Don't know anything about that heater but I assume it's either nat gas or propane. In either case sounds like a very rich mixture which on a gas heater can be caused by blocked combustion air flow or the pressure regulator went ape and the gas pressure is way high. Can you look at the flame and see what it looks like? Should be mostly blue with orange flame tips I believe, for the right fuel/air mixture. If the gas pressure went high you should see an abnormally large flame too. the soot is a sign of incomplete combustion.
Al
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