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HVAC Supply Air

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  • HVAC Supply Air

    I own a 2500 sqft home that is heated and cooled by a heat pump. All the rooms in the house are quite comfortable with the exception of one room that is supplied by an 8" round duct 18 feet from the trunkline. The room also has an 8" return to the heatpump. Problem is that this room does not seem to be receiving the velocity of air needed to heat and cool the room properly. I suspect that the 8" supply might be to big and is cutting down on velocity of air to the room. Would running a 6" duct in its place add more airflow to the room?

  • #2
    Re: HVAC Supply Air

    Close registers slightly in other rooms to increase the airflow to the problem room. Imagine Air like water in that it takes the path of least resistance...

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    • #3
      Re: HVAC Supply Air

      the 18 ft of flex does not help
      make sure it is straight with no kinks
      Charlie

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      • #4
        Re: HVAC Supply Air

        The 18 foot run is rigid round metal duct. I do receive airflow but it seems weaker than other supply vents. I suppose that cutting down airflow to other rooms would work but I don't want to affect comfort levels. I just question why all the other room supply ducts utilize 6" ducting over shorter distances and this one room utilizes 8" duct over a long run. I would think that a 6" duct would allow for more airflow at higher velocity. I am probably mistaken, yet curious.

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        • #5
          Re: HVAC Supply Air

          Originally posted by zzxx View Post
          I am probably mistaken, yet curious.
          Correct, you are mistaken. Its not velocity you want, its volume. An 8" pipe will move twice as much air as a 6" at the same static pressure. If load calcs and a Manual D duct design were done on your house then theres a good reason why theres an 8" running to that room and not a 6". Its because that room called for more airflow either on the heating, or the cooling side. I'd check to make sure the takeoff is not cut in a bad location where it wouldnt receive proper airflow. Im sure you have already looked for an adjustable volume damper.

          Andy

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          • #6
            Re: HVAC Supply Air

            Originally posted by zzxx View Post
            The 18 foot run is rigid round metal duct. I do receive airflow but it seems weaker than other supply vents. I suppose that cutting down airflow to other rooms would work but I don't want to affect comfort levels. I just question why all the other room supply ducts utilize 6" ducting over shorter distances and this one room utilizes 8" duct over a long run. I would think that a 6" duct would allow for more airflow at higher velocity. I am probably mistaken, yet curious.
            i agree totally dont decrease the duct size! either close other room dampers/ registers or add another supply duct to this room, or add a small inline booster fan

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            • #7
              Re: HVAC Supply Air

              Some info on inline duct booster fans.

              http://www.espenergy.com/in_line_duct_fan.htm

              http://www.hvacquick.com/suncourt/Duct_Fans.htm

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