Heat Recovery Ventilation (HRV) is an energy-efficient air exchange method that recovers heat from exhaust to reheat incoming air. It uses air-to-air or ground-to-air heat transfer and allows fresh air to be brought in for ventilation. HRV systems use ribbed plates to exchange heat and come in three designs: vertical flat plates, horizontal flat plates, or cellular flat plates. Earth-to-air HRV uses buried earth heating pipes to absorb heat from the ground and is commonly used in barns and greenhouses. A solar chimney extracts hot air by convection, and the exhaust is processed outside the building as cooled air.
Heat Recovery Ventilation, or HRV, refers to the air exchange method in which heat generated from exhaust in buildings is recovered and reused to reheat incoming air. The process uses one of two methods, air-to-air heat transfer or ground-to-air heat transfer. Regardless of the method used, a heat recovery ventilation system allows fresh air to be brought in for commercial 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 incorporated in a box to which ventilation fans are connected. When the warm exhaust air from inside the building flows over the ribbed plates, the air cools down and is then carried outside as cooled exhaust air. Simultaneously, the cold ventilation air also moves across the heat exchange plates causing it to heat up. The heated ventilation air is then fed back into the building.
There are three basic designs for a heat reclaim ventilation system. They use vertical flat plates, horizontal flat plates, or a checkerboard cellular design. Vertical dinner plates are about 50 to 70 percent efficient and work by using downward air currents. Horizontal flat plates are about 70-80% efficient and use passive airflow by alternating hot and cold ribbed plates.
Cellular flat plates are approximately 85 to 95% efficient and actually use countercurrent exchange technology. Countercurrent exchange refers to the process by which each cell flows in an opposite direction, creating a constant gradient across 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 heating pipes are buried under a structure and are most commonly used in barns and greenhouses in the United States. The heating pipe absorbs heat from the surrounding ground and is typically made of rigid plastic pipes that have been treated with antimicrobial chemicals to prevent the growth of bacteria. The pipes are buried between 6 and 10 feet (1.8 to 3 m) underground where the soil normally maintains a constant temperature.
As heat builds up, a solar chimney extracts the hot air by convection. The air then flows through the heat exchanger and is blown into the building as warm air. The exhaust is then carried back through the heat exchanger and processed outside the building as cooled air.
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