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  1. #1
    Join Date
    Nov 2007
    Location
    Livermore, Ca
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    163

    Branch size vs register size

    Greetings.

    Does the Manual D give register size with consideration to branch size?

    Example would be if a room requires 100 cfm using a 6" branch, does that mean that the standard 6x12 register will work. Do registers tend to be oversized from the duct to compensate for two-way, three-way or even four-way openings?

    My question stems from our system. The bedrooms are fed with 6" flex to a 6x12 three-way register with closing louvers. They were noisy. I removed them and it's very quiet. Well, less than with them installed. So maybe an 8x14 or 12x12 would allow the air to dump as well as the 6x12 with no register.

    This isn't meant to sound like a DIY. Just looking for knowledge. I'm going to get a Manual J & D performed. Need to know what to expect from the D and if there is still room for improvement at a personal level.

  2. #2
    Join Date
    Nov 2007
    Location
    Chicago
    Posts
    76
    Registers have nothing to do with branch size. The supply grille is sized for noise [mostly] and pressure drop. You want a NC of under 20.

  3. #3
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    Your reply is my point. If the room takes a 6" drop, the register is usually the standard 6x12. Does the Manual D address this?
    What does NC stand for.

  4. #4
    Join Date
    Dec 2006
    Location
    Arkansas
    Posts
    292
    Quote Originally Posted by hvacconsultant View Post
    Registers have nothing to do with branch size. The supply grille is sized for noise [mostly] and pressure drop. You want a NC of under 20.
    Supply grilles are sized for throw and spread with a velocity that will meet acceptable noise levels. Manual T is the guidance document for air distribution. One of the problems with most duct systems is they are only sized for noise and pressure, and the ability of given registers to distribute the air correctly to meet comfort level are ignored.

  5. #5
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    When I have the Manual D performed for our house, will it address the register size? Sounds like there is yet an other manual, Manual T?

  6. #6
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,271

    Wink

    Quote Originally Posted by regis101 View Post
    When I have the Manual D performed for our house, will it address the register size? Sounds like there is yet an other manual, Manual T?
    Diffusers are rated to a ( NC ) Noise Criteria by the manufacturer.
    i.e.
    http://www.titus-hvac.com/ecatalog/g...spx?fileid=898

    ... < 2 CFM / Sq In (300 CFM / Sq ft) should be quiet
    IF installed properly.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  7. #7
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    Nice link. Good info.

    Does the Manual D give register size? Seems to be a vital part of the equation. Seems senseless to have it done if the registers are not incorporated. What about in the end, the register ends up noisy. Can I go bigger and still be happy?

  8. #8
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,837
    It is a simple fact of life that there is a world of difference between a design built residential system and a mechanically engineered commercial/industrial job. With the latter, these is a blueprint that lays out the system including all duct sizes and supply outlets. CFM delivery is required and at some time during the commissioning process the entire system is flow hood balanced to design specifications. This attention to detail does of course, come at a price. Hence the substantially higher costs for an engineered job. But that doesn't leave the residential customer necessarily hurting.

    A high quality company at the residential end will do a Manual 'J' load analysis, a Manual 'D' duct design and use the register/diffuser/grille manufactuer's engineering numbers to deliver the proper results of both noise, CFM delivery and balance. Any experienced HVAC company knows pretty closely what the static pressure will be in his designed system. That by default should dictate how many and what size supply outlets need to be installed in a room. As to branch duct size, that's a function of the velocity. Again, deferring to noise, a good company will design for low noise. All of this must take into consideration the actual installation. Our company issues it's installers a 'cheat sheet' that contains engineering data taken from our favorite diffuser manufacturer, using the sizes and CFM delivery closest to the anticipated static pressure we will see in the finished system. It's a relatively simply matter to adjust volume dampers in each supply branch to bring any errant balance problems under control but quite honestly, when done to the numbers, we have precious few trips out with the flow hood for balancing purposes.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by skippedover View Post
    It is a simple fact of life that there is a world of difference between a design built residential system and a mechanically engineered commercial/industrial job. With the latter, these is a blueprint that lays out the system including all duct sizes and supply outlets. CFM delivery is required and at some time during the commissioning process the entire system is flow hood balanced to design specifications. This attention to detail does of course, come at a price. Hence the substantially higher costs for an engineered job. But that doesn't leave the residential customer necessarily hurting.

    A high quality company at the residential end will do a Manual 'J' load analysis, a Manual 'D' duct design and use the register/diffuser/grille manufactuer's engineering numbers to deliver the proper results of both noise, CFM delivery and balance. Any experienced HVAC company knows pretty closely what the static pressure will be in his designed system. That by default should dictate how many and what size supply outlets need to be installed in a room. As to branch duct size, that's a function of the velocity. Again, deferring to noise, a good company will design for low noise. All of this must take into consideration the actual installation. Our company issues it's installers a 'cheat sheet' that contains engineering data taken from our favorite diffuser manufacturer, using the sizes and CFM delivery closest to the anticipated static pressure we will see in the finished system. It's a relatively simply matter to adjust volume dampers in each supply branch to bring any errant balance problems under control but quite honestly, when done to the numbers, we have precious few trips out with the flow hood for balancing purposes.


    Pretty much they WAY IT SHOULD BE.


    Your grilles may be the wrong type,since they are noisey.
    Last edited by dash; 12-23-2007 at 07:53 PM.

  10. #10
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    Well that was a big circle, huh. I see the hoods all the time at work. I bend an eye to see them calculating, closing or opening the damper and moving on to the next.

  11. #11
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    I just want and need to be assured that when I hire someone to plug in the numbers I give them that the human error factor is nil.

    Most of the powers that be around here insist on having systems done using the Manuals. To this I agree. To this I will also have the duct designed. Hopefully the human factor is lessened due to the program. Hopefully the human factor is heightened to bring me, John Q Public, the results that we're paying for. I hate paying for stuff and still have questions.

    You can read up on my dilema by searching in "Threads Started" in my profile

  12. #12
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Programs don't eliminate humn error ,I think they actually can increase it.But don't let that be a reason not to have the calcs done!!!

  13. #13
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    I am having the calc's done. Sending off the paperwork tomorrow. I emailed the designer to ask about registers. He said that it'll be addressed. Also said most would just slap any 'ol thang on there.

    Whew. Close one

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