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Thread: Air Velocity

  1. #1
    Join Date
    Dec 2012
    Location
    Tarpon Springs, FL
    Posts
    8

    Air Velocity

    I was told by a HVAC company that I don't need to enlarge my current duct when replacing it with new carrier infinity 16 seer 5ton unit. The current main duct started at size 30x14 then reduced to approx. 16x8 because it's between floors and blocked by the beam in middle of house holding upstair bedroom. He states that because the main duct is straight from one end of house to the other, when duct gets smaller, air velocity will force it thru to all the vents at other end of the house. What's your thought on this? I don't believe him but like to have some expert opinion. Thank you!

  2. #2
    Join Date
    May 2008
    Location
    Central Fla.
    Posts
    311
    First question is how does it perform as it is? Ductwork design is an art/science. With that said the person designing it can do a killer job on paper, but when on the job the builder can shrink trusses between floors and move plumbing and electrical into the same spaces the ductwork has to go. At that point it is a free for all first come gets the space first and all the others have to work around whats there.

  3. #3
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    2,262
    How far does it go before it reduces? How many heat runs come off before it shrinks, how many total? Any issues with the current system and warm/cool rooms when the system is running. It sounds llike it is running fromt he furnace all one way, correct?

  4. #4
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,564
    Reductions in ductwork size are normal and needed to maintain air velocity. The reductions are made as the "runs" airflow is deducted from total. For example, if you need 1200 CFM total, and you've already connected a few runs (outlets), then you can reduce the size of the ductwork supplying the remaining outlets.

  5. #5
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    How did they determine you need 5 tons? Must be a HUGE home. Even with little insulation and lots of air leakage, 5 tons could satisfy a 3000sqft home if distributed well and depending on windows and shading. Florida is humid, but not nearly extremely hot, so sizing is even more crititical and oversizing will really hurt comfort since it won't dehumidify. In a humid climate, you're better off a little undersized than oversized.

    As for that smaller duct, it won't allow more than abotu 800CFM without overly high static pressures. So this duct just feeds part of the home? Does that part of the home need more than 50% of the total capacity?

    Have you considered dropping down to a 4 ton and installing a zone control system . Infinity zoning is one of the best zone systems on the market. If you have 2 levels, you should have zone control or 2 systems. Ideally, you have 2 zones on each floor, so you divide the levels of the home as well a the North vs. South facing rooms. With a 4 ton infinity zone sytem, the smallest zone only needs to handle 700 CFM on low stage without having to partially condition the other zones.
    Last edited by motoguy128; 01-09-2013 at 10:05 AM. Reason: oops, wrong post.

  6. #6
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    As for velocity pushing air... well air has both a dynamic and static properties. ON the static side, the blower generates pressure. At a gven pressure and size of opening, you will have a certain CFM. You have a pressure drop however caused by air movement, pipe reduction and friction in the ductwork (no matter how smooth it is). Dynamically, air also air has mass/inertia. So a duct located at the end of a branch, especially at higher velocities, will see higher pressure and get more airflow than a pipe in the middle. The air is recognizing what the air does after a curve, elbow and transition.

  7. #7
    Join Date
    Dec 2012
    Location
    Tarpon Springs, FL
    Posts
    8
    Thank you for such detailed explaination. This system handles only the 1st floor of the house which is approx. 2500 sqft. I've asked him that I prefer a smaller system but he insist that we need a 5 ton for downstair. He's the sales guy that's why I question his answers. Our second floor is approx. 2200 sqft with 3 skylights and high ceilings and he suggests 4 ton for upstair which we feel is right because the foyer area is 20 feet high. The carrier representative states that if the duct is too small the static pressure could go up too high and shut down the infinity system because of the built in safety device in air handler. Our dilemma is whether we need to tore out the ceiling and rebuild duct to accommodate the new system for the first floor. Old system was gone when we purchased the home so can't do the testing on static pressure. The main duct has two flex runs before reduce to 10x20 then 3 more runs then to 10x10 where the beam is then to 10x20 again and 6 runs towards the end. Thank you

  8. #8
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,004
    There is always a proper sequence order in which everything is properly accomplished; it appears that sequence is totally out of order.

    In your situation the first thing to do is a Home Energy Efficiency Rating/Audit. Then after those issues are resolved a room by room Manual J heat-gain & heat-loss calcs are performed.

    Then a Manual D is performed on the duct system as it relates to all the pressure drops as they relate to the blower's capacity ratings.

    Use an ACCA Manual D Friction Rate Work Sheet with a Friction Rate Chart...

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