Properly Sizing A Whole House Fan

The DOE Recommends Sizing A Whole House Fan - Like This:

Take Your Home's Total Square Footage And Multiply By 2 Or 3.  The Number You Arrive At, Should Equal The Flow Rate Of Your Fan In Air Flow (CFM).  Using This Formula, A 2,000 Sq. Ft. Home Then Would Require A Fan That Is Rated At 4,000 - 6,000 Cubic Feet Per Minute Of Air Flow!  AND HERE IS WHERE WE DISAGREE WITH THE DOE's SIZING RECOMMENDATION!

BIG PROBLEM #1:

Here Is The BIG PROBLEM with this formula!  A 6,000 CFM Fan Requires - 8 Square Feet Of Unobstructed Roof Venting.  Almost All Homes In Existence Today - DO NOT HAVE THIS MUCH NET FREE VENTING!  You Could Always Add More Vents and Venting - But That Could Get Expensive.  Furthermore, Do You Really Want To Put More "Holes" In Your Roof Than Are Already There? 

BIGGER PROBLEM #2 : Do You Like Mold?  What About Ice Dams?

The Traditional' Whole House Fan, Leaves A Large Uninsulated Hole In Your Ceiling!  Especially In Colder Climates - Heat From The Home Will Travel Into Your Attic.  Where It Will Meet With Cold Surfaces And Condensation Will Form. 

If You Have A Whole House Fan That Does Not Automatically Seal When Not In Use, Be Sure To Cover It During The Off Season Or Duct The Exhaust Directly To The Outside!