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Energy Recovery Fans

Date Added: June 21, 2012 12:24:30 AM
Author: Susan Terlitski
Category: Business & Economy: Supplies

Many HVAC system designers have realized the benefits of using Energy Recovery Ventilators (ERVs) in commercial and institutional buildings. However, as with many newly adopted technologies, questions of how best to apply this product are being raised. One application issue in particular is whether to apply an ERV as a stand alone unit on its own curb, or as a bolt-on accessory to a rooftop air conditioning unit. What is the difference between the two systems? Although both systems require an exhaust air stream as an energy source to pre-condition outdoor air, the primary difference between the two systems is where the exhaust air is taken from. The drawings at the right illustrate this difference and help visualize how the ERVs tie into the HVAC ventilation system. Some system designers may show a preference for the bolt-on ERV. This may be because they perceive that the installation cost will be lower, or they prefer having a single point of responsibility for the ERV/rooftop unit. In most cases, a system design that incorporates the stand alone ERV is superior to the bolt-on system. The fundamental reasons for this can be found by taking a look at more than just the outdoor air requirements. If we consider both the outdoor air and exhaust air requirements of commercial buildings, the advantages of the stand alone system become apparent. The stand alone ERV system provides the most efficient method of ventilating a building because it steals energy from air that has to be exhausted anyway. The bolt-on ERV is not tied into the exhaust system allowing the conditioned air to escape into the atmosphere. In many commercial buildings, the exhaust air code required is roughly half of the outdoor air volume. This means compared to the bolt-on unit, the stand alone ERV can recover energy from twice as much air leaving the building. The net impact is nearly double the effectiveness for the stand alone system. To illustrate this point, let's compare a stand alone unit to a bolt-on system based upon the following: • A commercial building requires 3,000 cfm of outdoor air based on ASHRAE Standard 62. • Based on codes, the minimum exhaust is 1,500 cfm from areas such as restrooms and conference rooms. • The building is to have a slight positive pressure and specifies total exhaust/relief air of 2,700 cfm (10% positive). • Outdoor air design point is 95db/78wb and room air is specified at 75db / 50% RH. Combining outdoor air and exhaust air ventilation functions into a single unit help drive an important practice in good HVAC design-building ventilation balance. In layman's terms: What goes in must come out. Traditionally, the outdoor air needs and the exhaust air needs were calculated in separate thought processes. The designer would specify the minimum outdoor air and exhaust air volumes required by code, but would stop short of comparing the totals to check the ventilation balance. Frequently, this led to over pressurized buildings where doorways were transformed into wind tunnels. In some cases, buildings became negatively pressurized and infiltration made indoor climate control difficult. Stand alone ERV systems inherently remind the HVAC engineer to perform the building balance check because the outdoor air and exhaust air volumes are located on the same equipment schedule. Additionally, the outdoor air and exhaust air fans are interlocked within the ERV control center. This translates into the following benefits of a stand alone ERV compared to a bolt-on: • The ventilation design is more likely to meet the engineer's intentions, typically a slightly positive building pressure. • System balancing after installation is far easier on a stand alone system. Exhaust and outdoor air duct runs provide recommended locations for airflow measurement. Determining exhaust and outdoor air volumes in the bolt-on configuration is practically impossible. • Exhaust fans and outdoor air fans are simultaneously energized so the building operates as it was designed. Separate controls, interlocks, and field coordination to maintain a balanced building are eliminated. (Note: In some traditional systems without energy recovery, exhaust fans were energized by occupancy sensors (i.e., motion detectors or light switches) in an effort to save energy. These systems prohibited a steady building balance and were counter-productive to proper ventilation and indoor environment control.) • Often, the system designer chooses to balance the air flow by increasing the exhaust air volume from areas such as restrooms and conference rooms. This results in fresher, better ventilated spaces and improved comfort for occupants. The installation cost of a stand alone ERV system is typically less than the installation cost of a bolt-on ERV system. Susan Terlitski HVAC Design Engineer Canada Blower Co. http://www.canadablower.com/ahu/index.html http://canadablower.com/hvac/index.html
 
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