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Combustion Analysis

Chris_85 Member Posts: 1
Long story short, or maybe the other way around.... I was at my father in-law's rental for a no heat. Luxaire low boy, tank style heat exchanger, probably with the square outer heat exchanger and breeched out the rear. Cast iron J-tube burner w/hat and single air shutter. Ended up replacing the gas vavle and checked manifold pressure @3.5". For s&g's I had my testo 325m, some extra time (there's something you don't have everyday!!) and thought I would stick the probe in the breech and see what what I got. Any way after playing around for a while I came up with flue temp 653F, o2 9.2, co2 6.6, co @0.00, and eff of 73.7. These numbers come after 15 min of operation, burner cover door in place, and air shutter cracked open only about 1/4". The book says to shoot for 7-9 o2 and 6.5-8 co2. I've worked on these dinosaurs over the last 20 some years and normally end up setting up the air shutter anywhere from 3/8 to 1/2 open, just enough to take the yellow out of the flame and not sound like a blow torch! I just recently picked up the analyzer mostly for dialing in Munchkins and Knights. (besides my co detector puked and decided to get an all-in-one). My actual question is...I still have a somewhat yellow flame. Is this ok? If I add more air, my 02 goes way up and my co2 comes way down and I start showing some co, although it never went over 6ppm. In pilot training they always say to "always trust your gauges", do I do that here even though I "think" I'm looking at a "dirty " flame? Granted this furnace/burner is the furthest thing from hi-tech, this is where all your experience can help my lack of. thanks


  • Dave_4
    Dave_4 Member Posts: 1,405
    I'd lean towards what...

    The instruments say - rather than observed colration, especially given the units age and condition.

    We found lots of dust and lint-like crap in the vicinity of the boiler as the major yellow-flame culprit, most of the time. Touvh one screw and you liberate 10 years of dust and lint that was in the area. Simply stepping in a spot undisturbed for a coupple of years, results in nearby and even 10 feet away dust getting sucked into the open flame - resulting in yellow tips; big time.

    If you wait half an hour, stay very still and allow the dust to settle, the yellow tips should disappear, validating your Testo is fine, the dust the cause.

    Whattaya think. Dust or incomplete combustion?

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  • Yellow tips is an

    indication of poor flame quality and low flame temp many times. The flame should be blue and soft. Make sure the input to the furnace is correct many times increasing the pressure slightly with instuments attached will show a better flame and give you good analysis with low CO. Those burners when adjusted correctly will roar slightly.

    What Ken is referring to is really orange flames caused by rust and dust stirred up by touching the pipes and scuffling around while working. Yellow flames on a natural gas or LP burner is an indication of poor flame quality and carbon monoxide. It is especially a problem if the flame impinges on a cooler surface then we have a sooted up system. Testo units tend to read low on CO because they deduct NOX contaminants before giving a CO reading. Bachrach and other testers do not do that and will tend to give higher CO readings.

    Adjust the primary air to get the flame blue after making sure your firing at maximum firing rate. Then adjust secondary air to balance out your combustion analysis. Those burners on those units are real tricky to adjust and if memory serves me they have a spreader in front of the burner which must be in good condition or flame quality will be very poor. When those burners (typically the spreader corrodes) go there are no replacements and repairs to them are fruitless. It then is time for a new furnace.
  • Jim Bergmann
    Jim Bergmann Member Posts: 24

    Just to clarify, Testo units do not read a low CO, they read an actual CO. NOx is a cross interferrent with all CO chemical sensors manufactured by anyone. Testo units filter out the NOx before it enters the CO cell.

    A yellow flame does mean poor fuel air mix, and low velocity through the venturi resulting in a poor fuel air mix. The secondary air is most likely the saving grace for the low CO production in this case. The burner would most likely get dirty faster do to the low air velocity. If the HWT is natural gas, the primary air shutter should be all the way open unless it causes the flame to lift off of the burner. If the flame does lift, back the shutter down until it stops. Air shutters are usually for propane conversion.

    Make sure you are referencing the air free reading and not the actual CO reading when evaluating the operation of the tank.
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