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Thread: Exhaust fan venting

  1. #1
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    Exhaust fan venting

    I will be installing a exhaust fan in a bathroom. The customer wants me to run the duct about 25 feet along the attic floor and up to a cupola at the top of the attic. I am concerned about the maximum effective length of a duct for the fan. I checked the Nutone website and the documentation for the fans do not give any information about maximum distance for the duct. Is there some other place this information is available? Surely, there is a chart showing duct size/length of duct/power of fan = AOK

  2. #2
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    Quote Originally Posted by Greg1707
    I will be installing a exhaust fan in a bathroom. The customer wants me to run the duct about 25 feet along the attic floor and up to a cupola at the top of the attic. I am concerned about the maximum effective length of a duct for the fan. I checked the Nutone website and the documentation for the fans do not give any information about maximum distance for the duct. Is there some other place this information is available? Surely, there is a chart showing duct size/length of duct/power of fan = AOK
    I always avoid fans in the ceiling unless they are a very short distance. 25 feet is a long way for a fan to push air esp. uphill- so to spreak.

    Have you ever installed an inline fan such as Fantech. Put the fan in the Cuppola near where it exits and install insulated duct down to a small grill in the ceiling. There fans are flawless and a fan generally draws air better than it can push air.

  3. #3
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    Call you buddy at an HVAC firm, he should hve this info or wait for Marc to sign on.
    Brian John
    Leesburg, VA

  4. #4
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    Nutone's info is quite useful.:grin:

    The unit will operate most quietly and efficiently when located where the shortest possible duct run and minimum number of elbows will be needed.
    Use a roof cap or wall cap
    Like that helps

  5. #5
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    There's a couple things I have to say about that. First, the IRC actually requires very darned little ventilation in a bathroom. Most bath fans are way oversized with respect to the minimum code requirements. If the bathroom already has a window, for instance, a fan is not even required.

    When you size the fan according to the manufacturer's charts (which mirror the HVI guidelines), I try to use the same rules on ductwork length that the IRC gives for clothes dryers. Max 25 feet, and subtract 5 feet for each hard 90. If I need to go longer than that, I jump up to the next size fan if the fan was already exactly sized to HVI guidelines. Chances are pretty good you're already dealing with an oversized fan anyhow. You could use the online ductloss calculator, but you will normally find that it closely mirrors the rule of thumb I just typed.

    I think that it is more important that the fan run the recommended 20 minutes after taking a bath/shower than anything else for effective moisture removal. Whether this is accomplished with a humidistat, an off delay timer or any other control means, it doesn't really matter. It is just important that it continues to run.

  6. #6
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    Quote Originally Posted by mdshunk
    There's a couple things I have to say about that. First, the IRC actually requires very darned little ventilation in a bathroom. Most bath fans are way oversized with respect to the minimum code requirements. If the bathroom already has a window, for instance, a fan is not even required.

    .


    Minimum requirements are not necessarily what is the best option in a given situation.

    Why would you want to open a window in the middle of January, or the middle of July?

    In Mass you must have mechanical ventilation window or not. I doubt we are the only ones with this rule.
    There are two kinds of people - those smart enough to know they don’t know, and those dumb enough to insist they do.-----Margery Eagan

    Open shop since 1988

  7. #7
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    I try not to do duct work since I dont have a duct license.
    "Electricity is actually made up of extremely tiny particles called electrons, that you cannot see with the naked eye unless you have been drinking."

  8. #8
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    Quote Originally Posted by stickboy1375
    I try not to do duct work since I dont have a duct license.
    I wonder if your deer license or fishing license will work.

  9. #9
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    Quote Originally Posted by stickboy1375
    I try not to do duct work since I dont have a duct license.
    I think that's a Federal stamp you get on your license, no?

  10. #10
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    If you don't mind, I think I am going to "duct" answering this thread.
    Instructor, Industry Advocate

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