A PIV fan is a general-ventilation system that often sits in the loft, with a vent on your landing. A PIV pushes fresh air into the property, which forces the moisture laden air out of the property, before it gets the chance to absorb more moisture and condense on a cold surface, such as a window or external wall. In layman's terms, Positive Input Ventilation is achieved when you push fresh air from outside the property, into the property.