A compensations duct assembly [100] is described that provides a gas tight connection between a backpass [9] of a furnace [3] and an air preheater pipe [10] having one end fixed relative to a floor. The compensation duct assembly [100] has an air preheater opening [133] that is connected to an air preheater duct [10]. A liquid seal [120] is employed to compensate for the movement due to thermal expansion between the backpass [9] and the compensation duct assembly [100]. The liquid seal [120] provides a gas tight connection under various conditions of thermal expansion.