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What kind of CO readings in oil flues?

BoilerproBoilerpro Member Posts: 407
Since I now have combustion test equipment, I am starting to get referals on the relatively few oil burners in my area. I am in an almost "I Know NAAATHING!" state in regards to oil burner tuning. Have a combustion analyzer, but no smoke tester. Whose do you recommen?


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  • SaggsSaggs Member Posts: 174

    Just started using a electronic combustion analyser and was wondering what is a "normal" amount of PPM of CO in oil fired equipment? I was checking an 86% efficient Viessman and it had 40ppm before cleaning and then 118ppm just after cleaning, both w/ a zero smoke and 11.5% co2. Also, any idea what a typical standing pilot 80% LP boiler might produce?
  • Jim DavisJim Davis Member Posts: 67
    Oil CO

    When it comes to oil it is necessary to record the Light-off CO, the run CO over several minutes and then the Shut-down CO. At no time should these numbers exceed 100ppm. When they do as yours, there is going to be a problem even with zero smoke. CO on oil is normally the oil droplets hitting the flame cone or combustion chamber. The exception is when you are underfired and then you produce CO much like gas.

    To diagnose your particular problem I would need all 3 sets of CO readings. Could be a problem if your analyzer has a slow response. Also the flue temperature would be helpful.
  • Lurkin' Murkin'Lurkin' Murkin' Member Posts: 136
    CO from underfiring

    Jim - can you provide more detail on how an under-fired oil boiler is more likely to produce CO? Most older oil boilers, that didn't have retention head burners originally, have the firing rate reduced with a newer burner.
  • Jim DavisJim Davis Member Posts: 67

    Not necessarily more likely but very possible when underfiring. The problem is the oil droplets seem to get past the hot flame zone and don't burn completely. I have seen up to 5000ppm of CO on underfired oil equipment. I discuss in class an oil furnace I tested once that had a 1.25 GPH nozzle instead of the 2.00 GPH that the furnace called for because someone said it was too big for the house. The CO levels on it were 1500 ppm at zero smoke. Even though the furnace was oversized the new burner was set back a 2.00 GPH. The CO was only 20ppm and at the end of winter the howmeowner said his oil usage was 600 gallons lower than it had ever been. Underfiring is never a good practice for safety or efficiency.
  • Lurkin' Murkin'Lurkin' Murkin' Member Posts: 136

    Thanks for the quick reply, Jim. Can't help but wonder if the problem of under-firing is more due to the wrong burner head or adjustment for the smaller nozzle size? An older piece of equipment may specify a certain input, based on poor burner performance - but just as many useful BTUH can be delivered to the heat exchanger with a modern burner, set up for a lower input. Baffling or reducing flue size may also be needed, though - probably too much for a guy with other jobs to get to...
  • Jim BergmannJim Bergmann Member Posts: 79
    Excess air and CO

    Jim is 100% correct, large amounts of excess air can and will produce high levels of CO, The excess air not only affects flame pattern, but it also cools the combustion process dramatically and lowers combustion temperatures meaning less fuel is going to be burned and a higher CO level will result.

    Jim Bergmann

    HVAC/R technical Specialist

    Testo Inc.
  • Steamhead (in transit)Steamhead (in transit) Member Posts: 6,688
    Jim D, on that boiler or furnace you mentioned

    I was wondering what type of unit it was, did it have a chamber and if so what type, and so on. Sounds like you had a flame that was too small for the firing area the unit had, and the firing zone never got hot enough for a good clean burn.

    Some of the Dead Men's Books I have say that the chamber size should only be determined once the load on the boiler or furnasty is known and the firing rate selected, and that the resulting chamber may not take up the full size of the unit's base area. Of course, this was more of an issue during the coal-to-oil conversion era than it is now, but may still be relevent since there is still so much oversized equipment out there.

    What say ye?

    Lurkin' Murkin, you're right- I think the usual practice is to down-fire 15-20% when replacing an older burner with a flame-retention type. The new burner makes a hotter flame, and can deliver the same number of BTUs to the boiler at the lower rate. I've used bricks as baffles too, like the Dead Men did. Haven't built a Heat Rake yet though.

