The ABCs of ERVs
ERVs are designed to provide energy savings in mechanical ventilation systems. They recycle energy from the building’s exhaust air to pretreat the outside air/ventilation air. This preconditioning of outside air reduces the load the HVAC unit must handle, and hence reduces the required capacity of the mechanical equipment.
The general rule for ERV pretreatment of ventilation air is that there is an HVAC equipment reduction of 2.5 tons (2.3 metric tons) for each 1,000 cubic feet per minute (CFM) of outdoor air handled by an ERV. For example, instead of installing a 75-ton (68-metric-ton) cooling system for a project that requires 40 percent outdoor air (12,000 CFM outdoor air and 18,000 CFM return air), one conceivably could install a 12,000-CFM ERV and a 45-ton (41-metric-ton) HVAC unit to work together.
This reduction in the size of the HVAC equipment helps offset the initial cost of the ERV. The energy savings pays for the rest of the initial investment within a relatively short timeframe—the payback period ranges from 3 months to 3 years for most systems, depending on the size of the system and the building’s geographic location. The ERV then continues to provide the owner with a positive cash flow over the life of the system, which typically is more than 20 years.
Additionally, an ERV provides savings in summer cooling and winter heating operations and can reduce the summer electrical peak load demand. There also can be a significant improvement in humidity control during both seasons.
ERV or HRV?
An HRV is a heat recovery ventilator and can be used in both the heating and cooling seasons. An HRV is a sensible-only recovery device. That means it can recover sensible (dry) energy but not latent (moisture) energy. This does make it less effective during the cooling season.
The decision to use an ERV (a sensible-plus-latent device) or an HRV (a sensible-only device) is a matter of psychometrics. The general guideline for commercial applications for most climate zones in the United States is to always use an ERV to get the greatest energy benefit, as well as greatly improve humidity control. In the summer, an ERV removes the humidity from the incoming outdoor air and shifts it over to the exhaust air, keeping it out of the building. In the winter, an ERV removes humidity from the exhaust air and transfers it over to the outdoor ventilation air, eliminating the need for humidifiers. The moisture always moves from the air stream that is the most humid to the one that is the driest.
There are applications where an HRV makes more sense, such as a locker/shower room. In this case, the indoor humidity always is much higher than the outdoor humidity and it is desirable to remove that moisture from the space at all times of the year.
Types of ERVs
There are different types of ERVs for different applications. Generally speaking, there are four media-component choices for an ERV: rotary heat exchanger (wheel); plate heat exchanger (fixed core); heat-pipe heat exchanger (refrigerant); and runaround coils (water). Each of these choices is a proven technology that has been around for more than 20 years.
The graphics above illustrate the impact an ERV has on the temperature of a building’s airflow in winter and summer conditions. The DB (dry bulb) measurement indicates the pure air temperature. The WB (wet bulb) measurement indicates the temperature in relation to the humidity level.
A wheel is a plastic or metal device that rotates between the exhaust and outdoor air streams. It picks up heat from one air stream and transfers it to the other. Metal wheels only can transfer heat (sensible energy), while some plastic wheels, when impregnated with a desiccant, can absorb and release moisture (latent energy) as well. Wheels are the most popular ERV because of their relatively low initial cost, reasonable pressure drop, ease of maintenance, and smaller physical footprint.
Fixed-core plates generally are larger and more expensive than wheels, but have no moving parts and can be used in certain applications (such as hospitals) where a wheel may not be permitted by code. Instead of a wheel moving between air streams to transfer energy, the air streams pass by each other through a series of channels, heating up or cooling down the material between the channels and transferring energy. This is much like putting a can of warm soda in a bucket of ice. The ice cools the metal can and the can cools the soda without the ice ever actually touching the soda itself. Fixed-core plates can be metal, plastic, or even paper. Metal cores can only transfer sensible energy, not latent energy. Plastic cores can come in sensible-only or sensible-plus-latent versions. Paper cores always are sensible plus latent.
