how many watts do i need ?

[xboxman]

i'm going to hook up a dc powered fan in the attic of my shop to blow hot air out in the summer and was wondering about how many watts i need to run this fan  it's a 300 cfm 12 volts dc at 1.42 amps  i came up with 17.04 watts does this sound right ?

[DanG]

With that small of a current draw I'd say 40% of the time it will be stalling because of back draft via negative air pressures due to prevailing winds. Just opening up existing vents and/or adding a few $8 DIY through-the-shingle vents would be good - IMHO one should explore all the passive alternatives before adding a fire hazard in a remote area.

I was looking at RV vent fans recently and it looked as if automotive radiator cooling fans 12 or 14 inches across are being used - a Junk yard fan for a few bucks could give good service to rapidly exhaust heated air but I wouldn't trust it to last more than a season as a continuous duty fan...

Wire run length will be a concern with 12 volts too - try for no more then 5% losses.

[Nando]

Yes you are right 17,04 watts, but what is the size of the attic, specially the volume.

For a 1500 square feet house and low roof the attic may have 30,000 cubic feet, so it will take 100 minutes to change the air once, meanwhile the solar insolation may be generating around 100 watts per square foot, reaching 3 to 10 watts/square foot internally.

For such volumetric space a 3000 to 5000 CFM is really needed to bring the temperature down by 10 to 20 F.

Nando

[adelaide]

free cos hot air rises u just need big enuf vent or door? or step back wards is small vent with fan, maby use the fan in winter to pump  warm air down to u

[johnlm]

Nando,

Are you sure your not off by 10X on the 30,000 cubic ft of attic on a 1500 sq ft house?  

John

[Nando]

Yes you are right I placed one too many Zeros, it should be 3000.

Nando

[xboxman]

my shop is 25 feet long and 20 feet wide  the roof comes down on the sides it's only 4 feet tall in the center it's kinda like the letter A but flater so i'm guessing it's less than 500 sq feet space  ( i didnt know how to figure this type of roof )

[Spelljammer]

All you have to do is take the length x width x height and then divide that by 2.  So, 25 x 20 x 4 = 2000.  Divide that by 2 and you get 1000 cubic feet.  So, every 3 minutes or so, your fan will rejuvinate all the air....in theory.  

[Spelljammer]

All you have to do is take the length x width x height and then divide that by 2.  So, 25 x 20 x 4 = 2000.  Divide that by 2 and you get 1000 cubic feet.  So, every 3 minutes or so, your fan will rejuvinate all the air....in theory.