Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Combustion air temps and oxygen content

Rocky_3 Member Posts: 231
Greetings and salutations, groovy guys and groovy gals. Trying to find a chart relating combustion air temps to oxygen content. Doing tune-ups here we have a lot of direct-ducted combustion air right to the burner. In middle of winter, we can see combustion air temps of -40 or so. So if we tune to these numbers, the oxygen content will be way different when the boiler is running in the summer with +75 degree air temps. Looking for some info along the lines of: " oxygen will decrease by (pick a number) % for every 40 degree rise in combustion air temps." I know I've seen a chart like this in the past, but cannot remember where I saw it at. Anyone have anything like this? This came about from a discussion some of my techs were engaged in this morning, and while everyone knows oxygen will change, just trying to nail down something more than just anecdotal numbers.
Thanks, guys and gals,


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,132
    edited May 2020
    Aren’t you the same guy that asked this on Facebook? I think @Robert O'Brien provided you with all the info you need.
    @captainco (Jim Davis) would probably also know.
  • Rocky_3
    Rocky_3 Member Posts: 231
    Yes, that is me. I just took a shot-gun approach and sent this out to several forums.
  • Jamie Hall
    Jamie Hall Member Posts: 21,113
    percent oxygen in the air changes very very little with temperature, if at all. What does change is air density. Cold air is considerably more dense than warm, and whether you are dealing with forced air burners or induced or natural draught, that will make a considerable difference.

    But you are not looking for changes in oxygen percentage. You are looking for changes in density. It's related to the absolute temperature, which is the Fahrenheit temperature plus 459.

    Combustion is related to mass air flow, not volume air flow -- colder temps, greater mass. Why cars have mass air flow sensors. Why turbocharged engines have intercoolers. etc.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,132
    edited May 2020
    Rocky_3 said:

    Yes, that is me. I just took a shot-gun approach and sent this out to several forums.

    Lol. The same people are in all the same forums.
    If your combustion air is dropping to -40 I’d say you need a way to warm that up. I don’t think many, if any manufacturer wants that temperature air entering the burner.
    If you’re talking about combustion air boot on an oil burner, longer length of pipe, even some up and down or 'elephant trunk' should get the combustion air closer to room temperature.
    You must be up in the Yukon or Alaska.
    Beyond the standard practice of adding air after establishing true zero smoke, for a cushion, I wonder if maybe the best thing may be to go back in the dead of winter and do another combustion analysis.
    I’d be curious to see how combustion changes.
    EBEBRATT-Ed Member Posts: 13,454
    It's not the temperature of the air it's the amount of change in the temperature.

    As @Jamie Hall said the colder the air the more dense the air becomes, the denser the air the more oxygen will be delivered to the burner.

    As @STEVEusaPA mentioned I wouldn't want a burner running with -40 deg air.

    I would thinks about an ERV (energy recovery ventilator) to heat the incoming outdoor air with outgoing basement air.

    Other than that you adjust the burner during warmer months and hope it doesn't get too lean in the cold weather.

  • Zman
    Zman Member Posts: 7,447
    The air density does change with temperature and can have a noticeable effect on combustion.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    HVACNUT Member Posts: 5,032
    The Buderus BE burner is set up by adjusting CO2, and therefore O2, according to combustion air temperature.
  • captainco
    captainco Member Posts: 755
    There was an ASHRAE study in 2006 that stated outside cold air for combustion was not as good as previously thought. Often I here air temperatures related to auto engines which is a bad comparison. Cars use cold air(nitrogen) to delay combustion to add more fuel before it is compressed with the air, giving them more power. However, in non-compression combustion cold air does not mix well because it is denser. Might be more air but just like cold fuel, it doesn't mix well. Therefore you end up with higher O2 readings and lower CO2 readings when the air gets cold.
    That is a great chart but I doubt combustion and burners are 100% linear.

    In industrial process it is common to pre-heat combustion air which allows for better mixing and O2 readings down to 1%.

    I have seen charts that show that for every degree the combustion air temperature goes up the flue temperature goes up one degree. But the warmer the air, the hotter the flame. Again the chart is 100% linear which I don't believe, but I don't have any way to prove it.

    Remember, as air gets colder it gets denser, but air is also 79% nitrogen which doesn't help combustion at all.
  • Rocky_3
    Rocky_3 Member Posts: 231
    Thanks to all who have taken of their time and chimed in here. Appreciate the edification on this subject. That's why I love these forums: aggregate knowledge of some very sharp individuals. Thanks again for the input.