Ventilation units are used to replace indoor air with outdoor air though the ductwork system already in the building. This allows the new fresh air to run through the filter, A Heat Recovery Ventilator (HRV) uses heat from inside the house and transfers that heat to the incoming fresh air. An Energy Recovery Ventilator (ERV) transfers moisture and heat from incoming fresh air to the outgoing airstream. This helps keep the building cool and dry.