What is Heat Recovery Ventilation?

Heat recovery ventilation, or HRV, refers to the method of air exchange where heat generated from exhaust in buildings is recovered and reused to heat incoming air. The process utilizes one of two methods, either air-to-air heat exchange or earth-to-air heat exchange. Regardless of which method is used, a heat recovery ventilation system allows for fresh air to be delivered into commercial ventilation and residential ventilation. HRV is also more energy efficient than traditional heating systems.

In air-to-air heat recovery ventilation, ribbed plates constructed of corrosion-resistant material are built into a box that has ventilation blowers connected to it. As the warm exhaust air from inside the building flows over the ribbed plates, the air cools down and then is moved to the outside as cooled exhaust air. Simultaneously, cold ventilation air is also moving through the heat exchange plates which causes them to warm. The warmed ventilation air is then flowed back into the building.

There are three basic designs to a heat recovery ventilation system. They either use vertical flat plates, horizontal flat plates or a checkerboard cellular design. Vertical flat plates are approximately 50 to 70 percent efficient and work using downward drafts. Horizontal flat plates are approximately 70 to 80 percent efficient and use a passive air flow in alternating warm and cold ribbed plates.

Cellular flat plates are approximately 85 to 95 percent efficient and actually use counter current exchange technology. Counter current exchange refers to the process by which each cell flows in an opposite direction, which creates a constant gradient over the entire set of plates. By having a constant gradient, the heat exchange does not require as much energy thus making it more energy efficient.

In earth-to-air heat recovery ventilation, earth warming tubes are buried beneath a structure and are most commonly used in barns and greenhouses in the United States. The warming tube absorbs heat from the surrounding soil and is typically made of rigid plastic pipes that have been treated with antimicrobial chemicals to prevent bacteria from growing. The pipes are buried between 6 and 10 feet (1.8 to 3 m) underground where the soil normally maintains a constant temperature.

As the heat builds, a solar chimney draws the warm air out through convection. The air then runs through the heat exchanger and is ventilated into the building as warm air. The exhaust is then run back through the heat exchanger and processed out of the building as cooled air.