Buildings consume about 40% of the energy our nation uses. Among that, within commercial buildings, more than half of that energy is typically spent conditioning the building to comfortable temperatures and maintaining indoor air quality through ventilation. In all commercial buildings, fan systems distribute air for ventilation. Other fans route air from odor- or fume-driven spaces such as restrooms or kitchens and exhaust it to the outdoors in a means to confine and convey odor particles outside. Energy is spent conditioning the air from the outside as it is brought in for ventilation. Exhaust air leaving the building is typically already conditioned since the air was inside previously. If only there was some way to recover the valuable energy within the exhaust airstream and put it back into the ventilation airstream. It would take less energy to condition the fresh air using what would otherwise just be a waste stream from the building.
There is! It’s done with Energy Recovery Ventilation (ERV).
Energy recovery technologies offer a variety of ways to accomplish this. The International Energy Conservation Code (IECC) serves as a guiding force in shaping the energy efficiency landscape within the construction industry worldwide, including proving requirements on energy recovery in commercial buildings. While energy recovery can be beneficial, and often offer very short payback intervals, IECC Chapter 403 section 7 outlines when energy recovery technologies are a minimum requirement of the International Building Codes. (International Code Council, Inc., 2021) These codes are critical, as they are implemented to varying degrees in every state’s building codes within the USA as well as internationally.
The benefits of implementing these technologies in a commercial building are vast. Recovering energy the building has already spent and reusing it is effectively free energy savings. Implementing these technologies in air handling systems is typically very straightforward, and while an AHU with ERV will take up slightly more space, ERV technologies typically pay for themselves in less than 7 years.
When is ERV Required
While the benefits of ERV technology are obvious, there are times where the IECC Chapter 403 makes it a minimum requirement for design. Chapter 403 Section 7.4 outlines those requirements. In it, certain criteria are grouped in a chart that defines the maximum total airflow that a system can possess without requiring ERV technology. The code is written so that larger systems with higher outside air requirements, those systems with the greatest potential energy savings from ERV, are most likely to require it. (International Code Council, Inc., 2021)
These charts are first defined by the approximate annual runtime of the AHU. If the unit is anticipated to run less than 8,000 hours a year (essentially continuously), one set of requirements apply; if it runs more, another set applies. From there, two additional inputs define the particular maximum CFM allowable without ERV. The geographical location of the building defines what ASHRAE climate zone is applicable, and the minimum anticipated percentage of outside air with respect to the total airflow of the AHU drives which criteria box applies. Once the specific set of limitations is defined, the total airflow of the anticipated AHU can be compared to the limitation to derive whether ERV is needed.
There are a variety of exceptions to this process, which allow individual cases where ERV would be detrimental to the performance of the building or the safety of its occupants, which are also outlined in the chapter. Among them, exhaust systems that present heightened safety risk to the occupants, such as fume hoods or other hazardous exhaust airstreams, are excluded. Make-up air that is being directly and immediately vented to the outdoors, such as that in commercial kitchens, are also excluded.
It should also be noted that in some states (such as Massachusetts) stretch energy codes have been adopted that supersede the IECC requirements, and remove many of the exceptions for energy recovery.
Types of ERV
There are a variety of different types of energy recovery technologies, but all of them operate under the same principle: absorb as much energy from the exhaust as possible, move it to the incoming ventilation air, and release as much of it as possible. Some technologies take it one step further in being capable of absorbing both sensible and latent energy to mitigate energy spent heating and cooling as well as humidifying and dehumidifying. Some technologies are more effective than others at ensuring the exhaust and ventilation airstreams never mix. A brief outline of the three most common recovery technologies is outlined below.
Plate Heat Exchangers: This device utilizes metal plates to transfer heat between the outgoing exhaust air and the incoming fresh air. The plates allow for efficient heat exchange while maintaining separation between the two air streams. This method is particularly efficient in temperate climates where the temperature difference between indoor and outdoor air is moderate. Implementation of plate heat exchangers is particularly effective where project economics are a priority , and smaller unit applications are used.
Heat Wheels (Rotary Heat Exchangers): These rotating wheels consist of a matrix material that absorbs energy from the exhaust air stream and transfers it to the incoming air. The matrix media often consists of a desiccant material capable of absorbing both heat and moisture, making it the most overall energy efficient technology for humid climates. Extreme cold presents challenges to frost control within the wheel media since it has capacity to convey moisture.
Run-Around Coils: In scenarios where it's essential to keep the outdoor and exhaust air streams completely separate, such as in life science or healthcare buildings, run-around coil systems come into play. This technology employs coils in both the exhaust and supply air streams, connected by a circulating fluid. Heat is transferred between the two coils, ensuring energy recovery while preventing any direct contact between the two air streams, making it an ideal solution for applications with strict air quality requirements.
All three technologies offer pros and cons depending on a system’s particular priorities. The diversity in strength between them ensures that all three are commonly implemented throughout commercial HVAC design.
IECC Section 403 sets a comprehensive standard for the integration of energy recovery ventilation systems in building design. As the construction industry continues to prioritize energy efficiency and sustainability, ERV systems emerge as a key solution, offering benefits ranging from improved indoor air quality to substantial energy savings. By embracing and implementing the requirements outlined in Section 403, stakeholders in the construction industry contribute not only to individual building performance but also to a more sustainable and resilient future for our planet.
International Code Council, Inc. (2021, January 29). 2021 International Energy Conservation Code (IECC). Retrieved from ICC Digital Codes: https://codes.iccsafe.org/content/IECC2021P2/chapter-4-ce-commercial-energy-efficiency
Mechanical Project Leader