Energy recovery ventilators (ERVs) are heat recovery ventilators (HRVs) that exchange moisture between the two air streams. An ERV is a heat recovery ventilator (HRV) that recovers both latent heat and sensible heat. Latent heat is the amount of heat that must be removed from air to change the water vapor from a gas to a liquid. Sensible heat is the amount of heat involved in raising or lowering the temperature of air, not including any heat required to cause water vapor to change state (e.g., from a gas to a liquid).

A heat recovery ventilator (HRV) is a ventilation device capable of transferring heat (and sometimes moisture) between two airstreams, or from one airstream into water. People often refer to HRVs as energy-saving devices. This isn’t exactly true because HRVs are equipped with fans that require electricity to operate; therefore, they actually consume energy. In fact, HRVs, central-supply, and central-exhaust ventilators all use electricity to run their fans. Remember, all forms of ventilation consume energy. HRVs consume less energy by reducing, but not eliminating, the cost of tempering the incoming air (or by heating water, in the case of a central-exhaust heat-pump water heater). An HRV is first and foremost a ventilator and, second, offers a way to recover energy that would otherwise be wasted during the process of ventilating. (Actually, an HRV should save more energy by tempering the incoming air (or heating water) than its fans consume, but whether it does so, depends of its efficiency and the climate.)

Most HRVs are balanced ventilators in which heat is transferred from one airstream to another. These balanced HRVs are often called air-to-air heat exchangers (AAHXs). Central-exhaust heat-pump water heaters are HRVs but they are not AAHXs because they transfer heat from one airstream into water, not between two airstreams. Another term often used is energy recovery ventilator (ERV). All HRVs transfer sensible heat from one airstream to another or to water. If latent heat is also transferred, then an HRV can be called an ERV. While there are specific differences between these different terms, many people use HRV, AAHX, and ERV interchangeably.

Source: Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems - published by The Healthy House Institute.