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Attic fans in series? - Page 3

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Posted by Jim Wilkins on June 9, 2014, 4:32 pm

Depending on which way the Jet Stream meanders we can be downwind from  
you and suffer from your tropical humidity, or downwind from Hudson's  
Bay. Gulf winds are nice in winter, Canadian air in summer, but not  
the reverse.

Posted by Eeyore on June 28, 2014, 4:24 am

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?



Your Fans won't last more than 2-years.  Unless you put a thermo switch, and  
do maintenance.  I already tried it.


Posted by No Body on August 5, 2014, 2:01 am
Look a bit more carefully at the graph.. and the answer is there..

If you have a low static pressure and are near maximum flow rate, the
additional volume goes to zero!

This is because the 2nd fan is just making up for the "slip" on the
first fan.. Once you get to the maximum flow rate, the increase in
volume drops to nothing, at the right hand end of the graph.

I suspect that the delta-p on the first fan is already low.

You need a "different" fan for the 2nd one... one that can
provide additional acceleration of the incoming air, over
and above what the first fan can already do.

Your "first fan" is already running with low slip.. so there
isn't much the 2nd fan can do...

Measure the pressures before and after each fan with a U tube
manometer.. and this will tell the tale.

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