Chilled Water TES Hydraulics | P2S

Kent Peterson

Vice President

PE, ASHRAE Presidential Fellow, LEED AP BD+C, BEAP, MCIBSE

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Thermal energy storage (TES) is an effective means of shifting cooling electrical load from peak to off-peak electrical rates. Chilled water is the most common form of TES, using concrete or steel tanks to store chilled water at 39°F (4°C), which is the temperature at which water density is highest, encouraging stratification within the tank. Under normal conditions, a chilled water TES tank is always filled with water. During discharge, cold water is pumped from the bottom of the tank, while an equal amount of warm return water is returned to the top of the tank. Due to the increased density of colder water, a stable stratification of layers of water can be obtained.

Placing an open chilled water thermal energy storage tank in a chilled water system has several ramifications on the hydraulic performance of the system. This month, I intend to point out engineering issues that must be addressed under various scenarios. These are all real examples from actual facilities that I have designed, peer reviews I performed of designs by others, or retrofits of designs by others.

System Considerations

TES tanks are seldom ASME-rated pressure vessels due to the high cost, so they must be vented to atmosphere. This establishes the reference pressure for the system; essentially the TES tank is serving as a vented expansion tank. Heat exchangers could be used to physically isolate the TES tank from the distribution system, but this is typically not preferred since chilled water ΔT will suffer with heat exchangers and additional pumps would be required. This column will focus on how to control for pressure in chilled water systems with atmospheric TES tanks without heat exchangers.

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Sustainability. Efficiency. Inventiveness. All words that come to mind to describe the new Aztec Student Union at San Diego State. Officially named the Conrad Prebys Aztec Student Union, the replacement to the original 1968 Aztec Center is the largest LEED Platinum Student Union building in the state of California. This was accomplished in no small part due to the vision of SDSU students, who ultimately selected the structure’s hyper-efficient design. And it’s the students who are enjoying the fruits of that vision. The new Aztec Center serves as the hub of the campus, where students meet friends to eat, shop and enjoy themselves between and after classes.


Students at SDSU demanded a highly sustainable student union building. Our team delivered a range of innovative energy and water saving systems into a space delivering the utmost comfort and recreational possibilities for students. The multi-function building features a theater, ballroom, bowling alley and a variety of dining and shopping options, making it a popular student hotspot. Highly efficient air-conditioning and heating systems respond to occupant demand, improve indoor air quality and use over 40% less energy than California standards. This design safeguards occupant health and ensures comfort while reducing energy consumption. The building’s lighting system is an effective mix of natural light and efficient lighting. Daylight illuminates significant portions of the building’s interior and energy saving light fixtures and overhead lighting illuminate the space after the sun goes down. A photovoltaic (PV) system on the roof offsets a portion of building energy usage. The Aztec Center also comes outfitted with a rainwater collection system to reclaim and use water for irrigation. All utility systems within the building included sub-metering to make energy and water consumption patterns easily visible. The new student center boasts 49.8% annual energy and costs savings over the previous one.

The Crafton Hills College campus had not undergone major facility upgrades since its opening in 1972. P2S helped revive the campus by providing mechanical, electrical, plumbing and technology design services to the new Crafton Center. The new campus hub is located where the former library once stood.


P2S provided MEPT and Fire Protection Design for the new facility, which houses student services and amenities including administrative offices, versatile event and conference space, food service areas and study spaces. Scoring 80 total points from the United States Green Building Council, the building’s design achieved LEED Platinum Certification.


Originally founded in Los Angeles in 1920 as a school of theology, Vanguard University moved to its campus in Costa Mesa, California in 1950. Over the century, Vanguard has evolved into a leader in educational innovation within its denomination, preparing students for all types of ministry. The University is now preparing for the next 100 years by ensuring generations of students have places and resources needed for academic, relational, spiritual and physical growth. P2S provided MEPT engineering design, site infrastructure, as well coordination for technology, AV and security design for the new state-of-the-art Waugh Student Center. The new building is an extension of the classroom, supporting a relational and unfettered approach to the learning environment of Vanguard.


The 35,000 GSF building replaces the existing student union and cafe, and now features a 16,000 SF cafeteria including on-site commercial kitchen facilities, a 3,000 SF fitness center and an 8,200 SF multi-purpose event space, as well as a student lounge which includes a fireplace and café.


During the schematic design phase, P2S reviewed program statements, site requirements and budgets with the design team, compiled the MEPT criteria and advised the team of the impact to the utility systems, code and space requirements. Designs were completed for the HVAC, electrical, plumbing, technology and lighting systems as well as site plans for proposed gas, electrical and telecom utility improvements, and recommended equipment location and orientation. Chilled water distribution infrastructure for connection to future developments was also designed. Sustainable design strategies were adopted to improve building performance.


