A method for making a furnace for heating a glass sheet by lower electric resistance elements (50) supported by a furnace housing (12) below a roll conveyor (40) and by supplying a gas burner heated hot gas flow from a forced convection heater (56) that provides a dominant mode of external heat for heating the conveyed glass sheet from above. The furnace is manufactured by retrofitting an electric resistance heater type glass sheet furnace that initially has both lower and upper electric resistance elements (50) located below and above the roller conveyor (40). The lower electric resistance elements (50) are maintained while the upper electric resistance elements (50) are replaced with a gas burner forced convection heater (56).