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Energy recovery ventilators (ERVs) are a cost-effective way to bring more outdoor air into a commercial building while reducing the burden on the rest of the HVAC system. They work by:
- Drawing in fresh air through an intake fan
- Removing stale indoor air through an exhaust fan
- Using a heat exchanger to pretreat the incoming air to a more comfortable temperature using recovered energy from the outgoing air
While the heat that building occupants can feel (sensible energy) is usually top of mind when thinking about a heating and cooling system, the moisture (latent energy) that an ERV transfers is just as important. That is what distinguishes an ERV from a heat recovery ventilator, which only transfers sensible energy. DOAS (Dedicated Outdoor Air System) units, which ventilate buildings with conditioned outdoor air, can transfer moisture through energy recovery as well.
This article explains moisture transfer and why it should be a significant consideration for energy recovery in a building.
Sensible Energy, Latent Energy, and Total Energy
Sensible energy, also known as sensible heat, can be measured with a thermometer and determines whether a building’s occupants feel comfortable with the temperature. Latent energy is the heat energy contained within the moisture in the air. A space with a higher humidity level will have more latent energy. Typically, up to half of the total energy in a cooling and dehumidification system is latent.
When you add sensible energy to latent energy, you get total energy, also known as enthalpy. The greater the percentage of the total energy an ERV or DOAS unit can transfer between the exhaust and supply airflows, the more effective it is at recovering energy.
Moisture Transfer in Different Energy Recovery Technologies
The ability to transfer the latent energy in air moisture is a key factor in choosing an energy recovery technology. There are three main technologies used in energy recovery:
- Total energy wheel
- Can be made of either aluminum or polymer
- Manufactured with a desiccant that allows it to transfer latent energy
- Total energy core
- Made of corrugated polymer or fiber
- Polymer cores are more effective at transferring energy
- Sensible aluminum plate
As the names imply, both the total energy wheel and total energy core transfer both sensible and latent energy, whereas the sensible aluminum plate transfers only sensible energy. Table 1 shows what percentage of total energy each type of technology can transfer.
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| Table 1 |
Latent energy is only transferred from one airstream to another when moisture is transferred without condensation. This process maintains the latent heat of condensation. Rotating wheel technology is used in DOAS units and ERVs because of this moisture exchange function. The total energy wheel is therefore the most effective widely available technology in energy recovery, as it uses moisture transfer to recover up to 80% of total energy from the exhaust airflow.
The total energy core is less effective at 50–65% but maintains the ability to transfer latent energy between airflows.
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| An energy recovery ventilator using a total energy core during summer operation. |
The aluminum plate transfers no latent energy, although it can transfer 75% of sensible energy. Therefore, an ERV using this technology is functioning more like a heat recovery ventilator than an ERV with total energy recovery capability. In very humid indoor applications such as a swimming pool, this would be preferable, as then only the heat energy from the exhaust air would be recovered without the humidity.
Benefits of Moisture Transfer in Energy Recovery
Why is it important to transfer moisture rather than only sensible heat when recovering energy in most applications?
Energy recovery can reduce the strain on the equipment humidifying or dehumidifying a building so that it uses less energy and needs to be maintained less often.
Learn More
Find a Greenheck Representative: Reach out to a representative in your area to get more information about incorporating energy recovery into your project.
Energy Recovery Ventilators: Learn about the features in the energy recovery ventilators that Greenheck offers.
HVAC Energy Recovery Technologies—Wheel versus Core: Distinguish the two main technologies used for energy recovery in DOAS units.
Energy Recovery Product Application Guide: Find out the differences and similarities between energy recovery technologies and how codes and standards affect them. This guide has been recently updated with information on ASHRAE Standard 90.1-2022, including the fan power limit.
DOAS Energy Recovery
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