******This situation strongly suggests that something has altered the air/fuel mixture of your water heater burner. If nothing physical has occurred, such as removing and replacing the appliance, then the problem is most likely an obstruction of some sort that is preventing the proper mix of the LPG and the required air supply. Most often this happens when insects build a nest or web inside the burner tube. Another possibility is that the orifice has become partially obstructed, due to contaminates in the propane supply.
******First, remove the burner tube by taking out the hex-head screw that holds the burner assembly to the water heater. This is located just above where the flame is. Clean out the burner tube with a brush or compressed air and make sure that the flame deflector that is located on the end of the burner tube is square to the tube and aligned properly.
******The orifice is located on the gas valve, where the burner tube slips on. There is a small hex-head "nut" here with a very tiny hole in it. Clean it with alcohol and let it air dry. Do not poke anything through the opening as this will distort the proportions of the calibrated hole and result in an erratic burn characteristic.
******Once this is done, reassemble everything, and test fire the water heater. The burner flame should be mostly an "aggressive" blue color, with little or no yellow in it, and should have a mild roaring sound to it. If this does not fix the problem, then see a qualified LPG technician - you have other problems.
******The "soot" is unburned carbon and along with this you have carbon monoxide production. I would advise you to not use this appliance until the defect is remedied.
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