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  1. #1
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    Jul 2007
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    Chicago
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    Default outside intake air vs inside

    Does anyone have a chart or reference as to the increased efficiency of using outside combustion air on Cat IV furnaces versus inside air (non hooked up combusition air kits)? This would help me settle an issue with a builder who claims its only 1% and helps being fresh air in...yada 3x.

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  2. #2
    Join Date
    Mar 2007
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    Near Philly, Pa.
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    Exclamation Re: outside intake air vs inside

    As indoor air is consumed by a furnace, replacement air must come from somewhere. You can allow the furnace to inhale indoor air instead of piping it in and the lower part of the house will depressurize enough to draw in makeup air from the paths of least resistance. This will cause cold air infiltration in places they probably do not want it. As cold air enters the living space, it will cause a call for heat and run the furnace anyway. Piping in the combustion air reduces cold air infiltration while reducing the chance of backdrafting any remaining atmospherically vented appliances.
    Tell him to pipe in the air then spend his money on sealing the ducts and weatherizing the home. These unpiped Cat.IVs can be dangerous.
    Bob

    Keep the fire in the fireplace.

  3. #3
    Join Date
    Mar 2007
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    Dallas, Texas
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    4,112

    Default Re: outside intake air vs inside

    Quote Originally Posted by Ross Neag View Post
    Does anyone have a chart or reference as to the increased efficiency of using outside combustion air on Cat IV furnaces versus inside air (non hooked up combusition air kits)? This would help me settle an issue with a builder who claims its only 1% and helps being fresh air in...yada 3x.
    Can't help you Ross. Have you tried the manufacturer?

    Jim Luttrall
    www.MrInspector.net
    Dallas, Texas

  4. #4
    Join Date
    Feb 2008
    Location
    Chicago IL
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    1,937

    Default Re: outside intake air vs inside

    I don't have a link for fact info. I have looked into it and read various things. The efficiency difference from what I have read is minimal, a few % 1-3,4. I've never seen anything that states some high rate.
    I don't believe that efficiency output is the real issue or concern, either in the field or by the manufacturer's from what I have read. I have to agree with Bob that the real issues area safety and proper operation.
    There are usually two points I discuss with the client:
    1- starving the unit for air and poor operation; You know when they build the furnace into those nice tight little closets with solid doors.
    2- creating negative pressure in the house as Bob discussed

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  5. #5
    Bob Bob's Avatar
    Bob Bob Guest

    Default Re: outside intake air vs inside

    It is indeed a safety issue and is code here, depending on where the furnace is located. In an attic? no problem and not required (unless the attic has no outside air access). In a closet in the living space of the house. Yes, you need outside air for combustion. Otherwise, it draws air from the living space and can use up oxygen in tight homes.

    As previously mentioned, it can also create negative pressure in the house (which is likely already present anyway due to various factors unless fresh air is introduced to the return air of the system).

    This can be mildly problematic (i.e., drawing unconditioned outside air into the living space, reducing the efficiency of the system...like leaving a window open) or, in tight homes, serious, creating negative pressure in the living space but not drawing enough outside air to equalize.

    This means that combustion fumes lack the required positive pressure to properly vent, which of course means that they can enter the living space. Flame roll-out can also occur. Fire or asphyxiation.


  6. #6
    Join Date
    Feb 2008
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    Default Re: outside intake air vs inside

    On the other hand you are introducing air changes if combustion air is being pulled in from interior.
    Remember there is also the influence of stack effect.
    Indoor air is more polluted than outdoor air.
    Guess its a trade off, spend a little more on introduction of make up air through cracks or seal the house so tight there are fewer air changes, next to impossible in older homes.

    What circa is the house?

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