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Thread: Air flow - generator enclosure help needed

  1. #1
    Join Date
    Jun 2008
    GTA Ontario Canada

    Air flow - generator enclosure help needed

    Picked up a brand new (second hand) attic fan with thermostat to use on the top of my generator enclosure.

    Fan moves 1200 CFM. This should be more than enough to keep the generator cool and prevent fire. In my view it will even air cool it better than it standing on its own. Well at least i hope.

    So now i got this fan on the top of a large box. Approx 4' x4' at the base and around 3' high.

    Sides will be enclosed tight with layer of insulation as if it was an R4 wall ie 2x4 stud bay with insulation.

    Floor is a plastic pallet i got free. I could one day regret using this but for the moment i am in love with the pallet. Think future when it goes to ground level i have automatic rot free base that is raised off the ground by 6 inches.

    Roof will be another pallet. Before you ask no plans this is flying by the seat of the pants. Dont have time to do drawings at this stage. Got to wing it Xmas is around the corner.

    So now comes issue of air intake. Exhaust fan is 14.5 inches in diameter so i figure given this is going to be a tight box i gotta put some intake vent on it and want air intake starting at the bottom and distributed around so all sides of generator get cooling and air flow.

    By my calc PXR squared says area of exhaust vent is 165 square inches.

    I am thinking if i run a strip of soffit vent around the sides on all four sides including the door I should be ok to have matching air intake and most important not have bugs and critters get in.

    Soffit vent best i can tell is 2.75 inch wide but looking at the pic i say at best 40% perforation. So i figure for each 2.75 the effective open area is 1.1 inch. If sides are 4' x4' square that says i got 212 inchs of opening.

    Should be quiet a rush of air passing through those vents when the fan gets going.

    Just wondering if sofit vent will collapse or not if its supported all along its lengths and secured with staples. Think here of two pieces of 2x4 laying horizontal and strip of soffit vent secured between them along the edge.

    Questions do you think it will hold ?

    Do you think this would work and be enough intake ?

    Dont want a place where the likes of insects would find their way in and set up home and dont want to have to clean filters all the time. Soffit vent holes are kinda self cleaning in a way given they are pressed in. I could simply hose down from time to time.

    Also trying not to make too large of an opening that lets all the noise out.

    Any comments critiques or additional ideas welcome.

    Alternatives i have thought of using are floor registers, Houshold air intake vents for ventilation systems , vent covers for washroom exhaust fans when mounted on walls.

    Most are designed with view to exhaust and close to keep insects out or assume non insect environment like interior stuff.

    Also looking for vent to be weather bearing.

    Thanks for any input.

  2. #2
    Join Date
    Nov 2006
    SE Minnesota
    A few thoughts.

    I think your soffit would work but I would be inclined to back it up with hardware cloth on the inside to provide a little stiffening without degrading air flow.

    I think I'd do something to prevent water from being sucked in if it is raining. Similar to the way a dorade box keeps water out of a boat.
    Irony: The opposite of Wrinkly

  3. #3
    Join Date
    Dec 2006
    RETIRED(!) in Austintown, Ohio
    Pretty much what Dave said...

    One additional concern: In my area, I'd have to protect the wiring from mice, voles, etc., and also the enclosure itself would make a wonderful nesting area for squirrels, chipmunks, field mice - and maybe even the occasional possum. You might want to think about that, too.
    Jim D.
    Adapt, Improvise, Overcome!

  4. #4
    Join Date
    Jun 2008
    GTA Ontario Canada
    Jim you just got me thinking when you said mice and voles. The flex rubber SOOW cable i have to link generator to temp panel is rubber and these critters think its food for some reason from what i recall about people having issues with them in cars parked in garages.

    I think i will get a piece of conduit to do the run from the wall to the enclosure. Thanks for the tip.

    Dave excellent input. Just what i needed. when you refer to "cloth" i presume that is a wire mess not material. I aint seen a material version yet in these parts will have to look at depot.

    And the trap arrangement is exactly what i had in mind given i want to also keep the sound in and not have it going out the vent as best as possible.

    Thanks for the input guys i appreciate it keep it coming anyone else with more input. Just saved my cable from mice. boy would they look funny taking a bite of a cable like that when its live. Sad part is they may never make it to the copper but sure would play havoc with the insulating properties and cause me grief.

  5. #5
    Join Date
    Dec 2012
    Bedford, NH
    OK, let me show some ignorance here.

    First of all, I'm not sure why you want an enclosure for a generator in the first place; generally that isn't a good practice unless a suitably large enclosure. Is it to control the noise?

    Some other questions:
    What kind of generator is it?
    Is it run by gas, propane, natural gas?
    If so, how do you plan to re-fuel it when it's covered?
    Is the cover, or whole box readily removable?
    How about access for maintenance, or for backup battery servicing?
    Is it grounded separately, or via the house service panel?
    If the net soffit area is 1.1' per running length x the perimeter (16'), then how did you get 212sq. in. of soffit area vs. 17.6 (1.1x16)?
    Is the exhaust fan run by the generator so that if the generator runs out of fuel the exhaust fan also stops running?

    Just a few initial thoughts that you probably have already taken into consideration, but........
    Thoughts entering one's mind need not exit one's mouth!
    As I age my memory fades .... and that's a load off my mind!

    "We Live In The Land Of The Free, Only Because Of The Brave"
    “The problems we face today are there because the people who work for a living are outnumbered by those who vote for a living."
    Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy, its inherent virtue is the equal sharing of misery." Winston Churchill

  6. #6
    Join Date
    Jun 2008
    GTA Ontario Canada
    Thanks for your thoughts Al. Answer to your questions are this enclosure is to hold a portable generator of around 5000 watts to provide temporary power to my workshop. The enclosure will be moved when i move. Its a gasoline generator and fan will run powered by generator. Enclosure will have a door (one full side )and generator will slide out when needed for servicing.

    The 1,1 is 1,1 inches (( not feet) of open space) times the 16 ft (192 inches) around the edge if i deploy the soffit vent which is 2.75 inches wide but i am estimating only 40% open .

    So 1.1 x 192 = 211 square inches.

    Oh and it will have its own ground spike for proper ground best i can do under the circumstances. No battries involved. Pull start. Maybe later i will retrofit a electric start.

    Thanks for the input. All welcome.
    Last edited by Rob Keeble; 12-12-2012 at 03:46 PM.

  7. #7
    Join Date
    May 2007
    Kansas City, Missouri
    I was thinking along the lines of Dave, that you may want to pull in air up higher. Mostly not because of rain, but to keep snow build-up away from the intake.

    To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault

  8. #8
    Join Date
    Nov 2006
    SE Minnesota
    Rob, yes, hardware cloth is a sort of wire screen. The wires are bonded together at the intersections so it tends to be fairly rigid.

    And yes, as Darren said, you want to keep snow out, too. The point of showing the dorade box was to illustrate that air could get in without letting water in.
    Irony: The opposite of Wrinkly

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