Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

combustion analysis



  • furnacefigher15furnacefigher15 Posts: 508Member
    atmospheric burner

    Primary air adjustment depends on the burner. Most of the older ones gave some kind of air damper built into each burner to regulate amount of air into the venturi.

    Secondary air is dictated by availability of combustion air, and amount of over fire draft.

    Gas pressure is the easiest adjustment too make.

    .01 inches draft is on the low side.

    What were the other readings? Also Start a new thread.
  • Tim McElwainTim McElwain Posts: 4,231Member
    Primary air which

    is the adjustment on the burner itself has really nothing to do with combustion analysis O2 reading it is simply to adjust the air/gas ratio to a 10% gas 90% air ratio for a soft blue flame not roaring or lifting on the burner, not flashing back into the burner. On most atmospheric burners excess air is controlled by the design of the equipment as long as doors and installation procedures are followed. The requirements for combustion air are for every 1 cubic foot of gas (1,000 BTU's) 10 cubic feet or air plus 4 cubic feet of excess air at the burner opening into the combustion chamber. Then add roughly 15 cubic feet of dilution air at the draft hood. Most atmospheric burners get up in the range of 50% excess air. Then compared to say a power gas burner replacing and oil burner we can adjust excess air and get it down into the range of 15 to 25%. Some of these burners can fire at 2% O2 and still be safe.

    Failure to put doors in place or properly install particularly boilers when elevating to deal with flooding issues are often the main causes of excessive excess air which lowers efficiency and can cause cooling of the flame which increases Carbon Monoxide potential. ANSI sets standard for CO not AGA which is an allowable air free of 400 PPM. This is really to high and we actually want to see under 100 PPM on Oil and Gas unless manufacturers specs state otherwise. A good dependable analyzer is your best friend on all of this as you can monitor O2, Stack Temperature and CO and based on those three along with draft have a pretty good idea of real maximum firing rate efficiency. Some analyzers have a NOX eliminator so air free CO readings will be lower with those.
  • drhvacdrhvac Posts: 189Member
    I mainly deal with gas

    80% and 95% furnaces, along with atmospheric boilers and furnaces. And like someone said, the furnaces with the in shot burners are the hardest to adjust. Besides the gas pressure, what else is there? And a couple of other things i notice:

    I put in American standard furnaces, I looked at all of there their installation manuals, service facts, etc. None of them have anything on what the co2, o2, excess air, etc. should be. Theere is no mention of it anywhere.

    Is to much or to little draft have to do mainly with flue pipe sizing assuming there is no blockages. If the draft is to high how do you reduce it. Everything is fixed? thx
  • icesailoricesailor Posts: 7,265Member
    CA Adjustments:

    I understand what you are saying about gas adjustments. I have done only oil and we have total adjustments. The old gas I see has almost no adjustments. Now, with the newer stuff, there are no easy adjustments to be made. I see equipment all the time that has been installed by installers who wouldn't know a CA if they saw one.

    Oil guys should really understand draft. Some don't. I'm being forced into gas testing. I saw two 2002 Munchkins last week without the 925 controllers in a duplex. One was running 90 PPM of CO and the other was 400+. What to do?

    If it wasn't for Tim McIlwaine and others here, I'd be in a quandary.
  • furnacefigher15furnacefigher15 Posts: 508Member
    draft too high

    If draft is too high, it can be reduced with the addition of a barometric relief damper.

    Generally speaking draft before any dilution air should be between -.02 and -.04 '' W.C

    In some situations there is not much else to adjust other than gas pressure, and flue design.

    I have heard of some people adding ball valves to the intake of non modulating 90% furnaces to regulate amount of fresh air being added introduced.

    As far as combustion targets, sometimes you need to call the manufactures.

