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re: solar attic gable fans
2 jul 2004
daestrom  wrote:

>> > >a small solar fan won't produce the air flow you need.  neither will
>> > >sofit vents.   sofit vents are mainly for moisture control, not cooling.
>>
>> > how much airflow does she need? soffit and ridge and gable vents with
>> > vent area a and height and temperature differences h and dt will produce
>> > cfm = 16.6asqrt(hdt). for instance, a = 4 ft^2 with h = 8' and dt = 10 f
>> > makes 594 cfm.
>
>> 4 square feet of ridge vents? thats some ridge. even the commercial
>> self opening vent systems i've seen arent likely to produce 4 sq ft.

i was mainly thinking about gable vents.
 
>my ridge vent is 1 inch on each side and 40 feet long (ridge is 45 feet, the
>length of the house).  that gives 2/12*40= 6 2/3 ft^2.  i don't know if this
>is 'typical'...

some are larger, eg 6" wide, but the bug mesh reduces the airflow.
manufacturers have data sheets with effective areas.

>of course, the extremely long and narrow shape is a detriment to air flow.

maybe not, if the air is coming from soffits. soffit air might be happier
exiting via a ridge vent than traveling the length of a ridge to exit
a soffit vent. 

>one common factor used for flow calculations through irregular shaped
>openings is the so-called 'hydraulic diameter'.  it is calculated hd =
>a/(4*perimeter).

i seem to recall 4a/p, eg 4x6/12/81 = 1.98 feet, with a = 3.1 ft^2.

>...for my ridge it would be 6.666/(4*160) = .0104.  that is less
>than 1/8 inch.

that would be in feet vs inches, using the wrong formula.

and where did the 160 come from?

nick




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