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  1. #1
    Join Date
    Oct 2006
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    CFM questions...


    My HVAC contractor has just finished installing ductwork inside my Studio in my basement. It's now time to install the diffusers for the supply and the grills for the returns.

    Because I need diffusers and grills designed to be "QUIET", I'm not sure if my contractor needs to take cfm readings at individual diffuser/grill openings in the duct or just use the estimated cfm he gave me. I've got 1 shot with him coming back to measure & tell me my ACTUAL cfm and I need to know for myself what needs to be done. Below are some questions that will help me know what's going on...

    1.) Exactly what information do I need to be able to determine the correct
    diffuser? (ie:cfm, diffuser size, grill size, etc etc.)

    2.) Is it possible to obtain the cfm directly from the diffusor opening without
    the actual diffuser attatched? If so, could you briefly tell me what needs
    to be done/used to measure this? (In my contract he's suppose to come
    back after the Studio duct was installed to measure my cfm. This question
    will tell me what I need to request him to do in obtaining the(cfm) number.
    My present contractor is not the original contractor who installed the
    HVAC system I now have in my basement. Since I did not use the
    original installer/contractor my new contractor only gave me a general
    cfm number. I'm somewhat confused because my present contractor
    stated that the diffusers need to be on first, but from my
    reading, you need to know the correct cfm to order the diffusers)

    3.) Although there is a 'general' cfm number for my HVAC system,
    is it better to install duffusors based on the ACTUAL cfm
    blowing from each INDIVIDUAL diffuser opening in the duct, or should I
    use the "general" cfm number?

    4.) What specific info is needed to select a return grill?

    Well, that's about it. The diffusers/grills won't be ordered until I'm sure of my cfm and I make sure it's measured correctly. Any and all help is appreciated!


  2. #2
    Join Date
    May 2007
    DC Metro Area (MD)
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    Do you have a drop ceiling? Dash, beenthere and others can comment on the CFM questions, but ceiling type will play a role in diffuser selection.

    Here are some diffusers with data (CFM, Noise Criteria, etc.):

  3. #3
    Join Date
    Dec 2006
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    I'm somewhat confused because my present contractor
    stated that the diffusers need to be on first, but from my
    reading, you need to know the correct cfm to order the diffusers
    You are attempting to enter a deep level of aerospace engineering that isn't applied in HVAC because there is no measured benefit. Your contractor can't guarantee an exact written CFM for each duct before the diffusers are installed. Would you ask a sprinkler contractor to demonstate a sprinkler system without sprinkler heads?

    The diffusers (Registers) have a certain range they work within. Your contractor has to estimate what the final CFM throughput will be in order to order the diffusers. You are asking a chicken before the egg type question.

    You should be more worried about assisting the contractor by defining where you don't want the cold air to blow (i.e. over an intended location for a bed).

  4. #4
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Website for grille selection.They/yo will need to know what cfms each grille was designed for.

    Hopefuly bt doubtfuly the duct system was designed correctly to serve each space.

  5. #5
    Join Date
    Jan 2004
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    Register selection should be known before installing the duct.

    Then the duct can be sized accordingly.

    You can use the Hart&Cooley website to pick them now. But when they are installed, your CFM will change slightly from any reading taken before they were installed.
    Contractor locator map


    How many times must one fix something before it is fixed?

  6. #6
    Join Date
    Mar 2005
    reading PA
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    your contractor should be able to tell the cfms for each and every branch run of your ducting before they begin the install. a proper duct layout is based partially on the amount of cfms needed to condition a specific space. diffusers restricting the airflow is almost a non topic, they are designed to cover a specific sized opening without a large pressure drop. the size of the supply opening, branch run diameter, and especially the duct size will determine the cfms being delivered through each run.

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