The definition of an ERV (Energy Recovery Ventilator) in HVAC (Heating, Ventilation, and Air Conditioning) is a system that is designed to help reduce energy costs by transferring heat and moisture between the incoming and outgoing air streams. This is done by using a heat exchanger to transfer the heat and moisture from the outgoing air to the incoming air. This helps to reduce the amount of energy needed to heat or cool the air, as well as reducing the amount of moisture in the air. ERVs are becoming increasingly popular in homes and businesses as they can help to reduce energy costs and improve indoor air quality. ERVs are also used in commercial buildings to help reduce energy costs and improve indoor air quality. ERVs are a great way to save energy and improve indoor air quality, and are becoming increasingly popular in both residential and commercial buildings.