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| Sustainable Building Sourcebook | ||
| Chapter: Energy | ||
| Energy Recovery Ventilators | ||
| CSI Numbers: 15700 Heat Recovery, 15730 Heat Exchangers, 15762 Unit Ventilators, 15786 Desiccant Humidifiers | ||
| Introduction: | ||
Adequate ventilation is necessary to provide a healthy and productive indoor environment. An energy recovery ventilator (ERV) is an air to air heat exchanger or preconditioner, designed to reduce the energy required to heat or cool required outdoor air in mechanical ventilation systems by as much as 80 percent. These products exchange temperature and moisture properties from one airstream to another. The result is capturing the cooling or heating energy from the exhaust air before it leaves the building. | ||
| At-A-Glance Notes: | ||
| Technology: | ||
| The technology is well developed. | ||
| Suppliers: | ||
| Commercial ERVs are readily available in the Central Texas area, and there are several suppliers nationally. Residential ERVs have limited availability in the area but are readily available nationally. Residential air-conditioning contractors are not well versed in this equipment. | ||
| Cost: | ||
| The prices will range from $500 to $1,700 for residential sized units. There is a very large variance in type and size of commercial equipment. The payback on energy savings is very short when ASHRAE 62-1989 compliance ventilation rates are being maintained. The payback is immediate in new construction when air-conditioning equipment sizing and installations account for the added cooling and heating capacity provided by the ERVs. There are large health benefits achieved from providing adequate pre-conditioned fresh air. | ||
| Public Acceptance: | ||
| There is very little awareness of this type of equipment in the South. In the North, many people perceive heat recovery ventilators as beneficial. When understood, it is generally regarded favorably by people interested in new energy efficient homes and indoor air quality. However, comprehension of the technical aspects required to use and maintain a system properly is low. | ||
| Regulatory: | ||
| ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality. | ||
| Considerations: | ||
| ASHRAE Standard 62-1989 requires a 0.35 air change per hour for residential construction and per person outdoor air quantities for specific commercial building uses (these quantities vary between 15 to 50 cfm/person). It was thought that these ventilation rates would be adequate to remove any contaminants in the space and provide acceptable indoor air. However, inadequately conditioned outside air can itself be a contaminant. If the amount of moisture introduced with the outside air is not reduced to acceptable levels, high relative space humidity can result, which is a primary factor in the growth of microorganisms and fungi. Molds, dust mites and many other biological indoor pollutant sources thrive in humidity levels over 60 percent, so it is essential that indoor humidity levels be controlled below that level.
Design engineers, home builders, building and home owners must take care to introduce the required level of outside air, while ensuring that enough moisture is removed under all load conditions to keep the space relative humidity below 60 percent. Energy Recovery Ventilators will significantly reduce the operating cost of the HVAC system of a building or home designed in accordance with the ASHRAE IAQ standard. They will also improve humidity control. ERVs pre-treat incoming fresh air to modify its temperature and remove humidity before introducing it into the building. The removal of the humidity from the incoming air is essential to maintaining a healthy indoor environment in Austin. This is a major improvement over older heat recovery ventilators developed for northern climates that only heated the incoming air by passing it through an air-to-air heat exchanger. |
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| Guidelines: | ||
| How It Works
ERV systems are usually packaged systems that include supply and exhaust air fans, outdoor and return air filtration and an energy or enthalpy wheel. The wheel recovers both sensible (temperature) and latent (moisture) energy, it cools and dehumidifies the outdoor air during the cooling season, while heating and humidifying the outdoor air in the heating season. The wheels are either coated with a silica gel desiccant or a non migrating adsorbent desiccant on a molecular structure designed to trap water vapor from a moist air stream and loose it to a drier air stream. As the transfer media slowly rotates between the outdoor and exhaust air streams, the higher temperature air gives up its sensible energy to the mass of the wheel. This energy is then given up to the cooler air stream during the second half of the revolution. Just as the temperature is captured and released, so is the moisture. Since the opposing air streams have different temperature and moisture contents, they have different vapor pressures. This vapor pressure difference provides the driving force that causes the transfer of latent energy.
There are many ways to configure and install ERVs in commercial or residential buildings. They can be located inside the building, above dropped ceilings in attics; or exterior to the building, on rooftops. The systems can be coupled to packaged rooftop units, located near and ducted to air handling units, or de-coupled from the air conditioning system completely. Design or consulting engineers have a lot of flexibility with the systems. The size and configuration of these systems is almost without limit. Equipment is available for flows from about 200 cfm to 30,000 cfm. Custom systems can be designed for larger volumes. They require two connections to the outdoors - one to exhaust the stale indoor air and the other to bring in the fresh outside air. The inlet and outlet on the building exterior need to be distanced from each other to avoid cross contamination. Fresh air from the unit into the building interior can be supplied in two ways. A separate supply duct can carry the fresh air to a central location in the building (a de-coupled system). This supply outlet is best located where the air will not blow on people, since the air can be too cool or too warm depending on the season. Otherwise, the fresh air can be ducted into the HVAC ductwork, typically in the return duct. There are a few other configurations, but these are the most common. EfficiencyEnergy recovery wheel efficiency ratings are based on tests conducted in accordance with ASHRAE 84 and ARI 1060 standards. Published effectiveness values correspond to total energy transfer. Sensible only effectiveness will typically be slightly higher. The efficiency of ERVs refers to the amount of energy transferred to the cold dry air stream from the hot humid air stream. The Home Ventilating Institute (HVI) has a standard test for units manufactured in the United States. Not all manufacturers have been tested, so it is necessary to examine efficiency claims to see if they are HVI data or manufacturers data to get a true comparison. All units sold in Canada are subject to a standard test (R-2000), which is almost identical to HVI's test. |
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| Resources: | ||
| Professional Assistance: | ||
| See "Engineers -Professional" in Yellow Pages |
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| Components/Materials/Systems: | ||
American Aldes Bossaire Inc. Des Champs Laboratories, Inc. Energy Federation Inc. Honeywell Inc. Nutech Energy Systems, Inc. Positive Energy Conservation Products Shelter Supply Stirling Technology, Inc. Therma-Stor Products Group Venmar Ventilation Inc., Conservation Energy Systems, Inc. Vent-Aire Systems, Inc. |
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| General Assistance: | ||
American Society of Heating, Refrigeration, Air Conditioning Engineers (ASHRAE) Air Movement and Control Association International, Inc. |
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