    While combustion testing see where temp rise of furnace is (should be in range)

    adjust fan speed to get rise correct. (this will effect stack temp and efficiency)

    General targets for category 1 vented Natural gas or LP

    O2 - 6 to 9%

    stack temp 325 to 500 degrees F

    draft -.02 to -.04 '' W.C

    CO air free 50 or less is best below 100 OK (below 400 acceptable )

    Category 4 (high efficiency)

    General targets

    O2 - 6 -9%

    stack below 140 degrees ( 144 is condensation point)

    CO air free 50 or less best, 100 or less is good (400 is acceptable)

    power burners (non - condensing

    3 to 6% O2 (4 is Ideal for most)

    275 to 500 degrees F stack temp

    CO same as others, the lower the better 400 max

    CO2 operates in a direct relationship with O2

    CO over 50 indicates a problem, over 100 worse, over 400 remove your probe to prevent damage to the chemical cell in the probe.
  • Mike Kusiak_2Mike Kusiak_2 Posts: 604Member
    Atmospheric representative combustion numbers

    Here are some representative actual combustion readings taken recently with a Testo 327

    Burnham series 2, (model 205)

    T stack 465F

    Eff  80.2%

    CO 2 PPM

    Excess air 50.7%

    O2 7.6%

    CO2 7.46%

    Condensing dewpoint 124.2F

    Old National (1950's) gas boiler 178K BTU

    T stack 447F

    CO 3 PPM

    Eff  81.5%

    Excess air 42.8%

    O2 6.8%

    CO2 7.9%

    Condensing dewpoint 127.1F

    Burnham ES2 high efficiency atmospheric

    T stack 266F

    CO 8 PPM

    EFF 85.9%

    Excess air 37.5%

    O2 6.2%

    CO2 8.24%

    Condensing dewpoint 127.9F

    In comparison, a gas power burner can burn clean with much less excess air:

    Gordon-Piatt dual fuel commercial burner at low fire

    T stack 290F

    CO 0 PPM

    EFF 86.1%

    Excess air 23.7%

    O2 4.4 %

    CO2 9.24%

    Condensing dewpoint 131.9F
  • KCA_2KCA_2 Posts: 303Member
    edited December 2011
    Here are a couple of readings

    From atmospheric boilers...  What to adjust?


    Eff% 81.6

    CO2 2.9

    CO   240

    O2 - 15.6%

    Draft -.04

    Stack Temp - 185degF

    Excess Air - 262

    Air free  860


    Eff% 82.7

    CO2 - 5.4%

    CO - 2285  (Yikes)

    Draft - .01

    Stack Temp - 242degF

    Excess Air - 109.7

    O2 - 11.4%

    Manifold pressure - 3.66WC

    The CO in the raypac unit is wild...  I really could use some suggestions...

    All readings were done with a Bacharach #125 just calibrated....

    :-)  Kca
  • Tim McElwainTim McElwain Posts: 4,231Member
    edited December 2011


    Eff% 81.6

    CO2 2.9

    CO   240

    O2 - 15.6%

    Draft -.04

    Stack Temp - 185 degF

    Excess Air - 262

    Air free  860




    Eff% 82.7

    CO2 - 5.4%

    CO - 2285  (Yikes)

    Draft - .01

    Stack Temp - 242degF

    Excess Air - 109.7

    O2 - 11.4%

    Manifold pressure - 3.66WC



    Are these boilers copper finned tube boilers? If so they are susceptible to getting drastically plugged up and need a real cleaning if not already damaged beyond repair.


    Ignore the efficiency readings on both of them




    CO2 is too low way too low.

    CO air free way too high

    O2 and excess air way out of control. Did you take these reading with everything in place on boiler doors etc?

    The stack temp is too low and is probably producing condensate.

    Has this unit been cleaned at all?

    Did you take these readings after the unit had been running at least 10 minutes?

    What training if any have you had on combustion?




    CO is gone! Again has this unit been cleaned?

    O2 and excess air way out

    Stack temp is in the condensing range, anything below 275 will condense.

    I assume the rating plate calls for an outlet pressure of 3.5 so 3.66 is over fired most of the time. Have you clocked these burners at the meter to see what they are running at for input?
  • furnacefigher15furnacefigher15 Posts: 508Member

    Ditto everything Tim said.

    If i'm not mistaken laars and raypak use 4'' manifold as there norm.

    Like I said in a previous post, and as Tim just pointed out theses boilers soot up easily if not set up right, and take on damage easily as well. The burners are stamped steel, so they'll warp easily, and the copper fin design lends to quick sooting.

    These boilers also condense very easily if return water is not 140 or better, and the condensate, falls directly onto the burners cooling the flame, causing incomplete combustion, which causes soot and CO, and the soot build up will cause the heat of the flame to more or less reflect back down at the burners which causes warping, which will cause future CO.

    I've had a few of these, where I was called too late. Once the damage is done, sometimes the only fix is boiler replacement. I've spent hours upon hours to get a good burn after years of misuse to no avail.

