A space heater with a linear source of infrared radiant energy in heat exchange relationship with a heat exchanger formed of copper or aluminum material. The copper is pretreated to soften the copper and partially blacken the surface thereof. The aluminum is anodized and electrolytically colored dark. The space heater is thermally more efficient than a comparable space heater wherein the copper or aluminum has not been pretreated. The linear source of infrared radiant energy and heat exchanger are mounted in a heater core that is thermally insulated by an air jacket from an exterior case.