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CO re-infiltration

CO is venting out the roof or there are gas fired RTU on a building. Can the flue gases, specifically CO get back into the building. Obviously CO2 is heavier than air so that is not a question. But what about CO. It is always listed as slightly lighter than air which means at the same temperature it is lighter than air and if it is hotter it is even lighter.

But what about CO when it gets cold? Years ago I was researching some information on different gases and found an article that stated that because CO contains carbon, it can get heavier than air, especially below 32 degrees. Therefore if equipment is making high levels of CO and it is on the roof or going out the roof, it can easily re-enter the building through fresh air intakes or other vents or openings. This explains why CO is found on the opposite side of the building that the equipment may be on. I had to evacuate a commercial boiler room years ago because CO was coming down off the roof and back into the boiler room through the combustion air grilles.

I am thinking of all the RTU that might have been covered up by the recent blizzard on the East Coast and wondered how many CO poisoning are occurring that are going undetected?


  • Mark EathertonMark Eatherton Posts: 5,843Member
    I can't tell you how many ModCon concentric wall terminations I have watched doing the CO reversion shuffle. It's REAL obvious when its cold outside and there is just a slight breeze... Good thing it's a sealed combustion system, except for the open window that was ingesting the products of combustion... Per the manual, the termination was in compliance with the manufacturers recommendations.

    Just when you think you've seen everything there is to see, Ma Nature pipes up and shows you something you never thought you'd see...

    Right Jim? :smiley:
    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.
  • Jean-David BeyerJean-David Beyer Posts: 2,641Member
    Gasses, at normal temperatures, pressures, and gravity, once mixed, will not separate, no matter what their molecular weight might be. Now if you spill some liquid propane on the floor, the stuff will vaporize and the cold vapor will stay near the floor, but once it attains the same temperature as the air around it, it will mix and gravity will not separate it, even though its molecular weight is considerably more than that of nitrogen, oxygen, etc.

    Similarly, if you take the hot gasses from the vent of a home heating furnace or boiler, they will rise because of their higher temperature compared to the air around them. But as they rise, they will cool and when they get down to the air temperature, they will mix and stay mixed as well.
  • captaincocaptainco Posts: 428Member
    J-DB, I assume your comments are based on actual measurements you have made in the field rather than something that was just written in a book. If you stir a glass that has oil and water in it the oil and water will mix. But what happens when you stop stirring? Having measured gases, mostly CO and CO2, I have found that at normal temperatures, above freezing, CO is always higher at the ceiling than at the floor and CO2 is always higher at the floor than the ceiling.

    Many times in cold weather I have measured CO vented out the roof re-infiltrating the building below. In one case the CO came down the side of a commercial building and back through the combustion air grilles.

    Short term gases do not mix rapidly based on my measurements. Long term they may eventually diffuse and dilute.

    Take a CO2 meter and stick it on the floor and talk next to it. Put one up high at the same time. I think you will see things don't mix equally very fast.
  • ChrisJChrisJ Posts: 10,112Member
    Are you sure we can compare how liquids in an emulsion behave to how gasses behave?

    I'm sure there have been studies done on this but it's well above my head.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • captaincocaptainco Posts: 428Member
    Maybe, maybe not. Whether or not gases eventually mix is not the question or answer it is what will happen immediately versus over time. CO will fall when it is colder than 32 degrees. That is after it came out in the flue gases at 300 or 400 degrees and then cooled down. Had a factory in Kentucky, a school in Chicago and a restaurant in Cincy that all had re-infiltration from CO on the roof.
  • Tim McElwainTim McElwain Posts: 4,278Member
    I have seen what Jim is talking about in boiler rooms that had excessive outside air into the room and the air temp dropped below 30 degrees the CO was seeming to be heavier than air and much more difficult to ventilate.

    I decided to bring this to the attention of a friend of mine who is environmental engineer at a local college. He did a little testing (nothing documented) and found it seemed that CO in 70 degree air was measured in his test at a specific gravity of.89 as compared to air at 1.0. When the air temp was lowered gradually the specific gravity stayed the same until the air temp went to 25 degrees it then was at a specific gravity of 1.22 as it went to zero degrees it was 1.29. No further testing was done.

    This resolved some concerns I had over the years as to the time for CO to dispate after accumulation especially in garages with heating equipment installed in the garage.
  • SWEISWEI Posts: 7,356Member
    Great info, Tim. Thank you (and your friend) for providing those numbers.
  • Jean-David BeyerJean-David Beyer Posts: 2,641Member
    "J-DB, I assume your comments are based on actual measurements you have made in the field rather than something that was just written in a book. "

    Sorry: very bad assumption. But I learned in high school chemistry, and had confirmed in college chemistry, that once mixed, gasses stay mixed.

    I may not agree with Einstein about everything, but long ago he wrote a paper on the Brownian movement and why this is so, and I agree with it.
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