Abstract: An improved catalytic heater for heating an oxidizing gas, normally air, that contains a small percentage of a catalytically oxidizable gas, normally hydrogen, and a novel method of operation within the catalytic heater. A preheat catalytic chamber is defined within a primary catalytic chamber, and the air/hydrogen mixture is first flowed through the preheat chamber and then through the primary chamber, so that the gas will first be catalytically preheated as it passes through the preheat chamber and will be further heated within increased thermodynamic efficiency because of the preheating as it passes through the primary chamber.