During the design development phase, P2S updated and recompiled the preliminary designs as necessary and advised on space requirements and identification of major systems routing and updated site plans. P2S provided final construction documents which finalized the design and included system controls and construction details, reviewed cost estimates, implemented value engineering, constructability design revisions, and City plan checks. Additionally, our team provided construction support and close-out documents. The project was completed in early 2020.

CSULA’s LEED Gold Building 12 was once the home of the University’s Physical Sciences Department. The 9-story building was built in 1972 and was partially in use at the ground floor and basement levels. Following the Physical Sciences Department's transition to a new complex, the initial plans to demolish the building were changed after the University found that the interior of the building could be renovated. The project team saw an opportunity to put the mid-century concrete building to work by consolidating multiple services into a single, state-of-the art building. Retaining the building’s structure, the interior renovation and replacement of the existing building systems make for an improved on-campus experience for the CSULA community.


Certified as LEED Gold in November 2022, the building includes, lecture halls, conference rooms, workrooms, lounge/study areas, multi-purpose outdoor tables and open offices. In addition to the Administration and Student Life programs, it provides spaces for the Academic Affairs, Finance, University Advancement and President’s Office divisions. Floors one to four house the student ecosystem, while floors five to eight house the administrative services.

P2S provided MEPT engineering as well as AV and security systems for the building renovation. All of the building's existing interior partitions, finishes and equipment were demolished for a complete interior redesign. Seismic, fire/life safety and building systems replacements exceed current Title 24 standards, giving CSULA an efficient and comfortable space for its new occupants. P2S was responsible for MEPT schematic design, design development, construction documents, plan check approval, bidding and construction support services and project close-out.

The new Student Success Center (SSC) at UC Riverside (UCR) is a 3-story, 62,000 GSF building comprised of general assignment classrooms for modern pedagogies and technology, multi-purpose student life spaces, and dining services. The building is intended to serve as a new beacon on the campus to support student and organizational success through active learning, collaboration, and community building. The project was intended to not only address UCR’s continued student growth and corollary shortfall of instruction space, but to do so in a manner that upholds UCR’s academic mission with an emphasis on reinforcing student success. This project was certified LEED Gold on July 28, 2022.


From a sustainability standpoint, the team coordinated the construction materials to minimize loads within the building and choose climate appropriate solutions. These collaborative efforts allowed the team to minimize the infrastructure and maximize the usable square footage within the proposed footprint. The team worked to analyze every hour of operation to right-size the equipment and provide innovative solutions, some of which had not been utilized on the campus yet. To implement these systems, the team provided examples of where these technologies had been utilized for similar building types and clients. The building also features low-emitting materials that minimize concentrations of chemical contaminants that are damaging to indoor air quality, human health, productivity, and the environment.


Advanced energy modeling techniques were performed to evaluate the impacts of various energy conservation measures (ECMs). The project team evaluated the energy use intensity (EUI) impacts of multiple design strategies and integrated them with cost estimating from the contractor to understand the total benefits of the ECMs. In a number of cases, the team opted to move in a different direction. For those strategies that were employed, we used the following techniques:


-The project has several large lecture halls with occupancy ranging from 100-420 occupants, based on the room size. These lectures hall have high ventilation loads and raised flooring. For these spaces, we used indirect evaporative cooling and displacement ventilation to reduce the demands on compressor driven cooling. These strategies resulted in an EUI savings of approximately 4 btu/SF-year.


- For the large lecture halls, we provided four VAV zones, with the zone controls and reheat coils within the mechanical penthouse. This allowed us to eliminate any service within the space or the raised floor, to minimize the maintenance impacts to the space.


- The campus had an existing steam system that is failing and unreliable. The campus also has a requirement that we cannot have combustion for hydronic heating and domestic hot water. We used a modular heat recovery chiller to generate HHW for the building and export CHW to the campus loop. This strategy had a 7-point EUI reduction compared to the steam system.


- The project utilized a heat pump water heater for domestic hot water.


These solutions would not have been possible without collaboration of the building layout, envelope construction, shading, etc. to minimize the loads seen by the mechanical system. The contractor supported this collaboration by providing valuable input into the cost impacts of the strategies being proposed.

  • Services: MEPT, AV, Fire Alarm, Security
  • Size: 100,000 SF

In an effort to modernize key campus facilities, Mt. San Antonio College replaced the decades-old Student Life facility with a 100,000 SF new Student Center. Funded by Measure GO, the three-story building is a multi-purpose hub for campus life. The LEED Silver-seeking complex includes a wide range of areas from study and dining spaces to dedicated rooms for student clubs and government.


