What Is an ERV? Energy Recovery Ventilators Explained

Commercial buildings need ventilation to dilute contaminants, manage CO2, and support indoor air quality (IAQ). However, bringing in humid, cold, hot, or dry outdoor air (OA) increases heating, cooling, and humidity control loads. One strategy that engineers are increasingly adopting, due to code requirements and the need to save on operating costs, is energy recovery. Energy recovery can be integrated with a Dedicated Outdoor Air System (DOAS), a ventilation unit designed to bring conditioned outdoor air into a building. When an application needs outdoor-air ventilation with energy recovery but not the full temperature and humidity control of a larger DOAS unit, engineers may choose an energy recovery ventilator (ERV). An ERV is a ventilation unit delivering 100% outdoor air while using outgoing exhaust air to precondition incoming outdoor air before it enters the HVAC system. It can provide application-specific benefits that include:

• Smaller footprint than a typical DOAS unit

• Lighter weight to accommodate a wide range of projects

• The options of flexible, modular indoor installation or traditional rooftop installation

Read on to learn how an ERV works, what type of ERV may be best for your project, and why efficiently increasing outdoor air through energy recovery makes buildings healthier for occupants. How Does an ERV Work? An ERV ventilates while transferring both heat (sensible energy) and moisture (latent energy) between exhaust air and outdoor air. Preconditioning the outdoor air in this way reduces the energy required to heat, cool, or dehumidify it. For example, in winter, warm exhaust air helps pre-treat incoming cold outdoor air. In summer, cooler and drier exhaust air helps reduce the cooling load from hot, humid outdoor air. The ERV uses a supply blower to bring in outdoor air and an exhaust blower to remove exhaust air, as well as either a total enthalpy wheel or core. “Total enthalpy” refers to the ability to transfer both sensible and latent energy. A total enthalpy wheel rotates to transfer energy, while the total enthalpy core is stationary with no moving parts. Both technologies meet or exceed ASHRAE 90.1 guidelines around energy recovery, including restroom exhaust.

The total enthalpy wheel or core inside an ERV transfers both heat and moisture between airstreams to precondition outdoor air.

This table outlines some differences between the total enthalpy wheel and core.

Why Energy Recovery Matters Building codes and industry standards have strict requirements for ventilation in CFM based on how many square feet a room has or how many people are in the room. Beyond the minimum OA levels required by code, higher IAQ increases occupant comfort and lowers the health risks that come with heavily recirculated air. Less outdoor air causes higher CO2 levels to build up, making exposure to airborne pathogens such as influenza more likely. (Check out our recent blog post “Classroom CO2 Levels: The Hidden Problem in School Ventilation” for more on this topic.) Energy recovery makes bringing in more outdoor air more affordable by ventilating while also reducing cooling and heating demand. In the example below, a school in Dallas retrofits their old HVAC system to add an ERV. Their outdoor air percentage increases by 10%, while the cooling capacity their system needs to meet drops by 47% and the heating capacity decreases by 60%.

Energy recovery can efficiently increase outdoor air CFM within a range of school retrofit projects, as this table shows.

Rooftop ERV Vs. Indoor ERV So you know you need energy recovery in your building project, whether because of code requirements, an energy rebate, or another incentive to save energy and reduce operating costs. The first question is, should you use an ERV or DOAS? As mentioned, a Dedicated Outdoor Air System not only preconditions the temperature and humidity of outdoor air but conditions it to a precise level for comfort. However, if other equipment in your HVAC system, such as a rooftop unit (RTU), can perform that function, an ERV provides ventilation and energy recovery with a lower initial cost than a DOAS. The maximum airflow capacity of a DOAS is higher than that of an ERV, however, so CFM requirements for your building will also guide your decision.

Once you have decided on an ERV, you need to choose between a traditional rooftop ERV and a modular indoor ERV.

Rooftop ERVs are used when roof space, curb-mounted installation, and direct access to outdoor and exhaust air make a packaged rooftop arrangement the most practical choice. This is the most common ERV setup.

Indoor ERVs come in handy when rooftop space is not available, such as for a smaller building with a pitched or sloped roof, or a high-rise building where adding ERVs to mechanical rooms or ceiling spaces on different floors can provide the required ventilation without taking up the whole roof. Retrofits to add energy recovery to an older HVAC system with limited ductwork space are another good use case. Indoor ERVs can be assembled onsite, fit into elevators, and can be ceiling-hung in a mechanical room, attic, basement, or mezzanine.

Fan Power and Fan Energy

Because ERVs include supply and exhaust blowers, it is important to select an ERV with efficient fans that don’t consume all the power saved by energy recovery. Two metrics for fan efficiency that apply to energy recovery are fan power and Fan Energy Index (FEI). Fan power is expressed in watts/CFM, with lower values indicating less electrical power required to move a given amount of air. In many ERV applications, the goal is to keep fan power below 1 watt/CFM. FEI compares the electrical input power of a reference fan to the electrical input power of the actual selected fan at the same duty point. Values above 1.00, which are increasingly required in codes and standards, indicate lower fan energy use than the reference fan. Direct drive plenum fans with backward-inclined impellers conserve energy to comply with these metrics. They have reduced drive losses and maintenance requirements compared to belt-driven fans. Selecting an ERV that includes direct drive fans, rather than one with belt-driven fans, can help meet fan power requirements and keep energy savings on track.

Summary

• An energy recovery ventilator (ERV) is a ventilation unit that recovers the energy from exhaust air to precondition incoming outdoor air.
  

• This process reduces the energy needed to bring the outdoor air to a neutral temperature and humidity, lowering the operating costs of ventilation.
  

• Ventilation is required by codes and standards based on occupancy, square footage, and other factors. A higher outdoor air percentage in a building means higher indoor air quality (IAQ) and lower CO2 buildup.
  

• Greenheck ERVs recover energy using either a total enthalpy wheel, which has the highest total effectiveness, or a total enthalpy core, which has the lowest crossleakage between airstreams.
  

• Greenheck ERVs are available as either rooftop ventilators or modular indoor ventilators that can fit in a ceiling space or mechanical room.
  

• Using an ERV with efficient fans, such as direct drive plenum fans, helps the ERV save operating costs.

Contact your local Greenheck representative or email sales@rlcraigco.com for more information about using an energy recovery ventilator in your project.