    For the above reasons, I am not a fan of this type of boiler.

    However, when installed and set up properly from day one, these boilers are fine. The trouble is, they are rarely set up right. Part or the problem is that unlike mod/con type boilers for example the manufactures of the copper tube jobs do not seem to stress the importance of setup.

    I've found it to be extremely critical for copper tube boilers to have not only good water flow, but good and hot water flow. The use of a system bypass valve to maintain hot return water is crucial (especially in domestic and pool heating application).

    A cast iron, or fire tube boiler is much more forgiving to improper set up. They will soot just the same, but when the right tech comes along and cleans the boiler, then does a proper setup, the damage that was done in the past, stays in the past.
  • KCA_2KCA_2 Posts: 303Member

    Units are Copper tube...  Both units are rated at 4" typ...  Both units are approximately 12 years old.. and never been cleaned..  Neither unit is rolling out and pulling them apart to clean the Htex are real time consuming...  I've done many before...  I'll clean the Laars tomorrow and see what the new readings look like,,  I haven't had much training in combustion..  Not alot available except that I search online to self educate...  I'm trying to do that...  other than running the vent and adjusting manifold pressure there isn't alot to setting a atmospheric boiler up...  Both these boilers are P/S with Tekmar controls with injection...  Both radiant heat..  Both run at +- 110degf at 20degF outside because of the amount of tubing in the floors...  One uses the domestic as a buffer...  but thats another story... 

    After I clean the Laars I would expect that the CO would drop...  The Draft would rise?  and stack temp would rise?

      :-) Kca
  • furnacefigher15furnacefigher15 Posts: 508Member
    Return water too low.

    You need to use a mix down strategy to get a hotter return temp over 140.

    Stack temp wont go up till water temp goes up. Draft may not rise till stack temp is higher.
  • furnacefigher15furnacefigher15 Posts: 508Member


    Eff% 81.6

    CO2 2.9

    CO 240

    O2 - 15.6%

    Draft -.04

    Stack Temp - 185degF

    Excess Air - 262

    Air free 860

    O2 too high

    draft is tolerable, but minimally high. ( i would leave alone)

    You may have the wrong orifaces for the boiler if the gas is at 4'' and the O2 is still that high.

    need more gas or less air to lower the O2

    CO2 will increase as O2 goes down
  • Tim McElwainTim McElwain Posts: 4,231Member
    edited December 2011
    KCA, I am assuming these boilers

    are both shut off and not running especially with those levels of CO.

    What is your location as training is available from my training center located in Warren Rhode Island. Also sometimes Jim Davis from National Comfort Institute comes to different areas with his three day program. We come at this from different approaches but we both agree on the final outcome and 95% of our procedures are pretty much the same. I certainly bow to Jim's tremendous knowledge of combustion and combustion related issues.
  • KCA_2KCA_2 Posts: 303Member

    Both were running and for a good amount of time..  Easily 30 minutes or so.. 

    I'm in Colorado Springs.. 
  • Tim McElwainTim McElwain Posts: 4,231Member
    Those boilers should

    be shut off as they are unsafe!! DO IT NOW BEFORE YOU HAVE A TRAGEDY
  • drhvacdrhvac Posts: 189Member

    I have learned alot just through our discussion and some of the webinars i have took. Unfortunately I'm not close to Tim Mclain, or I would take his class. Every class I have taken over the years, even going back to my Lincoln Tech days back in the 80's, no body every said anything about combustion analysis. Between that, and the cost of the equipment,  I think that is why the majority of heating contractors don't do it. I've read a couple of Dan's books on radiant heat that was very helpful. Is there any similiar books on combustion analysis that any body knows of ?
  • Tim McElwainTim McElwain Posts: 4,231Member
    Dr HVAC

    I have the books you are looking for contact me at [email protected]
  • KCA_2KCA_2 Posts: 303Member
    edited December 2011
    After Cleaning


      I cleaned the exchanger and the burners on this Laars 160 boiler...  NG..

    Water temp in +- 180degF at return...

    The readings are as follows:

    Item:            Was              Is Now..

    Man Pressure  3.6"WC       3.78"WC

    CO            160-240PPM      5 PPM   Wow what a change...  Better draft?

    O2             15.6%              15.6%

    Draft          -.04                   .03 - .08  (I don't understand this...)

