NOVELTY - Preheating a catalyst (18) in a catalytic heater (10) involves preparing a gaseous mixture (42) of hydrogen and nitrogen, and dispersing the mixture into contact with the catalyst for a period to raise the temperature of the catalyst to a level that will promote oxidation of a fuel gas (32) in the presence of the catalyst.    USE - The method is used for preheating a catalyst in a catalytic heater.    ADVANTAGE - The temperature of the catalyst in a catalytic heater can be effectively raised to the required level to react with an incoming fuel by contacting the catalyst with a gaseous mixture of hydrogen and nitrogen. Mixing the hydrogen with nitrogen effectively controls the richness of the hydrogen and lowers the temperature of reaction with the catalyst, while creating a gas that is easily distributed through existing manifolds and gas controls for the catalytic system, thus creating a safe method for preheating the catalyst. The mixture closely resembles the density properties of natural as opposed to those of 99% pure hydrogen gas.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a gas catalytic heater comprising a body, a catalytically active porous layer disposed within the body, a sealed plenum chamber having a gas permeable portion facing toward the catalytically active layer, a first supply of a combustible fuel gas, a second supply of a starting gas mixture composed of hydrogen and nitrogen, gas inlet for introducing the fuel gas and the starting gas mixture into the plenum chamber, a first gas conduit mechanism connecting the first supply to the gas inlet, a second gas conduit connecting the second supply to the gas inlet, a first valve associated with the first gas conduit for opening and closing the first supply to the flow of the fuel gas to the inlet, and a second valve associated with the second gas conduit for opening and closing the second supply to the flow of the starting gas mixture to the inlet.    DESCRIPTION OF DRAWING(S) - The figure is a partially cut-away, perspective view of a section of a catalytic heater showing the gas inlet manifold system used for feeding the fuel gas into the catalytic heater as well as the preheat or starting gas mixture.    Catalytic heater (10)    Catalyst (18)    Fuel gas (32)    Gaseous mixture (42)