A flameless combustion heater system is described that comprises a flameless combustion heater; an oxidant inlet pipe; a fuel inlet pipe; and a preheater for preheating the oxidant or the fuel, said preheater comprising an oxidation catalyst. A method for starting up a flameless combustion heater is described comprising passing a fuel-oxidant mixture to a preheater comprising an oxidation catalyst to preheat the fuel or oxidant stream being fed to the heater. A method for controlling the temperature of the flameless combustion heater system is also described that comprises controlling the amount of fuel and/or oxidant that passes through the preheater.