Heat pipes are somewhat limited because, as with HRVs, they cannot recover latent energy. They are sensible-only energy-recovery devices. Heat pipes are copper tubes with refrigerant inside of them. These tubes run between the two air streams (exhaust and outside air). One air stream heats the refrigerant in the tube, causing it to evaporate. That refrigerant vapor then moves down the pipe to the other air stream. When that other air stream cools the pipe, the refrigerant condenses, warming the cooler air stream in the process. The newly cooled refrigerant then flows back to the warmer air stream.
Runaround coils share some similarities with heat pipes, but often are preferred when the exhaust and outdoor airflows are separated by large distances. This type of system requires the installation of a water coil in the exhaust air stream and a second one in the incoming outdoor/ventilation air stream. These two coils are piped together, filled with a water/glycol mixture, and then the mixture is mechanically pumped between the two coils. Heat is picked up in one air stream and released in the other. Like heat pipes, these systems only are capable of transferring sensible energy, never latent energy.
There is an HVAC equipment reduction of 2.5 tons (2.3 metric tons) for each 1,000 cubic feet per minute (CFM) of outdoor air handled by an ERV, such as the rotary heat exchanger pictured above.
Other than energy savings, the main market driver behind increased use of ERV technology is the impact of codes and standards. ASHRAE Standard 62.1, “Ventilation for Acceptable Indoor Air Quality,” from the Atlanta-based American Society of Heating, Refrigerating and Air-Conditioning Engineers, is a major pillar of the International Mechanical Code from the Washington, D.C.–based International Code Council and the U.S. Green Building Council’s LEED rating system, as well as many local ventilation codes. This standard dictates how much ventilation air must be brought into a building. For most buildings constructed since 1989, this equates to 15 to 20 CFM per person. Prior to 1989, it typically was 5 to 10 CFM per person. As older buildings are being rehabbed and brought up to code, more outside air needs to be brought into these buildings and conditioned.
ASHRAE Standard 90.1, “Energy Standard for Buildings (Except Low-Rise Residential),” is an important component of ICC’s International Energy Conservation Code, LEED, and state energy codes. This standard dictates how much energy a building is allowed to consume. The difficulty is that one standard/code (ASHRAE 62.1) tells us to bring in outside air, which is costly to heat, cool, and dehumidify, while another standard/code (ASHRAE 90.1) tells us to not use any energy to do so.
An ERV wheel rotates between the exhaust and outdoor air streams, picks up heat from one air stream, and transfers it to the other.
Additionally, ASHRAE 90.1 currently requires ERVs on systems that bring in 70 percent outside air if the system is larger than 5,000 CFM. There is a new addendum coming for ASHRAE 90.1 (addendum e) that will significantly increase the stringency of that requirement based on regional climate zones. In some cases, systems bringing in only 30 percent outside air will require an ERV to comply.
For those who follow the green-building movement, there is a new standard on the horizon: Standard 189.1, “Design of High Performance Green Buildings.” This is a joint standard between ASHRAE, USGBC, and the New York–based Illuminating Engineering Society of North America. Standard 189.1 further increases the stringency of the ERV requirement to some systems that bring in as little as 10 percent outside air. And don’t think it will stop there—approximately every three years, it is likely that ASHRAE 90.1 and Standard 189.1 will continue to become more stringent as each standard strives for net-zero energy use for buildings.
ERVs are a cost-effective means to reduce energy consumption without reducing indoor environmental quality. They can and should be used in most commercial building applications, such as offices, schools, fitness centers, churches, auditoriums, and other buildings that have fairly high occupant density. If the energy-conservation story is not enough to impact HVAC designs, the upcoming code story will be.
Ryan Hoger is a product manager at HVAC Solutions, in Chicago. Hoger is actively in involved in several local ASHRAE and USGBC committees. He can be contacted at firstname.lastname@example.org.