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

SteveB Member Posts: 19
Does anyone know how many cubic feet of air per hour is required per gallon of oil burned per hour? (oil fired boilers)Thanks in advance.


  • Brad White_9
    Brad White_9 Member Posts: 2,440
    At a minimum

    24 cfm of air per gallon of oil burned. I use 30 to account for excess air at 20% and some room ventilation.

    Both oil and gas are similar with oil requiring 7% more air per BTU.

    Another rule of thumb which is supported is to use 10 CFM per BHP. 1/3 of a GPH is about one BHP so the 30 cfm rule holds.

    EDIT: Memorandum posted FWIW

  • Robert O'Brien
    Robert O'Brien Member Posts: 3,537

    1365 Cu/ft per gallon. I believe this a theoretical minimum and more should be provided

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    That would be on an hourly basis

    just to be clear. (CFH versus CFM) I will post my memorandum in .pdf format above. Item 6 within that has it as 1431 cfh, close enough for Government Work but as you stated, more is necessary for excess air and cooling the room (as-if!)

    Thanks, Robert-

  • CC.Rob
    CC.Rob Member Posts: 128
    can this be easily translated to the residential level?

    This is very intriguing, as I've been wondering about adequacy of combustion air in my basement, particularly if I affect it by changes to the building envelope. Boiler is Burnham 206, ~ 164k btu in, 118k IBR out.

    What kind of cfm should be provided in a typical basement, and how might I go about determining the existing cfm?

  • Brad White_9
    Brad White_9 Member Posts: 2,440

    Fuel is fuel. But one hopes your house does not burn the levels in commercial/institutional work! Heck, you could heat your house from the waste heat alone.

    Your Burnham 206 at 164 MBH input (136 net MBH output) would require about 40 cfm when operating.

    Keep in mind your AHJ (building inspector, Authority Having Jurisdiction) may want the NFPA-54/ANSI Z223.1 National Gas Code "passive" solution.

    The CFM method we are taling about is for engineered systems In Lieu Of passive high/low combustion air.

    If your AHJ is going to accept an engineered system it will have to be interlocked so that the boiler fails if airflow is not established. Also will probably require a P.E.-stamped document of some sort.
  • Ed_26
    Ed_26 Member Posts: 284
    combustion air

    Don't forget you will need makeup air also - for exhaust fans, dryer etc. especially if you are tightening the building envelope.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Excellent point to bring up...

    With all of the focus on "net combustion air requirements", it is not good to lose sight of the bigger issue.

    Thanks, Ed.

  • Jim Davis_3
    Jim Davis_3 Member Posts: 578
    How much needed? How much required? Do I have enough??

    One of my favorite subjects along with venting. According to most combustion books natural gas requires 10.5 cu.ft of air per 1000 btus at perfect combustion. An additional 4.5 cu.ft of air is needed to operate clean and safe(approx). If you have a drafthood appliance another 15 cu.ft. of air is required for dilution. That is 30cu.ft. of air requirement if we are at nominal design. This only works if the draft in your chimney never goes above -.02"--03" because then more is needed for dilution. Barometrics require about 50% less dilution air than a drafthood. What about the already existing room air. 50cu.ft. of room area can also be used per 1000 btus. Do you subtract this area from the total a bring in the difference? ]

    After 28 years of field testing the #1 combustion air problem I have found is passive openings sucking out of the room, whether its grilles or pipes. 50% is usually the maximum passive air can be assumed to work in the first place. Fan powered combustion air is the only functional combustion air there is as was stated in 2000 ASHRAE report. Unfortunately the AHJ doesn't seem to care in many parts of the country but that hopefully is changing or at least some of the codes will!!
    How do you know if you have adequate combustion air? Check the draft of the chimney and measure O2 in the flue gas. Chimneys can't suck out of rooms without air. If you start running out of air you start seeing the O2 fall. If other things in the building interfer with combustion air(this is determine by a fall in the draft reading) then enough air must be provided to eliminate this problem. You can have all the combustion air in the world but still not have enough for the equipment. Chimneys and buildings are having a constant battle on who gets the air. It doesn't matter what the rules or codes might be, it is our job to make sure the equipment wins the battle because air can't read!!
  • CC.Rob
    CC.Rob Member Posts: 128
    thanks all. good info

    Thank you gentlemen. This makes it much clearer. I've also read enough here and elsewhere to not necessarily trust my "AHJ." The present system is interlocked, however.
  • SteveB
    SteveB Member Posts: 19

    The mechanical inspector actually suggested using an interlocked blower to make up the shortfall on this project's boiler room ventilation. I will be using a standard 200 CFM blower from Grainger with a proving switch and relay. It works out to be much cheaper than a packaged make up air unit. And he'll accpt it. Thanks to all for your excellent replies.
This discussion has been closed.