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  1. #1
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    Manual D duct design on small blower

    If one were to install a central dehumidifier, it has a blower with airflow rather like a 1-ton AC. Would it make much sense to design a duct system using Manual D principles? I figure almost nobody does it, but would it make sense?

    Thanks -- Pstu

  2. #2
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    Quote Originally Posted by pstu View Post
    If one were to install a central dehumidifier, it has a blower with airflow rather like a 1-ton AC. Would it make much sense to design a duct system using Manual D principles? I figure almost nobody does it, but would it make sense?

    Thanks -- Pstu
    That's exactly the way to do it,though we don't often install bandaids like that,it's exactly how we design ERV's.


    BTW all the eRVs we installed have 6" takeoffs and needed larger ducts.
    Last edited by dash; 06-24-2008 at 11:36 PM.

  3. #3
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    Especially helpful is your observation that in your experience ERVs need ducts larger than the takeoffs. It is all too easy to see the takeoff and figure the manufacturer is telling you what size will do.

    Best wishes -- Pstu

  4. #4
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    ERVs/HRVs have static pressure ratings like any other fan.

    just like an air handler, just because it has a 10 1/2 by 22 opening, doesn't mean thats the right size.
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  5. #5
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    Thanks! If OK with you I would like to change gears and consider a specific example. Hopefully you will tell me if I am reading and understanding Manual D in the right way.

    A Thermastor UA-150XT mentions two points in its airflow table, 415 CFM at zero static pressure and 365 CFM at 0.4 inch water column -- I used 400 CFM because that is a nice round number. The return side of this little air handler has a 10-inch collar intended to connect to a return grille in the central part of a house. That implies a 10-inch duct, and in fact Thermastor says it is OK to use 10-inch for total duct length up to 25 feet.

    The return side also has a 6-inch collar but its use is optional. There might not be installed a fresh air intake, or it may be shut off via a motorized damper much of the time. So I left that out of the calculation of main return duct size needed.

    But I look at Manual D (page 3-6, 2004 edition) and see 600 fpm recommended return duct velocity, with 700 fpm maximum. Looks to me like 400 CFM in a 10-inch duct means velocity in the 730-740 fpm range. So I should go up a size in order to obey the Manual D requirement, right? In spite of what the Thermastor manual says. A 12-inch return duct would bring velocity down to around 520 fpm.

    My read of the friction loss chart on page A2-10 of that book, says the 10-inch duct would have a friction loss of 0.15 (units are inches of water per 100 feet). The 12-inch alternative would have friction loss of 0.05 or just a hair under. The return duct length I envision would be only 8 feet so the pressure drop will be 0.004 not counting a return grille and filter, am I right? It looks like the hardware in the airstream will count for a lot more pressure drop than the ducts themselves, if I understand this right.

    The fresh air inlet would need a length of 20 feet and might carry 100 CFM when in operation. It would be dampered to that airflow. That comes to about 530 fpm, a friction loss of 0.15, and a pressure drop of 0.030 not counting grille and filter. A fresh air inlet would not enter into the main return-side calculations, or would it?

    I look forward to your comments. I am also trying to analyze the supply side but let me get clear on the return side first, if that is OK with you.

    Thanks very much -- Pstu

    P.S. I wonder if anyone could tell me the consequences of using the smaller high velocity duct. Assume everything will be flex and not hard metal duct. Probably shoulda planned on some bends in the duct adding to effective length?

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