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Thread: Duct size...

  1. #1
    I need help to answer a duct sizing, barometric damper question for a combined passive make-up air and exhaust type ACH system? (I've talked to the two HVAC shops in town and neither can help).

    The data I need is what size of hard duct (and shape?) to feed continuous 50-60cfm bath fan (ACH) and provide make-up air for 250-300cfm rangehood, dryer, bath fan on full speed (110cfm)? I've got a number of ways I can duct this system. They range from 10' with 90 degree turn at intake/outlet(low/high), to 37' with 4-90 elbows (inlet/outlet plus two). I may temper the incoming air with a loop off of the radiant floor boiler.

    Would you mention some barometric damper outfits with high quality/function products?

    Some other data:

    Propane cook top as only open combustion (elec. oven, boiler -radiant slab- with outside air intake and DHW loop).
    Two people, two bedrooms
    7500 HDD, east foot of Rockies, dry year round, no AC necessary.
    Tight house (guessed at .25 ACH), 1725 sq.ft., 35% SSF (passive solar).
    Only potential for intake is on predominently leeward side.

    Thank you,

    Mike Krall

  2. #2
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Here's a formula that will get things started.

    Q= 1.07 X A X the sq. root of 3p

    Q=cfms required
    1.07 is a constant
    A=area of the opening needed at each end of the duct
    3p= 3 pascals of pressure difference.

    So if you needed 300 cfms of passive makeup air:

    350 cfm= 1.07 X 188.9 X 1.7321

    The duct size would be at least 16" round ,based on your rescription.

  3. #3

    Duct size...


    Thanks, dash...

    For the given CFM, the pressure difference has the only potential wiggle room. Are the 3 Pascals of pressure difference hard and fast for a passive supply duct or would it be different for the two examples (10'& 2-90's, 37' & 4-90's)?

    Mike Krall

  4. #4
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    The equation is actually for the grille size required for a passive transfer grille .Note that the area given ,is the "free" area of the grille ,usually about 75% of the total grille size,best to use the grille mfrs. data ,if possible.

    The duct size needs to be increase ,when the distance and elbows increase.

    More than 3 pascals ,will tend to cause infiltration,when the exhaust fans are on,which is what I'm guessing you are trying to avoid.

    It's not likely that all fans ,and the dryer will be on at the same time,at least not often,so you may want to consider that in your sizing.

    Where do you intend for the makeup air to enter the home?

  5. #5
    Join Date
    Feb 2003
    as Dash states: consider the most likely scenerio: 2 ba fans for 15- 20min, or
    dryer fan for 40min,
    or cook fan for 30min.

    yes, sometimes may come up a little short, but if one is cooking, exterior doors may be opened due to coming/going.
    What is YOUR family pattern?

    2 intakes, 1 for ba, 1 for kit & dryer? -- keeps pipes smaller

    [Edited by cem-bsee on 06-24-2005 at 10:58 AM]

  6. #6

    The intake would be about floor level below neutral plane (no basement, single story) and other end is in or near ceiling either behind refrigerator (10'w/ 2-90's), in pantry near freezer, (23' and 3-90's), in non-sitting area between LR and Kitch.(37' and 4-90's).

    I don't know that I'm trying to avoid infiltration, but I don't do this for a living. The make-up aspect of this is to control where the air comes into the house so I figured when the rangehood was on, the pressure differential would open the barometric damper further than the 50-60 cfm continuous bath fan (ACH) does (one BR, Cem-bsee).


    Thanks for pointing out the useage times. Our useage is two people, simple living, conservation minded.

    I see your point about two intakes. The Dryer is on north wall in Ut.Rm. which is across the hall, two doors, 19', and on the same level (north half floor 6" up from south half floor) as the Bath Rm. The rangehood is in the south half some 35' away from the dryer and the passive intake and dump are in between. Does this work with your suggestion?

  7. #7
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    If you use one "damper" and duct for all,the bath fans alone ,likely won't be enough to open it.

    If you duct it to the return of the HVAC system,then the fresh air can be "conditioned" and filtered.

  8. #8
    There is no 'return to the HVAC', Dash, heating radiant slab (Weil-McLain 'Ultra' boiler).

  9. #9
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    My bad ,when you are in Florida,it's easy to think everyone has a ducted A/C system.

  10. #10
    My part of Wyoming, you want it cooler, you open a window. In the heat of the summer (mid-July to mid-Aug.) opening them after sunset will cool a house enough to get you through the day... usually.

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