Abstract: The invention relates to a combustion catalyzing system for commercial grade fuels. The system comprises a water reservoir which is fed with air through an inlet arranged to be immersed into the reservoir water. A vapor phase water outlet conduit is provided which extends from the reservoir top and is routed to a combustion zone to be catalyzed. Along the path followed by the vapor phase water which flows through the outlet conduit from the water reservoir, an oil-operated flow regulating device is provided which is effective to control the rate of emission of steam bubbles from the reservoir. Upstream of the flow regulating device, a motor-driven valve controlled by a humidistat located in the proximity of the combustion zone to be catalyzed may be provided for delivering a larger or smaller amount of steam to the flow regulating device.