Abstract: A single forced air furnace (10) may be installed in any of an upflow, downflow, or horizontal airflow orientation, with discharge of flue gases either opposite to or concurrent with the primary airflow path. The combination of design features which provides this versatility includes the following features: an inducer assembly (24) for withdrawing flue gas from the furnace heat exchanger (20), which inducer assembly (24) may be positioned to direct the discharge of flue gas in a selected one of a plurality of directions; an optional internal flue duct (64) for directing the flue gas through the furnace vestibule chamber then through the furnace housing (12); and the fact that the furnace housing (12) is dimensioned and configured so that the furnace may be supported on any one of four different sides (O, I, S.sub.1, S.sub.2) of the furnace housing (12).

Abstract: A forced air furnace (11) has a heat exchanger (35) with one or more tube pack assemblies (10). Each tube pack assembly (10) has a burner (21) situated to fire into a primary fire tube (30) which is mechanically connected through a transition cap (36) to a plurality of secondary tubes (37, 38, 39, 41) returning combustion products from the primary fire tube (30) to a collection manifold (24) at the intake end of a draft inducer assembly (23) for discharge of the combustion products to a flue. In the preferred embodiment, the transition cap (36) is connected to the primary fire tube (30) and to the secondary tubes (37, 38, 39, 41) by a mechanical interlock.