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  1. #1

    How do dampers affect velocity and flow?

    Hopefully someone can enlighten me ...

    Dampers are used to control the volume of air that enters a branch, but how do they affect velocity? I would assume the air speeds up as it flows through the damper opening, but would then slow back down after continuing down the branch.

    Here's the problem ...

    I have a room that requires 57CFM of cooling according to manual j. It calls for a 4 inch duct at 600fpm. Everyone I talk to recommends using a 6 inch and installing a damper, but with 57cfm in a 6 inch duct; velocity will drop to 300fpm which will not allow proper room mixing. Should I just resize the register to increase velocity, use the 6 inch duct or go with the manual d recommended 4 inch???

    I'm having the same issues on other outlets calling for 5 inch, 3 inch, etc. Should I go with the so-called experts or the manual d/j????

    Thanks.

    Jack

  2. #2
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    Who are the "experts" you are refering to? 57 cfm with a 6"? A little overkill.
    Jim

  3. #3
    Join Date
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    one would be hard pressed to measure the effects from the normal designed damper for HVAC residential ducts -- so far as causing flow problems --

    are you still in the design stage?

    many installers will just use 1 size duct, then add dampers --
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  4. #4
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    You'll have some turbulance right at and after the damper. But if your register isn't within 2' or so of the damper no problem.

    For a room requiring 57 CFM, I would use a 5" and damper. Depending on length of the run. You'll be hard pressed to find 3" pipe. I don't use anything less then 5". On jobs that only have one supply that would size out less the 6" inch, quite often we use just 6" and a damper for that one run. If all supplies are sized to the FR that the Manual D said to use for the duct system, very few supplies come to less then 5".
    Select registers and grilles for the throw you need.
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  5. #5
    Quote Originally Posted by tinmarine View Post
    Who are the "experts" you are refering to? 57 cfm with a 6"? A little overkill.
    The "experts" are just some colleagues that have been in the business for 20+ years. Perhaps they are just taking the easy way out by using 6 inch branches for every room.

  6. #6
    Quote Originally Posted by beenthere View Post
    You'll have some turbulance right at and after the damper. But if your register isn't within 2' or so of the damper no problem.

    For a room requiring 57 CFM, I would use a 5" and damper. Depending on length of the run. You'll be hard pressed to find 3" pipe. I don't use anything less then 5". On jobs that only have one supply that would size out less the 6" inch, quite often we use just 6" and a damper for that one run. If all supplies are sized to the FR that the Manual D said to use for the duct system, very few supplies come to less then 5".
    Select registers and grilles for the throw you need.
    So you would use a 5 inch duct for a 25 square foot powder room?

  7. #7
    Quote Originally Posted by cem-bsee View Post
    one would be hard pressed to measure the effects from the normal designed damper for HVAC residential ducts -- so far as causing flow problems --

    are you still in the design stage?

    many installers will just use 1 size duct, then add dampers --
    Still in the design stage. I'm aware many installers will just use one size, but my question is "is this the correct way of doing things?". What would you do for your own house? I want to make sure I do the job correctly and get proper airflow to each room.

  8. #8
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    Quote Originally Posted by jackmorgan View Post
    So you would use a 5 inch duct for a 25 square foot powder room?
    I wouldn't waste money on a 25 sf room.

    Just keep the door open. Tie the fart fan to the light. When door is closed, fart fan runs and pulls air under the door. Otherwise, door is open and room is conditioned by adjoining space.
    Perhaps you should have read the instructions before calling.

  9. #9
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    Quote Originally Posted by jackmorgan View Post
    So you would use a 5 inch duct for a 25 square foot powder room?
    Yes I would. If the room really needs to have a supply.
    The register selection will determine throw.
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  10. #10
    Quote Originally Posted by jrbenny View Post
    Just keep the door open. Tie the fart fan to the light. When door is closed, fart fan runs and pulls air under the door.
    Fart fan, lol.

    I just had an entirely new duct system run for my first floor. I have an old very chopped up house with 1,000 register locations. (Okay, actually 10 per floor.) No room called for more than a 6", several called for 4-5. They ran 6" to all rooms and dampered every run. I thought about it a bit, and actually like that idea (unless there's a good reason not to.) What if I want to change a bedroom to an office? Suddenly it needs more AC...open the damper up a bit and it's good to go. If it were a "perfectly" sized duct, I'd be running a new one or living with it hot.

  11. #11
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    Powder Room = sink and toilet.

    'Tis a bit different than a bedroom that might become an office.
    Perhaps you should have read the instructions before calling.

  12. #12
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    The 6" with a damper ,will have the same "Grille" velocity and throw when dampered to the same cfms as the 4",yes slower in the " duct but not the grille.

    I'd use the 4" duct and not have to bother with the damper setting.I'd also install a supply in the powder room,and not depend on cooling from the adjacent room.If you depend on cooling from the adjacent room,be sur to add the cfms to that room,which may over cool/heat the room a little.

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