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  1. #14
    Join Date
    Jul 2005
    Location
    Cleveland, Oh
    Posts
    10

    bypass sizing

    For most applications, a 10" bypass can pass upto about 1000 cfm.

    Calculate bypass based on velocity, not friction loss. The Bypass duct goes from the area of highset positive static to the areas of lowest negative. Also there is very little friction loss for there is very little duct work.

    Therefore you get a lot of velecity through the bypass damper.
    @approximately 2000 ft/min allows 1100 CFM through a 10" damper.

  2. #15
    So.....I can put the 2nd level on a zone (6 branch runs), the main level on a zone (10 branch runs), the master bed on a zone (4 branch runs) and the basement on a zone (1 louvre register in trunk and add 4 branch runs) and install either a 10 or 12" bypass. Is this correct?
    This will then operate without excessive air noise/velocity and will not be detrimental to the heating/cooling equipment?
    Just trying to get some answers from those with experience prior to installing and having a bunch of additional costs 'reworking' the duct work, etc.

  3. #16
    Join Date
    Jul 2005
    Location
    Cleveland, Oh
    Posts
    10
    Mike:
    Yes jobs just like yours are done everyday.

    If you decide to change to variable speed equipment or if you think you will in the future, you will need a modulating bypass damper. This is a good idea anyway because it will only open as much as you really need.

    They are easily adjustable and will ensure your system remains effective and is quiet at the same time.

    MAV

  4. #17
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,074
    Quote Originally Posted by zoneguy View Post
    For most applications, a 10" bypass can pass upto about 1000 cfm.
    LOL.

    In your earlier post, you said a small bypass.
    I consider an 8" a small bypass, which is good for around 400 CFM.
    So I was thinking you meant an 8" my mistake.
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  5. #18
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Bypass needs to be sized by velocity on the ductolator.1500 to 2000 FPM.

  6. #19
    The 4 runs of 6" duct to the master bed will only handle 400 cfm at 500 fps. (I believe 500 fps is toward the high end of velocity) This would leave about 1200 cfm to bypass. Is this too much air to bypass? Would this not require a 12" bypass with a flow of 1500 fps? Is the bypass located on the supply trunk before any branch runs and routed to the far end of the return? If so, would this velocity get noisy over a 20 foot run?

    The arzel web page has a bypass calculator and it seems to use 1000 fps for branch runs. Would this not be noisey? Would I need an automatic bypass instead of barometric? Again the arzel site says the barometric is for similar sized zones.

  7. #20
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Quote Originally Posted by Madison Mike View Post
    The 4 runs of 6" duct to the master bed will only handle 400 cfm at 500 fps. (I believe 500 fps is toward the high end of velocity) This would leave about 1200 cfm to bypass. Is this too much air to bypass? Would this not require a 12" bypass with a flow of 1500 fps? Is the bypass located on the supply trunk before any branch runs and routed to the far end of the return? If so, would this velocity get noisy over a 20 foot run?

    The arzel web page has a bypass calculator and it seems to use 1000 fps for branch runs. Would this not be noisey? Would I need an automatic bypass instead of barometric? Again the arzel site says the barometric is for similar sized zones.
    Madison Mike, one detail. The ACCA Manual D book states the upper limit for supply airfow should be 700 fpm for flex, a couple hundred higher for hard duct. 500 feet per *second* is getting too close to bullet velocity<g>.

    You can buy Manual D for a few dozen dollars, here is one website which had a reasonable price just a couple days ago:
    http://www.constructionbook.com/acca...-4/hvac-ducts/

    While this is a lot for a slim book, it may be cheap vs. the value of getting the job done right. I believe Dash has remarked these air speed limits may be a bit archaic, perhaps under the right circumstances the limit could actually be higher with no ill effects. I'm a homeowner, he's the knowledgeable pro.

    Hope this helps -- Pstu

  8. #21
    Ahhh yes.....fpm and not fps. I suppose those numbers would create a 'little noise' in the duct work, but the exterior doors would seal a bit better.
    So velocities closer to 1000 fpm are acceptable?

  9. #22
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Manual D is available from www.acca.org for less .

    Noise issues depend on the type of ,turns/elbows that are close to the grille,size and type of supply grille.

    Flex can be used at higher velocities then 700 and will be less noisey,then metal.

  10. #23
    Join Date
    Jul 2005
    Location
    Cleveland, Oh
    Posts
    10
    The thing about air noise is that it is largely determined by the duct layout and the quality of the register. Noise is also very subjective; noisy to one person would be "The Sound of Comfort" to another.

    So the answer is to design a system with enough flexibility to adjust. Sizing the bypass (if it is required) at between 1500 and 2000 fpm and using a modulating bypass system like the Arzel® ModuPass™ will give you the solution yuou are looking for.

    MAV

  11. #24
    I believe arzel makes a barometric bypass, automatic bypass and the modupass. Will the modupass provide any benefit if I do not have two stage equipment?

  12. #25
    Join Date
    Feb 2004
    Location
    Wadsworth, OH
    Posts
    316
    Quote Originally Posted by Madison Mike View Post
    I believe arzel makes a barometric bypass, automatic bypass and the modupass. Will the modupass provide any benefit if I do not have two stage equipment?
    The ModuPass gives a much higher level of control which in turn makes for a much simpler and quicker bypass set-up process. Over bypassing diminishes the positive effects of zoning that make it so successful. The better the control the better the system operates, it's that simple.

    Please be careful trying to maintain design parameters per manual D when you are talking about zoning. That design criteria is for a non zoned system that is designed to meet average loads and not adjust itself to meet the variable loads that occur from hour to hour, day to day and season to season.

    In order for a zoned system to be effective you must allow the system to operate outside some of these "non zoned design standards". You must allow the positive zoning dynamics to happen to achieve the comfort levels that you are looking for and keep the equipment operating within the manufacturers limitations. This is critical. Pushing air at slightly higher velocities has many positive benefits and the difference between air noise and noisy air leaves plenty of room for increased air flow and an even greater increase of delivered capacity.

    jr
    "When you perceive zoning not as a bandage but as an enhancement, you truly understand the dynamics and limitations of forced air heating and cooling"

  13. #26
    Join Date
    Oct 2002
    Posts
    14
    The lesson I learned from my experience with retrofitting a zoned system: forget about the cost (within reason) - its the comfort that matters.

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