Sizing And Selecting Equipment
So, what does this mean to contractors?
“You have to design them right, size them right, install them right, and maintain them right,” stressed Hourahan, who provided a five-step approach to achieve this end during a presentation (“Standard of Care: Proper Indoor Environmental Quality”) he made at the 2003 Air Conditioning Contractors of America (ACCA) convention.
As vice president of research and technology for ACCA, Hourahan had a lot to squeeze in his 75-minute talk.
“You have to design to provide for wellness, rather than to prevent illness,” he said. “You should design to achieve fundamental requirements, rather than just to meet codes and standards. You should design for the last day of occupancy, rather than for initial occupancy. And, you should verify design with building diagnostic methods for virtual and actual building.”
Rules To FollowNot too long ago, chemical contaminants from building materials and inadequate ventilation were the major contributors to poor indoor air quality (IAQ), said Hourahan. The “problems” were resolved, he pointed out, by reducing product emissions and increasing ventilation levels. Today, the IAQ focus has evolved to address microbiological contaminants, which can bring about both health and building structural concerns. As a result, today there is an increased attention to moisture and humidity management, said Hourahan, along with renewed attention to vapor barriers and the need to control indoor relative humidity.
“The IAQ drivers [today] are health awareness and health care costs, increasing concerns for humidity control, pressure to reduce building maintenance costs, occupant productivity, terrorism, and regulations,” said Hourahan.
In his estimation, the best way to design and operate healthy buildings is by following these steps:
1. Establish building design criteria.
2. Determine the design loads.
3. Do not arbitrarily increase the load.
4. Ascertain system capabilities.
5. Evaluate latent loads.
Use The ManualConcerning the first step, Hourahan said the HVAC system should match the building application.
“Before undertaking a load calculation, it is important to ascertain the type of HVAC systems that are compatible for a building and its use,” he said. “This includes determining special space requirements or occupant needs or expectations.”
According to Hourahan, one should know, among other items, duct location and level of sealing and insulation, ventilation or filtration needs for asthmatics, and appearance issues “that have an impact on the type of system and on how the mechanical equipment can respond to the design and building requirements.”
He added that the simple replacement of “like for like” is not always the best solution.
“This assumes that the original equipment was properly sized,” he said. “This approach doesn’t consider whether the building functions have changed or that there have been upgrades made to the building.”
This means the contractor/designer has to perform a load calculation. The best way to do this, he recommended, is using ACCA’s Manual J for residential buildings and Manual N for commercial buildings. The association just released its eighth edition for residential load calculation, which Hourahan noted includes an electronic spreadsheet that can be used in calculating a load according to the principles of Manual J-8.
In this process, Hourahan noted it is critical that the designer carefully evaluate building construction parameters and verify assumptions about the related building details. This includes envelope tightness, solar orientation, insulation type and level, duct tightness and location, glass type, and overhangs and shadings.
“Observing design conditions is an important step,” he added. “Outdoor design conditions should be the 1 percent cooling dry-bulb design point for the specific geographic location where the building is located. The indoor design conditions should be based on customer needs and requirements.”
This means also ascertaining sensible and latent loads.
“The load calculation is based on peak load conditions. For summer cooling, this generally occurs on a sunny, hot day, and the peak sensible condition results from the peak dry bulb observed. However, what happens during the evening when the sun sets? What if it is raining?”
Do Not Increase LoadWhat frustrates Hourahan most is the fact some designers/
contractors arbitrarily increase the load “for safety reasons” once a load calculation has been determined and the sensible and latent loads established.
“Adding a ‘just-in-case’ safety factor of 25 percent to whatever is not acceptable,” he said, asking the contractor crowd to avoid overly conservative assumptions on building construction parameters, avoid failing to include diversity factors, and avoid rounding up loads to next equipment size.
Step four — verifying system capacities — means selecting the right equipment, using manufacturer’s application data.
“It is crucial that the selected equipment has the capacity to handle the latent load — at full-load operation and part-load operation,” he said.
Some of the cautions one should be aware of include making sure ducts are correctly installed, that drain pans are properly positioned and plumbed, and not to use crawl spaces as plenums, he said.
“If standard equipment cannot satisfy full-load or part-load latent requirements, then consider different equipment types,” suggested Hourahan. “Equipment manufacturers offer innovative options and approaches for modulating moisture.”
On his list was optimized equipment that utilizes multispeed/variable-speed indoor fan units and compressors to enable longer system runtimes. There is also hybrid equipment that uses wraparound heat pipes, desiccant materials, or enthalpy control, he said.
In the end, Hourahan noted that the correct application of equipment and operational strategies can handle challenging comfort conditioning requirements and provide for proper moisture control. However, he added that many practices, procedures, and approaches used in the past do not provide adequate moisture control in geographic areas of moderate-to-high outdoor relative humidity.
“The correct application of equipment, operational strategies, and customer support can handle challenging comfort conditioning requirements and provide for proper IAQ,” he concluded.
Publication date: 05/26/2003