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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?


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,133
    Most likely excess air caused by a cracked heat exchanger.
    I guess the nozzle/assembly was smoking because the cracked heat exchanger (blower motor air) combined with sooted up heat exchanger (poor or positive over fire draft), blew back some unburnt oil onto the nozzle assembly (or partially plugged nozzle).
    The back pressure kept the chamber nice and toasty, heating up the nozzle assembly, almost enough to ignite the oil.
  • Steamhead
    Steamhead Member Posts: 15,999
    What are you going to replace this unit with?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • SuperTech
    SuperTech Member Posts: 1,919
    I've seen the flakey soot in boilers as well, the last one I saw didn't have enough air.

    That draw assembly was definitely smoking due to the plugged exchanger. It was a pain in butt to get that exchanger clean.
  • SuperTech
    SuperTech Member Posts: 1,919
    We gave the customer an estimate for another Thermopride as well as a Hallmark furnace. We also gave a quote for fixing the terrible ductwork, a box plenum with flex all over it.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,133
    edited February 2018
    Yeah but if your numbers change when the blower comes on, that's a sure sign of a cracked heat exchanger, at least enough to investigate.
    I really don't get too hung up on 'types of soot'. It's black, brown, or the gooey mess of condensing. I'm more interested in dialing it in properly, and having no soot. Soot is by-product of poor combustion. So that's where I focus my efforts-draft, combustion air, etc.
    EBEBRATT-Ed Member Posts: 13,457
    All I do is put soot on my hands. If it's black oily stuff your just burning like crap.

    To me good soot (is their such a thing) should be like cigarette ashes. No oil left all burned up and not much of it.

    When the front end of the burner overheats it's usually bad draft, plugged hx, flue or chimney or in this case a bad heat exchanger

    Good soot, can't believe I said that