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Attic fans in series? - Page 2

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Posted by clare on June 9, 2014, 6:27 pm
On Mon, 9 Jun 2014 07:53:56 -0700 (PDT), "I can't do that Dave"

 In a situation where you are not running any appreciable duct length,
and virtually no flow restriction, you are running on the right end of
the graph where you will get little or no advantage from running the
second fan - which is whay I asked what your rationalle was for
running 2 fans. The information you posted shows quite plainly that
you will NOT gain anything by using 2 fans in series in that
application. Now, if you mount one in the attic access, and another in
a gable end vent or rooftop vent, you may see enough improvement in
air flow to make it worth while (or you may not).

Posted by amdx on June 8, 2014, 10:11 pm
On 6/8/2014 1:56 PM, I can't do that Dave wrote:

I'm with the others, I don't know if two in series is all that helpful.
I do recall running push pull 4" fans on a sealed box instrument. I did
enough testing to know it pushed more air than a single fan.

I want to add, I had a house with a fan in the access hole in a hallway.
It really worked great, we would crack the two windows in our bedroom  
about 3 inches, with the attic access fan running it created a nice cool
breeze blowing over the bed on those hot August nights. I remember it  
fondly. :-)           Mikek

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Posted by Jim Wilkins on June 9, 2014, 2:47 pm
 Hi all,

We have been given two 2200cfm fans (Lakso 20" box) and am thinking of  
mounting them in series and then fitting the assembly to the man-hole  
opening into the ceiling crawl space. The idea is to use a timer to  
run them during the night when it is cooler outside suck air through  
the place. They are close to the same size as the opening into the  
ceiling so series makes sense.

Does anyone have details on how far apart to mount them for the best  
efficiency? I have been testing with a small 12v fan and voltmeter in  
front of them and moving them closer or farther apart. It does not  
seem to make much difference to the voltage developed by the small  
wind-driven 12v fan on where they are placed.

With one fan, the voltage generated is pretty stable at 1.47vdc. With  
them in series, it is around 1.98vdc and I can't seem to find any  
sweet spot.

I have found this graph, which shows almost double output for a large  
percentage of the flow volume but I am not seeing that.


Thoughts and helpful suggestions?



You may be limited by the vent area in the roof. I run one of those  
big loud fans through a screen door during the evening and early in  
the morning, and a smaller quiet one in the attic hatch all night.

My quick formula for the equivalence between temperature and humidity  
is 3% equals one degree F. I think the actual number is around 2.8%,  
so 2.5 works for mental math too. That means outdoor night air at 60F  
and 98% has the same comfort value as 70F and 70% indoor air, and  
blowing it in could cost more in electricity for the fans than you  
gain in not running the A/C.

Although I'm in northern New England the summer highs are in the upper  
90's and the dew point can reach 75F.


Posted by I can't do that Dave on June 9, 2014, 2:56 pm
 On Monday, June 9, 2014 7:47:42 AM UTC-7, Jim Wilkins wrote:

Ummm, Arizona, it's a dry heat!! LOL  

Other than when it is raining, I have never seen the Humidity above 31%, but thanks for the numbers.

Posted by amdx on June 9, 2014, 4:14 pm
 On 6/9/2014 9:56 AM, I can't do that Dave wrote:

   I rarely get my house below 50%, it may go a little lower later in  
the summer when the air conditioner runs more. At one time, before the  
new HVAC, I ran a dehumidifier, with that I could get to the low 40's.
I'm in Northwest Fl. Recently, I saw we had 95% humidity at 65*.

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