Venting/ Exhusting a DIY Manifold assistance. Prefer HVAC cert. or expert

I'm revamping my exhaust and venting system.
I have a diy manfiold constructed.
I have 2 lines from cool tubes running into the manifold each with it's own axial duct fan rated at 250 cfm. I have also my exhaust with a carbon filter leading off the 3rd line running at 170 cfm into the manifold. Now what I'm wanting to do is to have all of this air mass that goes into the manifold climb 10 feet and disperse out into the attic space. What I'm wanting to know is do I need an additional fan in this system on the exit side of the manifold? Also what size should it be, and should it be all active or can I do a partial active/passive on this exit. What I'm concerned with is when the cool tube lines are shut off is any air from the exhaust back tracking through those 2 lines instead of going up into my attic space. 1 cool tube line runs 12 hours, the other at 16-18 hours , and my carbon filter exhaust is always running on 30% unless high temps trigger it on.

Thanks
~~ Flams ~~

An extra 250CFM fan attached after the 3 ports on your manifold to direct that air up to the attic would
be fine just to make sure the air is directed. A non return back draft damper http://www.fansdirect.com.au/admin/upload/RSK-Non-Return-Dampers_large.jpg
would also stop any air coming back from the attic you may not want.

can you explain that a bit more depth please? wouldn't the 250cfm fan that you propose restrict the airflow that is coming in from the manifold? Or any air moving in faster behind that fan will just pass through it unrestricted?

Also how do dampers work? The air pushes them open from one side? Is there a min pressure required to open these things? Thanks
~~ Flams ~~

there are manual dampers then ones controlled by temp and others like that work on remote control

ok real easy response to the back draft damper............ your dryer vent outside has backdraft louvers on it. A backdraft damer like hes explaining works off of weights on the damper to adjust the amount of pressure needed to open it.