ERV systems can transfer two types of heat: sensible and latent. “Sensible heat” refers to the heat content of the air itself, and it is measured with a standard thermometer. Latent heat is the amount of energy required to first evaporate water into vapor (humidity) and then remove that water from the water vapor via condensation. Measure latent heat with a wet-bulb thermometer.
Ventilation systems that transfer both sensible and latent heat (a process called total enthalpy) are referred to as ERV devices; those that only transfer sensible heat are called heat recovery ventilation (HRV) devices. The main difference between the two is in their treatment of moisture in the air. ERVs can modify humidity levels; HRVs can’t. For this reason, HRV systems may be desirable in tightly sealed spaces that are operating within humid climates, especially if the space has continuous vapor barriers that cause moisture from indoor activities to become trapped. Because an HRV can’t transfer moisture from the exhausted air to the incoming fresh air, the result is dehumidification in the conditioned space. ERVs, by contrast, need to ventilate more to reduce humidity levels. Typically, HRV systems do not meet the 50% energy recovery effectiveness level required by the International Energy Conservation Code of 2015.
Each type of ERV and HRV device–enthalpy wheels, fixed-plate heat exchangers, heat pipes, and run-around loops—offers different benefits.
Enthalpy wheels. Most often employed in the cooling season in humid climates, an enthalpy wheel is made up of heat- and moisture-adsorbing material (such as a desiccant) that rotates between the incoming outside air and outgoing exhaust air ducts (Figure 1). As the enthalpy wheel rotates, it removes water vapor from the moist outside air and transfers it to the dry, conditioned exhaust air that is leaving the building. Simultaneously, the wheel precools the hot incoming outside air and transfers that heat to the cool, conditioned exhaust air.
Figure 1: Mechanics of an enthalpy wheel
Enthalpy wheels are ideal for saving energy because of their high heat-transfer effectiveness, but they can also increase the pressure drop in HVAC ductwork, which will require additional fan energy. The values in this image represent typical conditions for a hot, humid climate in the summer months.
The enthalpy wheel saves electrical energy by precooling and dehumidifying the intake air, thereby reducing the load on both the refrigerant compressor and the air-handler fan. When conditions allow for partial loads, you can either reduce the wheel speed or add a bypass duct to lessen the load on the fans even further.
Enthalpy wheels are among the most common types of ERV systems; they typically yield high effectiveness values—the efficiency with which heat is moved from one place to another. This technology can have total effectiveness values of 75% or more, but proper cleaning and maintenance (at least once a year) is essential to ensure that dirt and debris don’t build up on them, which can reduce heat-transfer efficiency and increase the pressure drop in the ductwork.
Note: For low-humidity applications, rotary heat wheels perform similarly to enthalpy wheels, but transfer only sensible heat.
Fixed-plate heat exchangers. Unlike enthalpy wheels, fixed-plate heat exchangers don’t have any moving parts. Instead, they drive intake and exhaust air through an alternating series of separate, sealed parallel plates, effectively transferring heat between the two airstreams (Figure 2).
Figure 2: Configuration of a fixed-plate heat exchanger
As with enthalpy wheels, fixed-plate heat exchangers can employ a bypass duct under part-load conditions to reduce fan energy consumption. The values in this image represent typical conditions for a hot, humid climate in the summer months.
Fixed-plate heat exchangers are available in a number of different configurations (two common options are horizontally or vertically oriented plates) and can be made to transfer moisture as well as heat by using desiccant materials to separate the airstreams. However, many fixed-plate heat exchangers use materials that only result in sensible heat transfer, such as aluminum or plastic; as a result, they yield lower net savings and higher indoor humidity levels than the moisture-transferring versions.
The total energy effectiveness of fixed-plate heat exchangers varies based on factors such as size and configuration, but it’s possible to find units with effectiveness values that are comparable to those of enthalpy wheels. And because no moving parts are involved, fixed-plate heat exchangers can use less energy and have lower maintenance costs than enthalpy wheels.
It’s important to maintain a clean air supply when using a fixed-plate heat exchanger. In applications where lots of particulates (for example, dust or smoke) are present, the unit could become clogged and be difficult to repair, particularly if it’s hard to reach.
Heat pipes. When two air streams pass through the heat pipes—one on the supply side and the other on the exhaust side—the temperature difference between the two will heat or cool the refrigerant.The refrigerant then changes phasefrom a liquid to a vapor and back again, which transfers energy from one side to the other. Most of the time, heat pipes transfer sensible energy, but if the surrounding air is cooled below its dewpoint, condensation forms on the pipe, which results in some latent heat transfer (Figure 3). The sensible effectiveness of heat pipes is 45% to 65%.
Figure 3: Heat pipe system
Heat pipes can also be used as indirect evaporative coolers—automated sprayers can spray water on the exhaust side of the pipe to precool the supply air.
Heat pipes can be a great option because they require no energy to operate and have no moving parts. This causes them to last a long time, with simple periodic cleaning for maintenance. You can even order them with a protective coating to guard against corrosion.
Run-around loops. To cool process water, the system must reject heat; a run-around loop (also known as a “wrap-around loop”) uses that rejected heat to preheat incoming outside air for ventilation purposes (Figure 4). In warm, humid climates, they can also reheat cooled, dehumidified air by transferring heat from outside. This technology circulates fluid between a system’s airstreams or heat sources, allowing for energy transfer between process loads and ventilation air without requiring that those elements be physically together.
Figure 4: Run-around energy recovery loop with dehumidification
A run-around loop recovers energy from exhaust air by transferring heat provided by warm outside air. This method only works in warm, humid climates.
Rooftop ERVs. ERVs can also be installed on rooftops like other HVAC units. Vendors currently offer rooftop ERV units that house enthalpy wheels that can pivot out of the airstream while the rooftop unit is in economizer mode.