P2S provided MEPT engineering services as well as Fire Alarm, AV, and security design. Our team’s scope of work encompassed the creation of schematic designs, design development, construction documents, obtaining DSA approval, and offering bidding and construction support services.


The team's responsibility included the design and construction documents for building HVAC, electrical, fire alarm, plumbing, technology, and lighting systems. P2S also provided site utility plans for proposed gas and electrical utility improvements. In addition to the technical aspects, P2S assisted in the preparation of the project’s LEED checklist, providing crucial inputs on MEP prerequisites and credits and identifying impacts of implementing LEED requirements into the project.


With these contributions, P2S helped bring the Student Center to life, ensuring it meets the demands of modern students while honoring the sustainability commitments of Mt. SAC.





  • Delivery Method: Design-Build
  • Cost: $370 Million
  • Size: 450,000 SF

P2S is providing MEPT engineering services for the Riverside University Health System (RUHS). The currently underdeveloped site will be renovated to include five buildings with a total area of approximately 450,000 SF and a parking structure. The project is pursuing LEED Silver to focus on reducing energy consumption and waste, managing resources efficiently, and reducing operating costs and environmental impact. The Village provides for an entire continuum of behavioral health and wellness care, from urgent treatment to supportive housing, including outpatient care, education, and social services. It includes various park settings, activity areas, and outdoor gathering areas. Amenities include a market, cafe, and pet hotel. It will provide much-needed community, social, behavioral, wellness, and medical services to the local population.

  • Delivery Method: Design-Bid-Build
  • Cost: $1 million
  • Size: 101,000 SF

North Valley Hospital is currently undergoing a large-scale HVAC renovation to improve occupant comfort and indoor air quality. Three large rooftop air handling units as well as multiple energy recovery ventilators, variable volume terminal units and exhaust fans are slated for replacement to upgrade an aging infrastructure. The goal of the commissioning process is to ensure that systems are operating at their peak and to reduce issues that are commonly related to design and installation. P2S is meeting these goals by providing design review, site visits, pre-functional verification, functional performance testing, test and balance verification and trend data review on the project. P2S also seeks to keep the owner involved in the process with regular commissioning meetings and project updates to track the progress and effectiveness of the commissioning process. This project is in progress, but is currently on time and on budget.

  • Delivery Method: Design-Build
  • Cost: $1.2 Billion
  • Size: 550,000 SF

P2S is providing mechanical and plumbing design for the 347-bed Inpatient Replacement Tower which will house acute care patient beds and essential services, such as inpatient radiology, pharmacy, patient kitchen, labor and delivery rooms, intensive care units, acute psychiatric, medical-surgical beds, and observation beds for short-stay patients. The new warehouse, approximately 11,000 SF, under the jurisdiction of OSHPD, will support the campus' supply chain operations. This project is targeting LEED Gold Certification.

  • Delivery Method: Design-Build
  • Cost: $3.4 million
  • Size: 1,500 SF

P2S Inc. was part of a progressive design-build team tasked with building a new hybrid bi-plane operating room (OR) at the University of Washington Harborview Medical Center. The space, which functions as an imaging suite and operating room, is within a functioning group of operating rooms and sub-sterile core environment. Construction required a complete renovation with multi-phase delivery to minimize downtime for the adjacent operating rooms, sterile core, and surrounding corridors. The existing floor slab and mezzanine levels were reconfigured to make the most efficient use of the space and to accommodate the structural requirements of new systems.

  • Delivery Method: GCCM - MCCM
  • Cost: $27.8 Million
  • Size: 100,000 SF

The approximately 100,000 SF renovation occurred in the blue building. Originally constructed in 1990, the blue building has a total of six levels of patient care. This project renovated the level two labor delivery recovery postpartum rooms, and the level four NICU and converted level five patient rooms to antepartum rooms. In addition to the renovated floors, a new Emergency Department dedicated to the labor and delivery function was designed to support newborns and families.

  • Delivery Method: Design-Bid-Build
  • Cost: $75 Million
  • Size: 150,000 SF

P2S provided MEP, Technology, and Low Voltage design services for the project. Additionally, P2S helped with utility service accommodation, site improvements for the surrounding buildings, and a 3D laser Scan of the interior of the first floor of the existing building. Connected to the 3-story building, a new 50,000 SF facility houses the Transplant Recovery Facility which has roughly triple the capacity of the previous recovery center in Redlands - making it the largest facility of its kind in the country.

