It's not activated by heat.... It's actual name is "active Carbon" its extremely porous and absorptive. Heat isn't required. The only way to properly set it up is so that the only air leaving the space goes through the fan that pushes to the filter. You can put the filter in front or behind the fan as long as your ducting is sealed. I put mine at the end after the fan becuase i use a cage fan and its better at pushing air then pulling. Fans vary in static pressure, you can do a test run to see if one is really better then the other, one way sucks, one way blows, put ur hand by where its either sucking or blowing and see which one has more air flow, I think generally most fans are more efficient at making pressure"blowing" then trying to suck air. The filters are actually set up to have the fans suck air through them becuase they have a dust cover on the outside, which would be useless if it was blowing.