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  • SaggsSaggs Member Posts: 174

    Not to jump off topic, but in addition to the co readings this electronic equipment gives you, you also get a reading for excess air. I usually see mine around 30 (%?). tis is on oil with a 350 stack and 11.5% co2. How important is this reading in the whole scheme of things or is it a byproduct of the burner setup? I guess it may show you if you have leaks in gaskets or flue sections as well?
  • Steamhead (in transit)Steamhead (in transit) Member Posts: 6,688
    Good question

    but in order to give you an answer, I'd need to know- is the 30% the reading you get at the point where reducing the air any further produces smoke? If it isn't, what was the excess air reading at that point?

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  • Norm HarveyNorm Harvey Member Posts: 684

    After a smoke test, the next reading I look at is the excess air. I set my burners air shutters to achieve 30% to 40% and then go on with the rest of my interpretations.

    If I am seeing higher than 15 ppm CO at 30% excess air during normal operation, I look for other causes that can influence my CO

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  • Jim DavisJim Davis Member Posts: 67

    Sometimes the problem with underfiring is the burner characteristics not being able to mix properly at lower inputs and requiring more excess air. A second problem is not creating a pressure drop in the heat exchanger by lack of gasses. This increase the velocity of the oil in the hot flame zone and prevents complete burning.

    The furnace was an Williamson oil furnace. I use to call them soot buckets. Big drum heat exchanger that took a lot of heat to heat up.

    An oil furnace that is running properly should have a plenum temperature of at least 145 degrees. The flue temperature of oil equipment even with a flame retention burner should be a minimum 300 degrees higher than what you are heating in most cases. Oil flame temperatures are 1000 degrees hotter than gas flames therefore the flue temperatures can not be the same and rarely should they ever be below 400 degrees. If they are the actual efficiency is much lower than you think.

    Excess air is a calculation from the O2 reading not a separate measurement. 5% O2 is always 30% excess air. O2 on oil should be between 4% - 6%. Any CO under 100 ppm at all stages is acceptable.

    CO on oil is produced for many different reason than on gas and in most cases indicates some type of impingement of the oil droplets on the flame cone or combustion chamber.
  • Lurkin' Murkin'Lurkin' Murkin' Member Posts: 136
    Excess air origination

    Excess air can come from many reasons. If the air setting of the burner is open too far, it could be the cause of the CO - but wouldn't the smoke reading reveal a yellowish spot from the unburnt oil? If there is leakage between the sections, cleanouts, or flue collector, the guy making the adjustments might try to compensate by closing down the air setting to lower the excess air reading - perhaps causing CO from lack of "primary" air within the combustion zone in the process. How beneficial would it be to take combustion readings at both the flue and combustion areas, to differentiate what the cause is?
  • TONYTONY Member Posts: 28

    This is interesting because I've seen some wierd things recently on a very underfired old steamer. I think it was originally rated for 2.75gph and it is now fired at 1.4, apparently been running that way for years. It seems to need lots of air to get a 0 smoke, I can't get CO2 much above 7.5%. When I try cutting the air back, the flame actually gets longer and hits the back of the chamber. The chamber is a quickie 200, so it should be in the right size range, and I changed the burner head form a F12 to a F6. What you said about inderfiring and gas velocity might have something to do with it.
  • RonRon Member Posts: 184
    Excess CO can

    be present long before yellow shows on the test paper. Our experience w/ air leaks in the heat exchanger area has not shown high CO numbers. High 02 & above 35% excess air, w/ modern burners, is when we start looking for air leaks.

    Our experience w/ oil & gas power burners is high CO is more often from too much air, rather than not enough. Exception. When the Testo or Bacharach CO numbers climb like an F-15, there ain't enough air.

  • DarrellDarrell Member Posts: 303

    Don't forget that nozzles are rated and stamped at 100PSI. The point being, sometimes, especially if you're burning cold oil, a smaller nozzle, that is one that is rated at less gpm run at a higher pressure to get the desired firing rate will atomize better. Steadier flame, less impingement, and less chance for big droplets to escape the pattern. If you're having trouble with CO or smoke that won't go away, and you have pretty much dialed in the other details...try changing from solid to diffuse or hollow spray patterns or back the other way. Most of the newer boilers have very short chambers and impingement on the back wall becomes a factor pretty readily.
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