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How to best identify the cause of different types of soot
Over the years I have had to vacuum out several plugged oil fired boilers and unfortunately a few gas fired ones too. Usually the biggest pain in the butt is a plugged furnace. I'm not a big fan of furnaces but as far as oil fired furnaces go I like Thermopride the best. Last week I came across an older Thermopride OL-85 low boy with issues on a call for a tune up.
I arrived and the furnace was running, I shut it down and began the tune up. I was surprised to find the draw assembly black and so hot it had smoke coming off it. The nozzle was larger than it should have been, .85 instead of a .75 I think. I found the heat exchanger plugged with soot, but it wasn't the usual fine black soot. This soot was real flakey and brown and black in color. After cleaning the heat exchanger I began the combustion testing. I set up the burner to 6% O2 and 11% CO2 with no CO, unfortunately after the fan came on the 02jumped to 10%.
I'm wondering what the cause of the flakey soot was. Was it from being over fired? Was it from the plugged nozzle I found? Maybe it was from excess air when the fan operates.
Why is soot sometimes irregular and what is the causes and solution for prevention?