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  1. #1
    Join Date
    Aug 2007
    Location
    Chicago
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    20

    knocking noise in ductwork after heat turns off

    This is the first time that I've turned on the heat in the home I recently purchased, and after the furnace shuts off, I can hear a knocking noise in the ductwork that eventually stops after a few minutes. It is a Carrier split system with a gas furnace. Do you have any suggestions on what might cause this and how to fix it? thank you

  2. #2
    Join Date
    Apr 2007
    Posts
    6,285
    Sounds like the metal is warping from heat and when it cools off it's contracting back down.

  3. #3
    Join Date
    Oct 2005
    Location
    phoenix az
    Posts
    429
    he said not to feed them after midnight





























    darn gremlins!!
    "Do not let what you can't do interfere with what you can do"

    "Do what you can do the best that it can be done and no one will ever know what you can't do"

  4. #4
    Join Date
    Aug 2007
    Location
    Chicago
    Posts
    20
    I don't understand your reply, skorepeo. Is my post considered trollish?

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by beethovengirl View Post
    I don't understand your reply, skorepeo. Is my post considered trollish?
    Just funning a little,go with the other answer ,sounds likely to me.

    It can be a sign of low air flow,which causes the the heated air to be much warmer then normal.Filter(s) clean?

    Where is the duct located??

  6. #6
    Join Date
    Aug 2007
    Location
    Chicago
    Posts
    20
    Quote Originally Posted by dash View Post
    It can be a sign of low air flow,which causes the the heated air to be much warmer then normal.Filter(s) clean?
    We just replaced the filter.

    Where is the duct located??
    I'm not hearing the noise coming out of any particular register. I live in a 3.5 story townhouse, and there is a return duct on every floor near the base of the stairs, which is directly above the furnace on the first floor. I believe the heating ductwork runs up adjacent to the return ductwork [I'm probably using all the wrong terms! ], and I'm hearing the noise from within that wall. The noise is loudest between the 2nd and 3rd floor.

    thanks for your help

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    After a long run time,stand near the furnace and listen to see what you can hear.

  8. #8
    Join Date
    Dec 2002
    Location
    Toledo, Ohio, United States
    Posts
    12,901
    Twilli thinks listen and you should hear this

    http://www.youtube.com/watch?v=9hUy9ePyo6Q
    No Heat No Cool You need Action Fast

  9. #9
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Quote Originally Posted by beethovengirl View Post
    This is the first time that I've turned on the heat in the home I recently purchased, and after the furnace shuts off, I can hear a knocking noise in the ductwork that eventually stops after a few minutes. It is a Carrier split system with a gas furnace. Do you have any suggestions on what might cause this and how to fix it? thank you
    Some installs just are noisy in that way. There can be sheet metal stuff that can oil-can when it warms up and cools down. My parents house in Dallas is next to a bedroom, and sleeping in that bedroom I could almost believe the furnace was coal fired, it was so noisy. It has been this way for decades.

    You might define the noise itself as a malfunction, but I don't think it leads to any other malfunctions. I am just a homeowner in S.Texas so my experience with heating is not very deep.

    Hope this helps -- Pstu

  10. #10
    Join Date
    Oct 2007
    Location
    Midwest
    Posts
    194
    Does it make the noise after the fan was running in manual without heat? If it is just dependent on heat then it is likely due to the expansion and contraction of a long run of duct. An expansion joint may fix it, as it will reduce the amount of movement.

  11. #11
    Join Date
    Oct 2007
    Posts
    226
    Hard to say without hearing it but I wonder if you have a variable speed blower?

  12. #12
    Join Date
    Jan 2006
    Posts
    546
    I have seen this happen many times, Customer buys new type heavy pleated filter and system air return is run almost a vacuum on duct work, System cycles off or on and ductwork will flex and make a popping sound.
    rick

  13. #13
    Join Date
    May 2006
    Location
    Hot Springs, AR
    Posts
    1,043
    Quote Originally Posted by skorepeo View Post
    he said not to feed them after midnight
    darn gremlins!!

    Happy Halloween!
    "Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great." -- Mark Twain

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