It doesn’t get much more sensitive in HVAC design than designing ventilation systems for the schools that our youth occupy during their learning years. These students are the future of America. These are the youth whom we want and need to be in optimum learning environments.
There are studies that have been done on noise levels in classrooms. Students may be hindered from hearing the teacher if the noise level of an HVAC unit is too loud. There are also studies and debates on how much ventilation air is acceptable. Without proper ventilation, the IAQ may reach the point of restricting learning abilities of the students. Without proper ventilation, the CO2 levels in the classrooms may rise to a point that the students become tired and are less able to learn.
Historically, when it comes to energy savings, the basic school systems have been provided with the least expensive types of HVAC products due to budget restraints. Energy savings was less of a priority, if a priority at all.
Designing the HVAC for new modern schools and retrofit of older schools requires balancing objectives that are covered in ASHRAE Standards 55, 62, and 90 along with USGBC LEED guidelines and building code requirements, which vary by location. The application of heat and energy recovery ventilators is one way to help with compliance of codes and standards. The reality seems to be that one day, all buildings will require some form of HRV or ERV device to meet the building codes in the United States. There are already products on the market that utilize solar heat to preheat outside air going into the building ventilation systems.
A heat or energy recovery ventilator is an essential piece of mechanical equipment that is used to pre-heat or pre-cool the outside air that is brought into a building for ventilation. The technology can be such that it can recover only sensible heat (called an HRV) or total heat, sensible and latent (called an ERV). The technology can be a rotating wheel or a fixed cube-type device. There are other technologies to recover energy; however, this article will focus on wheels and the cube-type media technologies. In some situations and applications, a water or DX coil may be more appropriate than a media type heat transfer material.
The material of construction of the wheel or cube varies by manufacturer, and each will promote their product based on their performance advantages. There are pros and cons to both technologies. Suffice it to say that the engineer needs to have a conversation with the manufacturer and then apply reasonable good engineering judgment to discern which media material best meets the application needs.
Sizing and Selection
When sizing an HRV or ERV, the engineer must consider the ventilation load and also the intended use of the unit throughout the year. In some climates and applications there is no need (and it is possibly a detriment) to install an ERV versus an HRV.
The applications of energy recovery ventilators and sizing of these units takes a clear understanding of the fundamental tool of the HVAC industry — the psychometric chart. The psychrometric chart, by definition, is a graphic representation of the properties of mixtures of air and water vapor.
Most manufacturers have software programs to select their HRV and ERV units using basic input data that is needed to plot the conditions on a psychometric chart. The product selection program does all of the analysis and provides a performance printout that can be used as the basis-of-design unit requirements. Care must be taken to review the performance data and allow for discrepancies between manufacturers. Not all products are tested under the same conditions; therefore it is difficult to compare product performance.
Obviously, the application of an HRV or ERV depends on a source of building air that is being exhausted or relieved as part of the HVAC system. The goal is to recover a portion of the sensible or total heat in the exhaust or relief air in a manner that is safe and saves energy. Sources of exhaust could be toilet rooms, locker rooms, showers, kitchen areas, or laboratory spaces.
Some municipalities do not allow the use of wheels in toilet exhaust air streams because there is a bypass air factor for all wheels even when purge sections are used. However, the exhaust air compartment in the unit is generally more negative than the supply air chamber, so this may not be an issue of concern. The AHJ should always be consulted on any HRV and ERV application to ensure the proper type of unit is utilized.
An ERV and HRV may or may not be rated and tested under independent testing standards such as the AHRI standards. If not, then certainly engineers and facility owners may have a concern about the performance claims by a manufacturer. Even when tested, the testing may not be the same from one manufacturer to another, therefore the comparison of units is difficult at best and practically impossible. This affects design decisions and comparison for evaluation. Also, keep in mind that no performance is guaranteed past the day of purchase. The performance data of all wheels and cubes that this author is aware of is not guaranteed beyond the first day of the unit installation and operation. The industry does not have any type of fouling factors like with piping or other products that give some guidance on de-rating of performance over time.
Proving and verifying performance after installation becomes a challenge as with any product that is tested under ideally controlled conditions. Commissioning or enhanced commissioning of new or existing ERV and HRV units can be a challenge to prove or disprove the unit is indeed giving the performance that the owner paid for. Many variables exist, including installation and maintenance, that may impact the air streams in the ERV or HRV. It may be worth considering specifying an annual recommissioning of ERV and HRV units to verify performance on an ongoing basis.
