My house is part two story, part one story, with separate attics over the two parts. We recently installed a quiet 1500 cfm "whole house" fan in the one story part (which covers the master bedroom and master bath) and love it.

Now I want to install a quiet 4500 cfm version in the 2nd story attic, and need about 6 sq ft (864 sq in) of net free area of exhaust capacity. There are 9 slope-mounted square vents in that attic, which I gather are 50-60 sq inches of NFA each. If I'm doing the math right, that means I need another 300-400 sq inches NFA (not counting the soffit vents). The gable end opposite the prevailing winds is easily available, but does not yet have a vent.

I'm thinking of installing a gable shutter (~200 sq in NFA), and possibly a powered gable vent wired to run whenever the whole house fan is running. My questions are:

* Is the gable shutter a bad idea?

* Is the powered gable vent a bad idea? In particular, I'm wondering if it would prevent the rest of the hot attic air from being forced out, by "short circuiting" the airflow.

* If I go with a powered vent, how much of the NFA requirement would it take care of? I can't find a conversion factor -- maybe because it varies based on static pressure differences?

Any advice would be much appreciated...

- Dave