Abstract: A compact, highly flexible and efficient, multi-positionable, multi-dimensional and multi-stage gas-fired heat exchanger system as described. The system is connectable in a forced air duct regardless of the angular position and size of the duct. The heat exchanger comprises one or more heat transfer tubes which are shaped and dimensioned as dictated by the customized use of the heat exchanger. The tubes are secured to a support panel as well as the gas burner associated with each of the inlet openings of the tubes. A position orientable gas valve is secured to a gas distribution manifold. A turbulator may be associated with at least some of the tubes to cause turbulence in the hot combustion flow in each of the tubes to modify the efficiency in heat transfer along one or more sections of the tubes by directing hot combustion gas along an inner circumferential wall of the tubes.