Mitigating the effects of extreme heat
By looking at the mechanical, electrical, and plumbing system design strategies of buildings, engineers can help lessen the effects of high temperatures on buildings.
- Consider the various engineering systems affected by extreme heat.
- Review the methods to mitigate heat strain on building mechanical, electrical, and plumbing systems.
As heat waves have become more frequent and intense, they threaten health, stress water resources, and increase energy demands. New York City is particularly susceptible because of its dense urban environment, which absorbs and traps the heat. Prolonged periods of extreme heat accompanied by high humidity can create a dangerous situation for vulnerable populations while straining the city’s utilities and infrastructure.
A presentation in April 2016 from multiple city agencies (health, planning, and emergency management) as well as architects, urban planners, and landscape architects at the American Institute of Architects New York Chapter (AIANY) Center for Architecture discussed the effects of extreme heat on buildings and recommended mitigation strategies to deal with them. The program, “Heat Waves: Preparing for and Managing the Effects of Extreme Heat on Health, Populations, Emergency Housing, and Hospitals,” was organized by AIANY, Design for Risk and Reconstruction Committee, AIANY Health Facilities Committee (HFC), and New York Society for Health Planning.
This presentation focused on the effects of extreme heat on the mechanical, electrical, and plumbing (MEP) systems within health care facilities. One of many presenters, it explained that building codes establish the criteria—defined as “design day criteria”—used in the design of building infrastructure. The following is applicable to both new construction and existing-building retrofits.
Mechanical design strategies
In New York City, the mechanical system outdoor design criteria is based upon the 2014 New York City building code, which lists ASHRAE 1% data or 89°F dry bulb (DB), 73°F wet bulb (WB). If the outdoor environment temperature increases beyond that, it creates a domino effect on a facility’s infrastructure. In essence, the mechanical systems will be rendered undersized. Major equipment that may be affected include:
- Air-cooled refrigeration equipment
- Air handling units
- Chiller plants
- Control panels
- Cooling towers
- Elevators and machine-room air conditioning equipment
- Outside air intake and ventilation
- Ventilation of mechanical rooms.
In 2015, New York City experienced 20 days of temperatures exceeding 89°F. Such extreme heat conditions can be particularly devastating in critical buildings, such as hospitals, where the operation of mechanical equipment and controls may result in elevated interior temperatures and humidity throughout the facility, particularly at perimeter areas, creating reduced patient and staff comfort levels. Redundant systems may not be capable of addressing these issues due to design day sizing. Facility managers may try to reduce the outdoor-air ventilation rate to reduce cooling loads, but that can result in code violations.
Designers can use several mechanical mitigation strategies:
- Evaluate the equipment at greater than design day conditions by considering the de-rated conditions in other parts of the world, such as India or Brazil, that have elevated design day criteria. Also, consider specifying air-cooled equipment at elevated entering-air temperatures.
- Consider adding supplemental systems to make up the shortfall of the de-rating due to temperature, and specify at the elevated conditions.
- Ensure there are redundant systems and that systems are de-rated for their design intent at operating temperatures.
- Provide connections for plug-in systems, such as exterior supplemental cooling (e.g., external chillers).
However, when considering such strategies, it’s important to realize that equipment requirements will be larger, and more costly, so great care should be taken in coordinating and confirming requirements. It also should be noted that these parameters are in conflict with New York state and city codes, and they should be considered as safety factors for critical buildings only.
Electrical design strategies
Extreme heat drives the use of air conditioning, thus the demand for electricity. When demand threatens to outstrip the grid’s capacity to supply electricity, utility providers must take precautions to reduce strain on the system. Heat also can damage the electric system, stressing electrical generation, transmission, and distribution infrastructure.
Local electrical utility Con Edison lists extreme heat as greater than 90°F for 3 days. In extreme-heat conditions, power-grid demand will increase, leading to potential localized failures. Con Edison responds by reducing voltage to address heat conditions that could result in a facility voltage reduction of up to 13%. Because most equipment will not function at 87% voltage, some will just shut down while others may sustain permanent damage.
Typical standby generators are designed as “continuous standby,” although this is somewhat of a misnomer because they can operate continuously, but not for an extended period of time. Continuous operation for a week or more will likely result in some operational issues.