second tier of adjacent gas ducts and are located between inlet and outlet air conduits and have common walls therewith.The bypass air conduits connect the tube bundles of all the gas ducts in a single closed (ring) circuit. It is possible to set the gas ducts either along the radius on a single circumference or in a joined-up arrangement with the gas ducts divided into two halves by a vertical partition.In an alternative embodiment, the bypass air conduits connect the tube bundles of all the gas ducts in an open circuit, the tube bundles of both tiers of the extreme gas ducts are half as wide and are arranged to form a single pass with respect to the air flow.The thermal efficiency of the air heater is the same as that of a Z-criss-cross type air heater.