    Excess Air  262                  268

    CO2           2.9 - 3.0              2.9

    Eff%          81.6                  79.6

    Stack Temp  185degF         215degF  with 180degF water in return both times

    Air free         860                  24   (What is this?)

    OK...  Doesn't seem to have helped to much..  did with the CO.. 

    Your experience is appreciated...

    :-)  Kca
  • drhvacdrhvac Posts: 189Member
    training cont.

    Hey Tim, Is that the right email/website? I tried but it wouldn't go through.
  • Tim McElwainTim McElwain Posts: 4,231Member
    That is my e-mail

    address it is not a website as I do not have a website. I sent you an e-mail so we should be connected now.
  • Tim McElwainTim McElwain Posts: 4,231Member
    edited December 2011
    I am posting here most of

    what I e-mailed to you:

    Keep me posted and let me know if you have any questions. I have found that many times even after cleaning those boilers they do not recover very well. They are drastically affected over the years by airborne volatile organic compounds from the environment they are operating in, such as laundry rooms, hair dressing, wood working shops etc all putting various chemicals in the air which react with the products of combustion, the copper fins and the water vapor in the flue gases. All are a formula for disaster with these boilers.


    Did you clock the burner on the gas meter to see what it is actually burning? What is the full Model number and serial number of this unit?


    You will need to use the ½ foot dial on the meter; we will let the half foot dial do two complete rotations using a stop watch to time how long it took to do the two rotations. Let’s say it took 20 seconds to do that, we then divide 20 into 3600 (number of seconds in an hour) this will give us cubic feet per hour since the meter measures gas flow in cubic feet and burners are rated in BTU’s we need to convert to BTU. So 20 divided into 3600 gives us 180 cubic feet per hour, we multiply that times the BTU heat value of the gas which is around 1000 to 1075 BTU’s per cubic foot that depends or what they use in your area. For simplicity of math we will use 1,000 times 180 gives us 180,000 BTU’s what the burner is actually firing at for input.


    What altitude is this burner operating at? That has to be calculated if over 2000 feet?


    The oxygen is way out of what it should be (5 to 7 % on this system would be normal) this then causes excess air to be too high. The excess air would be around 30 to 50 %. That would give a CO2 of 8.5% top 9.5%. Your draft should be around -.02. On a tester you typically get two CO readings one is a contaminated reading the lower reading the other is what is called a CO air free which is always the higher reading that is your actual CO reading. Cleaning helped the CO it looks like but it will get messed up again if it is not cleaned up. Is anything missing such as doors or panels etc. Those missing would allow excess air to be high. Why was the gas pressure increased? What does the rating plate call for? Clocking the burner will tell us if you are over gassed which can cause these high readings on air. As I increase the gas I increase the size of the package which means more air required.


    The other thing is the stack temperature is extremely low it should be between 350 to 450.


    Are you taking these readings sufficiently into the draft to get down into the sections of the boiler? If you take them after the draft hood in the flue with this system which I believe has a built in draft hood the readings will be way off.


    If you can send some pictures showing the boiler, vent chimney connection and show me with an arrow the location of the probe to take your readings. If you are not way into the boiler itself with the probe you will get the high readings you have.


  • furnacefigher15furnacefigher15 Posts: 508Member

    I'm glad to see the CO levels were able to be reduced.

    O2 is still very high. Where are you putting the probe?

    Should be before any draft dilution takes place.

    CO air free is a calculation that the meter does. CO air free is what ANSI cares about.

    CO air free = 20.9 (oxygen in free air) / (20.9 - O2 in stack) X ppm CO

    Example: 500 ppm CO @ 3.5%O2

    20.9 / (20.9 - 3.5 ) X 500ppm

    20.9 / 17.4 x 500

    1.201 x 500

    600.5 ppm air free
  • Tim McElwainTim McElwain Posts: 4,231Member
    KCA is at 7,000 feet altitude

    so different rules apply.Table F.4 in NFPA 54 Equivalent Orifice Sizes at High Altitudes (including 4% input reduction for each 1000 feet.) Orifice size reduction starts at any altitude 2,000 feet or more.

    There is also the possibility of the readings being taken at the wrong location.

    When you have a built in draft hood you need to get the probe into the boiler sections in order to get correct readings. Your return water temperature is also high what is the operating temperature of the boiler? A 20 degree delta t should give you a return temp around 160.

