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keggerman

Backdraft and Negative Pressure

keggerman
10 years ago

Our house was build in 2006 and we moved in two years ago. We live in Oregon and when the temp gets cold I feel our furnace kicks in way more than it should (off 15 to 20 min's then on for 15 to 20 min's). When it is off I notice a significant amount of cold are come in through the registers and cold air returns. I have inspected all the duct work and everything looks good except for some small leaks around the registers.

After a bit of reading online I'm suspecting my issue is due to negative pressure and a back draft from the exhaust pipe. If I crack a door and hold a piece of paper and can see a mass rush of air come in leading me to assume I have a negative pressure environment (this is with all fans off).

The house is 2 stories and around 2000 sq/ft. Has a gas fireplace on the main level that is shut off and everything else is pretty typical for a house of its age.

My thinking is I should install a backdraft damper on the exhaust pipe just off the furnace and add some extra insulation around the leaks I found near the registers. Is there anything else you would recommend I do to help my situation?

Comments (18)

  • mike_home
    10 years ago

    Where is the furnace located? Is it a conditioned space?

  • keggerman
    Original Author
    10 years ago

    It's located in the Garage. I'm assuming that means it's not in a conditioned space..

  • klem1
    10 years ago

    You have a supply duct open to the attic. Close or tape over a few to all supply grills DO NOT COVER RETURN.
    Turn fan on and start looking for moving air. If you still can't find the opening,there is a special oderless smoke that lingers and makes it easier to find. The hole/opening/disconect may be concealed between floor and ceiling or inside a wall.

  • mike_home
    10 years ago

    Check the duct work in the garage, both supplies and return. If there are ducts running in the attic check those also.

    I think this is where you will find the problem and not the exhaust.

  • keggerman
    Original Author
    10 years ago

    I'll go up in the attic and have another look, but everything looked good at first. How big of a leak should I be looking for? Is it possible the supply vents in the crawl space would be causing this issue?

  • mike_home
    10 years ago

    If the crawl space is much colder than the house then it is also a possible problem. Duct work in unconditioned space needs to be insulated.

    Look the the connections between ducts. This is also a source of leaks. Have the furnace running while you are inspecting and see if you feel air escaping.

  • ionized_gw
    10 years ago

    "Back draft" usually refers to a dangerous situation were air is being sucked in through a chimney of a passively-vented water heater or other combustion heating device. The negative pressure is usually supplied by exhaust fans or another heating device that is power vented. Back drafts can be caused by poorly-balanced forced air HVAC as well.

    Since this is happening with your blower off, this paragraph is just an FYI. A leak in a supply duct located outside of the dwelling envelope can depressurize all or part of a home. In a similar manner, a leak in a return duct can pressurize a home.

    When you describe the disturbance of the paper in the cracked door, Is this an interior or exterior door? What kind of water heating device do you have? Note that in two-story homes, the stack effect can pull air into the house from outside as the warmer air exits from leaks on the second floor.

  • keggerman
    Original Author
    10 years ago

    We have a gas water heater that is located right next to the furnace. I did the paper test on an exterior door.

    Would using a punk or incense be the best way for me to detect any air leaks?

  • klem1
    10 years ago

    "Would using a punk or incense be the best way for me to detect any air leaks?"

    NO NO NO! Those can start a fire. Besides,that isn't near enough smoke. When I said smoke, I didn't literly mean smoke. More like "smoke effect" as used in film and theater. Maybe you should bring in a pro before doing somthing like that. If you are diligant and precistant,you should find it with your paper test and looking around.

  • keggerman
    Original Author
    10 years ago
  • klem1
    10 years ago

    You're starting to make yourself look silly and tip your hand. I have nothing further to say.

  • jackfre
    10 years ago

    Google "DOE duct leakage" and read. I think you are a strong candidate for a reseal of your entire duct system. Oh, and buy some duct wrap and install that while you are at it. Look at Hardcast, Polymer Adhesive, Kingco and others in the duct sealing side. In your case, I'd suggest the mastic with a bit of fiber in it to help bridge the gaps. As well use the scrim tape to support the duct seal. Kingco's Glenkote is a good one. Polymers Airseal 33 would be good too.

    Do not be tempted to do this job with DUCT tape. It is good for about everything but what it is named for. It will dry up and blow away in about 6 months. Count on it.

  • ionized_gw
    10 years ago

    Only the very best duct tape will last 6 months on ducts, sometimes.

  • keggerman
    Original Author
    10 years ago

    I inspected the vents in the attic again and found the culprit. The vent at the furthest point had pushed off so I put some air duct sealant on it and re-attached with a zip-tie. The negative pressure seems to be fixed.. Fingers crossed..

  • energy_rater_la
    10 years ago

    good deal.
    hope you used mastic or mastic tape to seal
    duct. I prefer Hardcast brand 1402 mastic tape
    & ccwI-181 mastic with a paint brush to apply.

    seal all the ducts while you have the materials.
    and use the hardcast to seal supply boxes to
    floor/ceiling by removing supply grill & taping from
    flooring/ceiling into supply box metal lip.
    press well to seal. make sure tape will be covered
    by supply grill.

    best of luck

    best of luck.

  • keggerman
    Original Author
    10 years ago

    Not sure if this is a good idea or not so I figure I should ask it here before I actually do it.

    Would it be a good idea to use mastic tape on the inside of the supply registers where the flex pipe meets the supply box? I used it on the outside but thought it might be good for added protection against leaks if I put a layer on the inside as well. My only concern would be the content air flow may peal it off over time..

  • energy_rater_la
    10 years ago

    sure. I do it often. just make sure surfaces
    are clean & dry. use a cloth to press mastic
    tape down firmly. if you seal the mastic
    tape to the clean dry metal...it isn't
    going anywhere.

    best of luck.

  • jackfre
    10 years ago

    I would suggest you go to Hard Cast, Polymer Adhesives, and Kingco's web site as well as the afore mentioned DOE link and watch some of their tutorials. Do it once do it right. It is such miserable work and generally in tight locations that these sites can help you out

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