Help diagnose rumbling oil furnace?

Discussion in 'HVAC Heating & Cooling' started by thebordella, Jan 7, 2010.

  1. thebordella

    thebordella New Member

    Nov 9, 2009
    Ithaca, NY

    I am a newbie at furnace-related DIY, and this is my story. (Apologies in advance for the length.)

    My house has a Slant Fin Liberty L-20-P oil furnace (not steam, baseboard hot water). It is an older unit, about 20 years old. For the several years I've been living here, I've had a service guy come out when the furnace needed service -- replace filters, nozzle, resolve other problems that have cropped up.

    It was last serviced in November 2009, so not long ago. At that time it was making a terrible rumbling sound, like a train. Service guy replaced the nozzle and cleaned the carbon deposits off that whole injection assembly. Not sure if he replaced the filter. When he finished, the furnace was sounding fairly smooth running again.

    Last week we were out of town for 10 days, and sometime during that time the furnace apparently stopped running. This is upstate NY, so needless to say the house was very cold, but fortunately no pipes froze.

    The first solution I tried was to bleed air from the bleed valve. I have done this before -- in fact it was the only service I knew how to do. We have a one-line system and when the volume of oil in the tank gets low combined with very cold temperatures, sometimes air bubbles get into the line. When this does happen, bleeding the line is usually successful. But not this time -- it ran for a little while after the bleed, but stopped again. Additional attempts to bleed and start would run for only a few minutes.

    I left a message with my service guy but, while waiting for a callback (he can be very busy), I became impatient and started reading online about performing some basic service myself.

    This morning, I went to Lowes and bought a new filter for $4. Following the instructions, I was able to replace the old (gross) filter. Cleaned out the canister, used the new gaskets, etc. No problem. Re-started the furnace and it ran for several hours -- before conking out again. Argh.

    I then poked around online about replacing the nozzle. The old nozzle was a .65 80A, but this didn't seem exactly right to me because the label on the furnace says it is a .75 model. Went to the hardware store and picked up a .75 80A nozzle for $6.

    Following online instructions, I removed the nozzle assembly. The nozzle did not look too badly clogged, although the "retention ring" (looks like a cone around the nozzle) was quite thick with carbon. I replaced the old nozzle with the new and scrubbed and chipped the carbon off the cone, so that the air vents were open again. I also cleaned the electrode tips. Re-assembled everything, ran the furnace through a couple of air bleed cycles, and then let it ignite. Success, it seems.

    The furnace has not conked out yet, longer than my earlier efforts. So that's encouraging. However, when it does run, it is still sounding a bit rumbly. Not the worst rumble it's ever had, but more than at its best.


    I later found the Slant Fin manual online. According to the manual, the Carlin burner (the type on my model) actually takes a .75 70A nozzle. Could the service guy have gotten it wrong all this time using an 80 degree nozzle? How much of a difference does it make? Should I replace the 80A with a 70A?

    What other factors might contribute to the rumbling sound? Anything else a layperson like me can poke into?

    Also, some parts of the ignition assembly are pretty worn. That cone piece has been scrubbed of carbon many, many times by the service guy. The electrodes are kind of gnarly now, and he has commented that they are just barely in spec. My question is, can't I just buy new components? If they're as cheap as the filters and nozzles, I don't get the reason to keep scrubbing away at them for years and years if I can just pop new ones in every now and then. Where would I buy them? I don't even know how to refer to or shop for them. It seems like it might be cheaper to replace these things myself than one service call.

    Thoughts? Appreciate any and all advice.

  2. SJProwler

    SJProwler New Member

    Apr 2, 2007
    The reason you hear the rumbling since replacing the nozzle is that the airflow and fuel delivery must be calibrated using a combustion analyzer. Without the proper equipment you will be wasting oil, adding to c0 emissions, carbon buildup, and reducing the life of your equipment.

    Changing the size and style of nozzle is best left to someone with the equipment to calibrate them properly. Also your oil maintenance guy may have chosen that specific nozzle and calibrated your burner to use it to customize your burner to your specific needs.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    It may be that the larger nozzle produced more heat than you needed, and made the cycles too short for maximum efficiency. SO, it was changed to a smaller nozzle to decrease the output somewhat. this is all a guess
  5. nhmaster

    nhmaster Master Plumber

    Jul 30, 2008
    Tech. Instructor
    S. Maine
    That particular model of slant fin's boiler line is notorious for combustion problems. The serviceman may have reduced the nozzle size and raised the pump pressure to compensate. Anyway, without proper testing equipment you can not do anything properly. Call him back.
  6. awesomlyleet

    awesomlyleet New Member

    Jan 16, 2010
    Dixfield Me.
    Carlins should never run with an 80 degree nozzle. 70 at the very least. But in changing your nozzle size from .65 to .75 you threw off your oil air mixture an your boiler is now running a bit dirtier than it was. You basically richened it like a car motor. In doing so it would make it run a little rough.
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