Sooting is general caused by incomplete combustion. Which, most of the time is caused by a dust, lint and/or pet hair build up in the heater. It clogs the burners , as well as the air intakes. Thoroughly cleaning the interior of the heater and it's inner workings will generally solve the problem. Also, an less frequent ... low gas pressure to the heater can cause sooting. Burning scented candles while the heater is in operation contributes greatly to the soot problem and indoor air pollution.
The above should help you to further troubleshoot and solve the soot problem.
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Sooting is general caused by incomplete combustion. Which, most of the time is caused by a dust, lint and/or pet hair build up in the heater. It clogs the burners , as well as the air intakes. Thoroughly cleaning the interior of the heater and it's inner workings will generally solve the problem. Also, an less frequent ... low gas pressure to the heater can cause sooting. Burning scented candles while the heater is in operation contributes greatly to the soot problem and indoor air pollution.
The above should help you to further troubleshoot and solve the soot problem.