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Combustion make up air

James DayJames Day Member Posts: 185
What is the proper formula's for boiler room make up air.  I know you need at least 50cuft of air per 1000btu.  What size is the opening if i have to pull air from outside?  Thanks    James


  • Mark EathertonMark Eatherton Member Posts: 5,584
    It depends...

    It depends on which code you are dealing with, whether the air is coming immediately through an outside wall, or run horizontally for a little distance, or vertical.

    The general safe rule of thumb I always have used, and have never had any issues is 1 sq. inch per 5,000 btuH of total combined inputs. No allowance for deration due to altitude is typically allowed. 1/2 that air terminates within 6" of ceiling and other 1/2 within 6" of floor.

    The newer codes (IRC?) allows for one high penetration, but it is sized at 1 sq. in per 4,000 btuH.

    The dual placement is a throw back to the old days of solid fuel fired appliances that had a significant standby loss. The theory was that hot air rising would escape from the high outlet, and be replaced with cooler air coming into the lower. Only problem in reality is that the majority of the heat is radiant heat, which is not significantly affected by convection.

    The high air outlet is to allow any accumulations of natural gas to escape from the mechanical room. Works great unless you are running on propane....

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Tim McElwainTim McElwain Member Posts: 3,904
    What size is the

    room in which the equipment is installed? What is the total BTU of all the equipment?

    You ask about make up air, make up air is the air required to replace air being removed by mechanical exhausting. Air for combustion is different than "make up air" and typically requires two openings one 12" from the ceiling and one 12" from the floor.

    It all depends on were you are getting the air from is it all coming from outdoors?

    Get all that info and I can size it for you.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    Is there an "it depends" here?

    "Air for combustion is different than "make up air" and typically

    requires two openings one 12" from the ceiling and one 12" from the


    My boiler is in the garage. It happens to be a mod|con that gets its combustion air from outdoors, and exhausts to the outdoors with 3" diameter PVC pipe. The boiler is considered "sealed."

    Now the garage has two vents, a little over 12"x12". These were there when the house was built in about 1950. At the time, it had an oil burner that used air from the garage and vented out the chimney. Maybe there was a code then, and maybe the house complied with it.

    But the lower vent is about 5 feet above the floor (and above the ground outside), and it would not make sense to have it lower because the snow gets that deep and would obstruct that vent. Should not the lower vent be placed high enough to be above the highest snow likely to occur? If this is not allowed, what do people do?

    (My father's house in Buffalo, N.Y. had no vents at all, though it may have had lots of leaks. Coal burning hot air furnace in the basement. It would have to have had the vent for that basement near the top of the second floor. In 1977, the house was completely under the snow. I wonder if he turned the heat off. By then, that furnace had been converted to gas, but still no specific air intake for it.)

    The higher vent is several feet below the roof because were it any higher, there would have to be openings in the beams that hold the roof up.

    So neither of these vents are in compliance with what I think the code is. But I never had trouble running the old oil boiler, so I guess it was getting enough air. In fact, the former owner had deliberately blocked the lower vent (that I re-opened when I bought the house).

    Now with my new boiler getting its air directly from outside, and venting directly outside, I do not believe I really need any vent at all in that garage. I believe the code says I must have vents, but maybe not as large. When the inspector came, he said I should not block off the boiler from the rest of the garage. I had no intention of doing so. But I assume the inspector thought the boiler should have access to those two vents.
  • Tim McElwainTim McElwain Member Posts: 3,904
    Your Mod/Con does not

    need those openings so if you want close them up unless there is some atmospheric gas or oil system in the garage.

    Air for Combustion is a large study in itself, I have written a complete approximately  100 page manual on the subject. It took that much in order to cover just about any circumstance.

    As for height for termination above grade most codes list 12". That may leave a lot to question here in the North with snow levels in the 20" to 30 " level. No one will ever own up in any of the jurisdictions as to what is expected snow level in any particular area. It behooves property owners to police there air and vent terminations after snow. I personally have a Pellet stove which vents out the side of  my house it is 12" off the ground so I have shovel when it snows a lot. If I forget I have a lovely wife who never fails to remind me.

    If it is possible to survey an install at the time of roughing in the utilities I always tell them to terminate 30" above grade.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    Snow 5 feet deep.

    A couple of weeks ago, I got snow 5 feet deep around my house. I could not even open the doors to get outside. I had to open the kitchen door (tile floor in that room) and remove the glass from the storm door to get outside. Walking was almost impossible. When I put my foot down, it did not reach anything solid. It took me about 1/2 hour to get to the garage where the snow shovel was. The distance I can walk in well under a minute in less severe conditions. It then took me 2 hours to shovel to my front door. Then I called it a day. (I am 72.) Now my air vents (that my inspector did not want me to block) were above the snow. The air intake and exhaust pipe are over 8 feet from the ground. So I did not have to worry about those. The only thing was the condensate drain that just dumps out on the ground about 2 feet up. I never did clear that area. I guess I could now, but most of the snow is gone. There is a 1/2 inch tube from the condensate pump to where it goes outside. But the pipe to outside is about 1 1/2 inch PVC with a 90 degree elbow at the end. I know it was buried, but apparently the flow was unobstructed. If it were obstructed, the condensate would have gone into a 2 gallon bucket inside the garage where I would have noticed it.

    Someone said I should just run it into a pan and boil it away with a heater. I do not know if they were joking or not, but I thought that pretty funny, and showed an amusing lack of understanding about why one bothered to condense in the first place.
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