    This should be giving you a stack temperature over 300 degrees.


    You drill a hole in the draft hood then stick the probe through the hole into the boiler other wise you are picking up all the air coming in through the draft hood (dilution air) this will cause stack temp to be too low and excess air high along with O2 incorrect. None of your readings are any good until you do what I am telling you.
  • dano415dano415 Posts: 22Member
    fieldpiece SOX2?

    1.  The  fieldpiece SOX2 looks like a great deal?

    2.  I need to find out if the sensor is replaceable, and at what cost?

    3.  I honestly feel the, "You need to buy a Quality instrument when dealing with

    combustion analysis!!"  are company reps for the competition over priced O2 sensors?

    (I would really like to ban all Company Reps--period--from these boards)

    4.  Fieldpiece has deep pockets.  Why would they risk a lawsuit over a cheap meter?

    5.   A $200.00 meter is a great buy.  In two years it will pay for itself if your a DIY

    guy.  Plus you can fine tune your heat source more than once a year.

    6.  Bachyrach, Testo, and UEI need competition--badly. 
  • dano415dano415 Posts: 22Member
    edited December 2011
    fieldpiece SOX2?

    If anyone knows about the sensor, and if it's replaceable please post.

    Oh yea,  I would love to read the post on the HVAC technician who built his own O2

    sensor out of a wide band  Lambda sensor?  I can't find the post. 
  • drhvacdrhvac Posts: 189Member
    more testing questions

    I went on a service yesterday. This furnace is approx. 4 -5 years old, its a horizontal  60,000 btu 80% in an attic. As soon as I looked at it, I saw a problem with the venting. A 4" elbow comes out of flue, goes up 1', elbows again, goes approx 4' into the bull of a 6"tee. The 6" pipe is coming from another appliance downstairs. Once it meets up with the furnace in the attic, the other side of tee elbows up and out the roof of the house. The fact that the furnace in the attic vents to the middle part of the tee instead of a Y where it would flow out easier is wrong. That being said here were my readings.

    stack temp 283

    co2 5.28

    o2 11.5

    co air free 6

    excess air 108.3

    eff - 82.7

    draft - 0.1590

    Obviously the draft is wrong, it should be negative, but is has been working like that for approx 5 years now with no problems. If that was an atmospheric burner, it would be backdrafting out of the draft diverter. I suppose if it was a real problem with this furnace, the pressure switch would be tripping? Thx.
  • Tim McElwainTim McElwain Posts: 4,231Member
    I assume this is a

    fan assisted 80% furnace which gets its air for combustion from the attic, is that correct? If so technically it is a atmospheric fan assisted furnace. The top of the vent outdoors must be at least 5 feet above the top of the unit.  Units in attics sometimes have the same problem as roof top equipment as they can no develop much temperature difference nor height to get good venting. They are also operating with typically in the winter time very cold air for combustion which affects their operation.

    What was the point that you took the readings in relation to the furnace?

    The O2 and excess air are way out of sorts, the 283 stack temp is very close to 275 minimum before condensing and is probably condensing like crazy when the attic temperature drops.
  • drhvacdrhvac Posts: 189Member
    point of measurement

    was about 4" after the 2nd elbow coming out of the furnace. The gas pressure was on the money. Yes this was taking attic air for combustion, and it wasn't that cold when I was there, maybe 45 outside, and the meter says it was 67 in the attic. This is what I don't understand, how could you correct the co2 and o2 readings? The only thing you could adjust is the gas pressure which is good. There is no control of how much air the unit pulls in.  
  • Tim McElwainTim McElwain Posts: 4,231Member
    There are other things

    that can be done but this is not the place to discuss them. We cover these things in our courses and some things are better not discussed here in an open forum as people who are not trained may try them and they should only be attempted by those who have been trained and are experienced in such adjustments.
  • ChasManChasMan Posts: 459Member

    I have the same meter. I bought it new 3 1/2 years ago. The O2 sensor gave up this summer. I have used it perhaps 10 times, never dropped,m always stored inside. I sent it in to be calibrated. The results I got back from Wohler indicated that the draft reading was out. Now when I test draft where I showed -.-02 before I show 0 now. Also, my Stack temps went up 100 degrees.

    Just Great.

    I know these things are more indicators than lab quality instruments but come on.

    I ordered a Dwyer 460 for draft.
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!