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  1. #1
    Join Date
    Mar 2010
    Posts
    3

    Unhappy negative house pressure

    we have what I believe is negative pressure in our home. when we moved in years ago I caulked all the gaps, etc to tighten up all drafts and then yrs later had it sided with vinyl with foamboard underneath the siding. now when the a/c or heat is on it wants to draw air down the chimney and even after 15-20 hours push the water out of the upstairs trap in the bathtub and we wake up to sewer gas in the morning. If I run some water in the tub before bed, then it is usually ok unless hvac has been running alot. HELP - do I need a ventilator system to let more fresh air into the system?

  2. #2
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: negative house pressure

    Quote Originally Posted by bkf View Post
    we have what I believe is negative pressure in our home. when we moved in years ago I caulked all the gaps, etc to tighten up all drafts and then yrs later had it sided with vinyl with foamboard underneath the siding. now when the a/c or heat is on it wants to draw air down the chimney and even after 15-20 hours push the water out of the upstairs trap in the bathtub and we wake up to sewer gas in the morning. If I run some water in the tub before bed, then it is usually ok unless hvac has been running alot. HELP - do I need a ventilator system to let more fresh air into the system?
    I'm trying to follow how the water in the traps being pushed out from a negative pressue from the HVAC ?

    Which chimney is being affected ?

    What type of heating system do you have ?

    is it located --- in the attic ?

    Is there one large centrally located return air vent ?

    Is there a whole house exhaust fan ?

    Are there any attic exhaust fans ?
    "" an ounce of perception -- a pound of obscure "
    - Rush

  3. #3
    Join Date
    Jun 2007
    Location
    Fayette County, Ohio
    Posts
    5,797

    Default Re: negative house pressure

    AC should only be recirculating the air in the house and have little affect however if you have gas or oil heat (furnace or boiler) it will be pulling air out of the house to supply the flame and flue. You may need to add a fresh air supply for your furnace.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  4. #4
    Join Date
    Jan 2010
    Posts
    163

    Default Re: negative house pressure

    A number of factors in any combination could be causing this.

    Is the house 2-story? Is the top of the chimney lower than the highest part of the roof? If so, the stack effect of the high portion of the house may be strong enough that it's trying to draw air in at lower places. One of the lower places would be the fireplace, but only if the top of the chimney is lower than the highest part of the roof.

    There could be insufficient return air in some parts of the house. This, in combination with a large return in the same space as the fireplace would cause the space with the large return air opening to become de-pressurized in relation to the outdoors and to other spaces in the house.

    Do you have a whole-house exhaust fan? This could de-pressurize the house.

    Are there ducts in the attic? If so, and if they are leaky on the supply ducts, then the living spaces will become de-pressurized in relation to the attic and to the outdoors.

    As for the sewer gas smell, I suspect that the trap on the bathtub is not properly protected by a vent, and that the trap has always been siphoned at times when conditions are right (or wrong) in the drain/vent system. It's just that you notice it when the de-pressurization happens because only then is sewer gas being sucked into the living space. At other times, the stack effect of the vent system causes air to flow the opposite direction. The fact that you can eliminate the smell by refilling the trap proves that the de-pressurization is not strong enough to actually overcome the trap seal. (This is because trap seals are typically 2" deep, and the pressures created by the duct system, either positive or negative, are much less than that.)
    Last edited by rdesigns; 03-12-2010 at 09:59 AM.

  5. #5
    Join Date
    Mar 2010
    Posts
    3

    Default Re: negative house pressure

    Quote Originally Posted by rdesigns View Post
    A number of factors in any combination could be causing this.

    Is the house 2-story? Is the top of the chimney lower than the highest part of the roof? If so, the stack effect of the high portion of the house may be strong enough that it's trying to draw air in at lower places. One of the lower places would be the fireplace, but only if the top of the chimney is lower than the highest part of the roof.

    There could be insufficient return air in some parts of the house. This, in combination with a large return in the same space as the fireplace would cause the space with the large return air opening to become de-pressurized in relation to the outdoors and to other spaces in the house.

    Do you have a whole-house exhaust fan? This could de-pressurize the house.

    Are there ducts in the attic? If so, and if they are leaky on the supply ducts, then the living spaces will become de-pressurized in relation to the attic and to the outdoors.

    As for the sewer gas smell, I suspect that the trap on the bathtub is not properly protected by a vent, and that the trap has always been siphoned at times when conditions are right (or wrong) in the drain/vent system. It's just that you notice it when the de-pressurization happens because only then is sewer gas being sucked into the living space. At other times, the stack effect of the vent system causes air to flow the opposite direction. The fact that you can eliminate the smell by refilling the trap proves that the de-pressurization is not strong enough to actually overcome the trap seal. (This is because trap seals are typically 2" deep, and the pressures created by the duct system, either positive or negative, are much less than that.)
    It is a 2 story and all 4 bedrooms upstairs have return air vents as well as living, dining and family room and chimney is higher than roof - once I had a roaring fire in fireplace and happen to go to the unfinished side of the basement and air was flowing down the airvent to the gas furnace into the room. House being tight and I guess it was the easiest way for air to flow to the fire. I put a fireplace heat stove insert into the fireplace and it does not draw as much air so fireplace isn't the problem - only a symptom. The gas smell is the problem. If the tub is not used long enough the trap will eventually loose some of its water and gas comes into the tub area. If I crack a window in the bathroom, then no problem or if it is a day the furnace or a/c does not kick on then no problem.

  6. #6
    Join Date
    Mar 2010
    Posts
    3

    Default Re: negative house pressure

    Quote Originally Posted by canuk View Post
    I'm trying to follow how the water in the traps being pushed out from a negative pressue from the HVAC ?

    Which chimney is being affected ?

    What type of heating system do you have ?

    is it located --- in the attic ?

    Is there one large centrally located return air vent ?

    Is there a whole house exhaust fan ?

    Are there any attic exhaust fans ?
    We have natural gas furnace in basement. Return Air vents in all bedrooms, living room, dining room and family room. Small exhaust fans which we rarly use in bathrooms. There is a attic exhaust fan that comes on in summer when temps in the attic are high enough, but does not correspond to problem. Real problem is the gas smell from drain in tub when water has not been ran in tub for more that 12 hours or so. I believe the pressure of the air flowing into the bathroom from the furnace or a/c is pushing some of the water out of the trap. If it is a day the furnace or a/c does not come on then no problem.

  7. #7
    Join Date
    Feb 2010
    Location
    Eastern MA
    Posts
    75

    Default Re: negative house pressure

    If you are drawing air down the chimney, you are backdrafting the chimney. This is a very serious situation. Backdrafting the furnace or water heater can bring carbon monoxide into the house. It is also possible for the flame to roll out of the furnace or water heater.

    You need to have someone with experience solve this and not rely on the advice of people here who may or may not have the knowledge to diagnose this and even if they do, need to see the actual situation. Try to find someone who is certified by the Building Performance Institute (BPI). BPI.org

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