We occasionally get questions from potential customers about ducting a whole house fan directly outside, instead of having it discharge into the attic. Have a look at the graph below from our San Jose DMP demo site to illustrate why we recommend discharging into the attic space.
At the end of a hot day, the attic space is around 95 deg F (RED line). When the whole house fan is started (GRAY line spike to speed 6), the attic temp drops over 10 deg F in just a few minutes. What you are looking at is the flushing effect of the whole house fan pushing all that hot attic air outside. Without that flushing effect, the house would continue to have additional heat load from the attic. This is an additional (bonus) effect on top of the cool outside air being pulled into your living space!