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  1. #1
    Join Date
    Dec 2007
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    4

    Default Backdraft - Negative Air Pressure

    We experience cold air flowing down the masonry chimney when the fireplace is not in use - even when the damper is closed. When I do lite a fire, the fire tends to die down after 45 minutes or so and the fire seems to cause the hot water tank to backkdraft. How do I correct this? Would installing a 4" outside air intake to the return air on the furnace fix the problem? (I have a 90% efficiency furnace

  2. #2
    Join Date
    May 2011
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    251

    Default Re: Backdraft - Negative Air Pressure

    Normally a 90% High Efficiency furnace should already be drawing in outside air on it's own PVC pipe. Depending on the furnace size, pipe length and number of elbows, it may only need to be 3".

    I'm guessing this is a 2 story house? Possibly with high ceilings upstairs?

    You problem is likely related to stack effect. You have air leaks upstairs to the attic or out upstairs windows that are making the downstairs and basement negative pressure. Made worse if you run a bath fan or kitchen hood exhaust fan vented outside. IF there are more leaks upstairs than downstairs, you might start backdrafting the hot water heater. The flue pipe might also be partially blocked or poorly designed... or even undersized.


    IS the furnace exhaust run out of it's own PVC pipe? Or is it tied to the flue (I hope not... that's very ,very bad)
    1925 Two-Story Stucco Beaux Arts Neoclassical

  3. #3
    Join Date
    Dec 2010
    Location
    Houston Texas
    Posts
    2,969

    Default Re: Backdraft - Negative Air Pressure

    Your air intake to the house needs to be larger, at least equal in size to the exhaust on the furnace unless the intake air has a suction / blower to increase air flow. You are not getting ehough air to the furnace to replace the air for combustion AND the air leaking out of the house when it's heated.

    I lived in a house with the same problem. After extensive research by teams of engineers from Princeton and Drexel, it was determined that I needed to put a 6" round hole through the foundation of the house all winter long as the house was constructed far too tightly. It worked like a charm.

  4. #4
    Join Date
    May 2011
    Posts
    251

    Default Re: Backdraft - Negative Air Pressure

    My question still wans't answered. Unless a contractor or DIY'r too the easy way out, the furnace should have it's own outside air intake. IT should be sealed combustion if it's a high efficiency unit (90%+).

    Honestly, I'm waiting for codes to ban natural draft appliances for new installations... and only allow them for retrofits. I figure that will happen in the next few years. OF course, they should ban vent free appliances as well with exception to gas ranges. But even then, there should be an integrated fresh air system when those are running.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  5. #5
    Join Date
    Dec 2007
    Posts
    4

    Default Re: Backdraft - Negative Air Pressure

    The problem isn't with the furnace, the problem is with the fireplace. Yes, the furnace has pvc piping for air in and exhaust and the furnace works fine. When I light a fire, after a while it seems to be starving for air, and like I said, it causes a backdraft from the hot water tank which is telling me the fire is loooking for air and pulling air from the hot water tank flu which is completely separate from the fireplace chimney. I need to introduce outside air but I dont want to keep a window open. Would a 4" duct pipe (fresh aier intake) from the ouside that is connected to the return air duct of the furnace fix this problem, or would that mess up the furnace?

  6. #6
    Join Date
    Dec 2010
    Location
    Houston Texas
    Posts
    2,969

    Default Re: Backdraft - Negative Air Pressure

    It will mess up the furnace operations.

    You need to supply fresh air to the fireplace directly if at all possible. Can you core drill through the chimney to the outside? But that's pretty much like leaving a window open anyway.....

  7. #7
    Join Date
    Jul 2009
    Location
    Northeast
    Posts
    661

    Default Re: Backdraft - Negative Air Pressure

    Quote Originally Posted by rzxspz View Post
    The problem isn't with the furnace, the problem is with the fireplace. Yes, the furnace has pvc piping for air in and exhaust and the furnace works fine. When I light a fire, after a while it seems to be starving for air, and like I said, it causes a backdraft from the hot water tank which is telling me the fire is loooking for air and pulling air from the hot water tank flu which is completely separate from the fireplace chimney. I need to introduce outside air but I dont want to keep a window open. Would a 4" duct pipe (fresh aier intake) from the ouside that is connected to the return air duct of the furnace fix this problem, or would that mess up the furnace?
    I can't see why running a four inch return from the outside would hurt, there are commercial RTU's that don't use return air and are dependent on using outside air, some type code in MA. that say's there has to be so much outside air. All that said I don't know if it will help, but it could as your putting more positive pressure into the house.

  8. #8
    Join Date
    Dec 2010
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    Houston Texas
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    2,969

    Default Re: Backdraft - Negative Air Pressure

    Too much air to the furnace isn't going to be good for the furnace The air needs to go to the fireplace.

  9. #9
    Join Date
    Jul 2009
    Location
    Northeast
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    661

    Default Re: Backdraft - Negative Air Pressure

    What will more return air do to the furnace?? I'm always open to ideas

  10. #10
    Join Date
    Dec 2007
    Location
    Tennessee
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    1,745

    Default Re: Backdraft - Negative Air Pressure

    I had a very similar problem once so I suspect that you may have the same thing. does the exhaust of the water heater use the chimney stack? Does it and the fireplace flue exit at the top of the chimney near each other and most important, do you have a single chimney cap that covers both flues?

    This was the situation I had, except it was the house heater and not the hot water that shared the chimney. I had to raise the cap up about 3" so that the fireplace smoke didn't get sucked down the heater flue and vice versa. The best answer is to have separate caps with their own spark screens/critter guards around them.

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