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ERVs are designed to recycle energy from the building’s exhaust air, using it to pre-treat the outside air as it enters. A building’s exhaust is usually wasted as its sent back into the atmosphere. By using it to pre-condition fresh outdoor air for both temperature and humidity, it dramatically reduces a building’s energy costs, and it lowers the workload of the heating and cooling system. And because the workload on the HVAC equipment can be dramatically reduced, often times you can downsize new or retrofit equipment for an immediate return on investment.

There are different types of ERVs, depending on the application needed for your building. In general terms, there are tour types:

• Rotary heat exchanger (wheel)
• Plate heat exchanger (fixed core)
•  Heat pipe heat exchanger (refrigerant)
• Runaround coils (water)

A rotary heat exchanger (wheel) is a plastic or metal device that rotates between the exhaust and the outdoor air streams. It picks up heat from one air stream and transfers it to the other. While metal wheels can only transfer heat, plastic wheels can absorb and release moisture as well. Because of their relatively low initial cost, ease of maintenance, and smaller footprint, wheels are the most popular ERV.

A plate heat exchanger (fixed core) are generally larger and more expensive than wheels, but have no moving parts, which makes them safer applications for certain situations, such as hospitals. Instead of using a wheel, a fixed core allows air streams to pass through a series of channels, heating up or cooling down the material between the channels and allowing energy to transfer.

A heat pipe heat exchanger are more limited that the previous two. Heat pipes are copper tubes with refrigerant inside of them. The tubes run between the exhaust and the outside air stream; one heats the refrigerant, causing it to evaporate, the other cools the pipe, allowing the refrigerant to condense, warming the cooler air in the process. The newly cooled refrigerant than flows back, and the process begins again.

Runaround coils offer similarities to heat pipes, but are often preferred when exhaust and outdoor airflows are separated by great distances. A water coil is installed in the exhaust air stream, and a second in the incoming ventilation air stream. The two coils are piped together and filled with a water/glycol mix to keep it operational. As heat is picked up in one air stream, it is released in the other.

What makes them a growing choice in commercial applications today? Besides the energy savings, there is also a push the the US Green Building Council’s LEED system that dictates how much ventilation air must be brought into a building. As older buildings are being rehabbed and brought up to code, more outside air is needed. This is where ERVs come into play.