the in-line is on-line

Our code-required mechanical ventilation system has been deployed (finally!). It's a 19 watt , 100 cubic ft / minute centrifugal in-line duct fan mounted in the crawl space. It pulls air from the crawl space and exhausts it thru the exterior wall to the outside ...that pulls air from the living space thru vents in the floor to the crawl space ...which in turn pulls outdoor air thru fresh-air makeup vents into the living space. (Nowadays, you have to make wind-tight, vapor-tight, well-insulated walls and then put holes in them to let fresh air in so you don't get sick from stale air.) The flexible duct is used to reduce noise (this fan stays on all the time). Most of the duct is rigid 4" pvc and goes to the far corner of the crawl space to improve air circulation in the crawl space. Generally, a fan like this would be used to fix a radon problem (which we do not have in this area).