Performance ratings of products by AHRI Standard 1060 third-party testing include leakage ratings at various pressure differentials and thermal effectiveness ratings for sensible, latent, and total energy transfer for both heating and cooling. These conditions are difficult at best (and at worst, impossible) impossible to replicate after installation, so calculations are necessary to determine if the unit is indeed performing as promoted in the manufacturer’s literature.
Installation, Operation, and Maintenance
An ERV or HRV, like any other mechanical device, needs to be maintained. The maintenance of the ERV and HRV depends on the materials used and the technology of the wheel. Some media may require more labor-intensive maintenance than others. Some media requires basic vacuuming, and others require washing or perhaps even steam cleaning.
Some ERV’s for inside installation are essentially outdoor units less the weather enclosures yet not much modification to account for limited inside serviceability. This gives engineers a challenge on retrofit projects and may be a product enhancement opportunity for manufacturers looking for a way to provide units that are sensitive to maintainability for the enduser. Design features that minimize the size of the ERV, minimize the access clearances, and provide multiple unit duct connection locations are just a few of the indoor unit criteria some engineers and architects look for.
The design of the intake of the outside air is important to understand so as to not take greater risk than necessary in impacting the media. Care must be taken to follow the manufacturer’s installation, maintenance, and operation instructions to optimize the length of the effectiveness of the media.
EPA Design Tools
The EPA has a section called “IAQ Design Tools for Schools” on their website. (http://www.epa.gov/iaq/schooldesign/ervassumptions.html). This page has a section called “ERV Assumptions” which has a System Financial Applicability Map.
They also have software (http://www.epa.gov/iaq/schooldesign/saves.html#ERV) called the Financial Assessment Software Tool (EFAST). Although the map gives a good starting point for consideration of ERV applications, the engineer should still do the due diligence of lifecycle cost analysis of the application based on specific project locations and conditions and operating conditions. Arbitrary application of any products that are marketed by manufacturers as energy-saving products with no detailed analysis can be costly to an enduser.
The LEED compliance initiative across the U.S. affects almost every aspect of a building design and HVAC systems. Although engineers have worked diligently for years before LEED came into being to design energy efficient buildings and HVAC systems, the LEED program has helped the industry focus on these essentials in good stewardship of materials and resources more than ever before.
The LEED-NC for new construction and major renovations gives credit for achieving increasing levels of energy performance above the prerequisite standard to reduce environmental impacts associated with excessive energy use. Some manufacturers have claimed that their customers have realized a reduction in total building energy use of 10-15% by using energy recovery as part of their HVAC systems. In these cases, according to the Energy and Atmosphere Credit 1: Optimize Energy Performance, this customer would receive two points for new construction and four points for a major renovation.
The USGBC also recognizes the contribution of higher outdoor air ventilation rates with better IAQ. The Indoor Environmental Quality Credit 2: Increased Ventilation gives one point for outdoor air ventilation rates 30% above the minimum required by ASHRAE 62.1. The credit also suggests to use heat recovery, where appropriate, to minimize additional energy consumption associated with higher ventilation rates.
The EPA has a Tier 1 and a Tier 2 level of compliance for ERVs. These can be seen at their website.
According to Energy Star, “Tier 1 (effective January 1, 2010) Products to be sold as Energy Star qualified must be tested and meet SRE requirements at 32°F (0°C) and -13°F (-25°C). The net supply airflows (in cfm) used during testing at these two different temperatures must be within 10% of each other, and specified in product literature and labeling.”
For qualification under Tier 2, the climate zone map is used to determine the product criteria to comply as Energy Star.
The Basic Components
In general, ERV/HRV units consist of a supply fan and an exhaust fan, and an energy recover wheel or cube. A unit may also come with associated motors, filters, and other accessories and options that are needed to accomplish the required unit functional criteria. It is essential to understand the needs of the application before selecting and specifying the units. The list below is a general list of some features of units that may or may not be standard by various manufacturers. Most manufacturers will have certain basic features as part of their unit, then additional options and accessories can be added as needed. This method of marketing ERV/HRV products helps keep the first cost of the products at a minimum. This list is not exhaustive and not in any particular order of priority.