David1950 wrote:

You better learn about intake. You're pulling air in from your
passive vents near the ridge as well.

I've looked at that, and although you'd think so, it's not really a
factor.

With the fan running and no cover to impede air flow, I placed strips of
newspaper over the passive vent grill surfaces. I have 3 of these vents
on opposing sides of my almost pyramidal roof (the roof is not quite
square, so I have a 4-foot long peak instead of a point-peak).

The passive vents are pretty much exactly like these:

http://www.single-family-home-remode...roof-vents.jpg

About 12" on a side.

Anyways, with the fan running, the paper strips would barely stay put
against the passive vents, but yes they were held there by a negative
pressure. This negative pressure was very weak - the strips wouldn't
stay put for any longer than about a minute before a weak gust of wind
would blow in from the soffit area and overcome the negative pressure
and blow the strips from the vents.

The powered vent looks like this (with the cover on):

http://www.atticvents.org/wp-content...ttic-Vents.jpg

Air is supposed to flow as a sheet, not some hacked up system you
believe you "designed" by throwing a power vent up there.

When I turn on the fan in the mid-morning, when the sun has already
elevated the attic temperature into the 90's, I watch my temperature
readout slowly start to decline by about .2 degrees every 10 to 15
seconds. So don't tell me the fan isin't doing anything - I can see
how it's reducing the temperature. And when I'm on the roof with the
exhaust blowing right into my face, it's just as hot as the exhaust vent
on my gas clothes dryer.

And yes, air is probably flowing like a sheet along the underside of the
roof decking because of where the soffit and vents/fan is located.
There simply isin't another path for the air to take inside the attic.

You're probably better off not running the power vent,

The roof will easily get 20 degrees hotter without the fan.

pulling air _only_ from the soffit.

I've proved to my satisfaction that this effect of the fan pulling air
into the attic from the passive vents is not happening to any measurable
extent. I would guess that the warm air inside the attic really does
want to rise, and there is more than enough soffit venting to allow
suitable outside airflow to enter the attic as the hot air is evacuated
by the fan. I probably would get a different result if I tried this
newspaper trick in the middle of the night, when the temperature inside
the attic is equal to the outside ambient air temperature.

This is of course based on if you have at least the minimum
intake & exhaust venting needed for the square feet involved.

Did you ever calculate how much venting is actually needed?

I have re-worked the eve and soffit area along one side of this roof a
few years ago, cutting out a hole in the existing plywood soffit between
every rafter and then replaced the existing aluminum soffit with one
that is fully vented along it's entire length. The other 3 sides of the
roof have a soffit vent maybe every 4'th or 5'th rafter.