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.

By Christopher McHugh, PE, LEED AP, CEM, AKF Group, New York City March 21, 2017

Learning Objectives

  • 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
  • Generators
  • 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.

Prime power systems (used for cogeneration systems) operate continuously for extended periods. They are designed to change filters while the engine is running and have oversized cooling systems. Catalog information for most engines/generators provides both a prime power and standby rating. A generator normally is rated for a maximum operating ambient of 104°F. If outdoor conditions approach this value, it can be expected that generator performance will degrade, as the ventilation air passing across the engine will exceed the 104°F limit.

Adequate fuel availability is critical to continued operation. For hospitals, 96 hours is required. The facility will need to have a means for emergency fuel delivery. The fuel itself can be an issue in that the generators employ turbochargers that circulate return fuel as a means of cooling them. The fuel is recirculated and mixed at the main tanks. Extreme heat can create a condition where, eventually, the fuel becomes too hot for proper ignition and cannot cool the engine. This will reduce deliverable capacity from the generator and/or will initiate alarm conditions on the generator.

For reference, electrical equipment is generally rated for a maximum operating temperature of 104°F and electrical wiring is generally rated at  a maximum operating temperature  of 67°F or 194°F at the cable itself at rated electrical capacity based on outside ambient temperatures of 86°F.

Site-specific parameters can further increase temperature conditions. Sunlight increases the temperature beyond ambient conditions. (For instance, electrical wiring and equipment installed onto rooftops that are exposed to direct sunlight). The New York City 2011 Electrical Code – Administrative Provisions and 2008 NFPA 70: National Electrical Code (NEC) amendments provides guidance for appropriate de-rating under these conditions.

Congestion within equipment, such as cable pull boxes or switchboards, also can restrict the required airflow, creating hot spots. Ventilation alone will not be sufficient to accommodate greater than 100°F outdoor conditions. Mechanical ventilation and cooling systems do not reach every part of the system equally.

More failures can occur as systems are pressed closer to their operating limits due to high temperatures or loading. In addition, indeterminate factors, such as the existing electrical installation, can increase the likelihood of such events as overloading, improper installation, inadequate maintenance, and/or operational requirements that differ from the original design intent.

Electrical mitigation strategies include:

  • De-rating of capacity is a solution for most high-temperature conditions. Higher-ambient conditions can be mitigated by de-rating wiring capacity. The New York City 2011 Electrical Code – Administrative Provisions and 2008 NEC amendments provides formulas to do so.
  • Oversizing generator radiators and/or de-rating the generator capacity for elevated temperatures. Consider normal design parameters in other parts of the world with high-temperature conditions, such as India or Brazil.

Plumbing design strategies

Extreme heat can result in a drop in residual municipal street water pressure, with reduced volume/flow due to high water use, particularly at peak times including mid-mornings and evenings. In addition, it can create elevated entering street water temperatures and elevated rooftop gravity-tank temperatures

These extremes will affect water availability for critical uses, pressures at process equipment, water temperatures, and interior pressures at pumps and tanks.

Mitigation strategies to consider:

  • Holding tanks
  • Process cooling for make-up water
  • A constant-pressure booster system
  • Capture cold condensate as precooling.

Architectural systems

The following will help augment the successful operation of MEP systems in buildings:

  • Minimizing the impact of solar contribution within the building
  • Addressing building orientation (reduce east/west-facing glass areas)
  • Specifying improved glazing systems (use good double or triple glazing)
  • Using high-mass building materials, such as concrete
  •  Reducing roof loads (green or high-albedo roofs)
  •  Reducing interior and exterior load impacts where possible
  • Using façade elements, such as frits and sunshades; external south shading
  •  Providing interior thermal or blackout shades
  • Selecting better-than-code efficient lighting and equipment to reduce interior loads
  • Considering possible use of climate walls (conditioned barrier between internal and external walls).

By putting several strategies into play, the effects from extreme heat can be mitigated. Consider:

  • Identifying areas/systems that are required to operate during periods of extreme heat
  • Evaluating equipment at greater than design day criteria to quantify the de-rated condition: Design infrastructure replacements and new buildings to design day criteria based upon other parts of the world. Coordinate carefully and prioritize needs, as equipment requirements will be larger and more costly.
  • Adding supplemental systems to make up the shortfall
  • Ensuring there are no redundant systems and systems are de-rated for their design intent
  •  Providing connections for exterior supplemental cooling and power
  • Performing a critical-failure analysis on the facility to identify weak links.

Resources like the “AIA Post-Sandy Initiative” and the “Urban Green Building Resiliency Task Force Report” can provide additional assistance.

Christopher McHugh is AKF Group’s partner in charge of commercial base building services, which encompasses MEP design for central utilities and energy services, core and shell, and tenant fit-outs.