  • Delivery Method: Design-Build
  • Cost: $223 Million
  • Size: 505,000 SF

The Martin Luther King Jr. Behavioral Health Center project is an adaptive repurposing of the former MLK hospital building. The 5-story, 505,000 SF building was demolished to the structural members and floors, then rebuilt with new MEPT services and distribution systems with supply service points of connection in the existing campus central utility plant. The renovations provide mental health services, including urgent care, outpatient, psychiatric health, substance abuse, and rehabilitation. The design-build team was led by HOK and Bernards in partnership with Los Angeles County Public Works.

  • Delivery Method: Design-Bid-Build
  • Cost: $106 Million
  • Size: 135,000 SF

The new VA San Diego Facility is an outpatient clinic that consolidates and expands the Veterans Affairs existing medical space in San Diego. It is expected that 600-800 patients will be treated daily at the facility. P2S provided mechanical, electrical, plumbing, and technology design as well as construction support services.

  • Delivery Method: Design-Bid-Build
  • Cost: $500 Million
  • Size: 600,000 SF

Kaiser Permanente Baldwin Park Medical Center is set to undertake an estimated $300 million infrastructure upgrade project. As part of the Design-Assist team, we’re providing our mechanical and plumbing design expertise for this replace and rebuild effort at the hospital, central utility plant building, and medical office building. Our team is working with the project general contractor and subcontractors in a “big room” setting, regularly coordinating with Kaiser’s Facilities Strategy, Planning and Design (FSP&D) Department and the project team to ensure design functions as intended.

We’re replacing existing air handling units, chillers, heat exchangers, cooling towers, humidifiers, fan coil units, exhaust fans, and controls. In the first phase, we are providing design options which will include sustainable design for each system to lower the building Energy Use Intensity (EUI) from 230 to 160. The strategies will include heat recover chiller design to save heat energy on the domestic hot water system, convert constant air volume distribution to variable air volume system, design air handlers with separate cold and hot decks to optimize economizer mode to name just a few of our improvements. The new Direct Digital Control (DDC) system will optimize the building’s energy use and will automate the entire HVAC and plumbing systems to minimize maintenance. This project received a CEC grant for its sustainable design to de-carbonize a large healthcare facility.

  • Delivery Method: Design-Assist
  • Cost: $230 Million
  • Size: 215,000 SF


Scripps Memorial Hospital Encinitas began construction on the new acute care Lusardi Tower and Lusardi Pulmonary Institute in June 2023. The $263 million project represents the final phase of a multi-phase expansion (Phase 1 included the Leichtag Foundation Critical Care Pavilion, completed in 2016). The project comprises a 224,000 SF new construction building with a full basement and 64 new med surge/ICU patient beds. Other major departments include six operating rooms, radiology (IR, cath lab, MRI, Spec CT), med surg, PACU, kitchen/cafeteria, pharmacy, clinical laboratory, and sterile processing. The tower will open by 2025, taking the hospital to over 230 patient beds.


The project consists of multiple construction phases and increments to minimize the impact on the existing acute care facilities on the campus. Multiple make-ready projects were required to house impacted staff and operations before demolition.

  • Delivery Method: Design-Bid-Build
  • Cost: $22 Million
  • Size: 60,000 SF

Our healthcare team worked on various aspects of this project designing systems for both the shell/core building and the tenant improvement buildout. We designed dry utilities and the greywater water system for the core building and coordinated with the LA Department of Water and Power for expanded electrical power utility at the building. For the tenant improvements, we expedited the process for the architect and designed low-pressure air distribution and ventilation systems; power distribution and lighting design; fire alarm and fire protection systems; and submitted all Title 24 energy calculations.

  • Delivery Method: Design-Bid-Build
  • Cost: $13 Million
  • Size: 5 Campus Buildings

The project for Huntington Memorial Hospital in Pasadena, California, incorporates utility assessments and upgrades across multiple buildings across the hospital campus includes a new 12kV electrical utility service distribution lines on campus, to support the complete Master Plan development, and the rerouting of existing building service lines through underground tunnels and seismically complaint spaces. Building emergency power services are upgraded through new 4160V site distribution for increased capacity to support interior renovation projects and the connection of existing HVAC systems. Our work also included upgrades to the existing cooling systems in the Central Utility Plant for additional chilled water and condenser water capacity. Our team executed various designs around active acute care operations without impacting patient care including cutovers associated with exhaust and supply air systems in several buildings.

  • Delivery Method: Design-Assist
  • Cost: $50 Million
  • Size: 72,000 SF


Scripps Memorial Hospital Encinitas needed a new building to accommodate its growing demand for capacity. P2S helped design the new $94 million, 72,000 SF Leichtag Foundation Critical Care Pavilion (CCP) and central plant. In line with the Hospital’s focus on future development, the design called for constant re-assessment and forward-thinking considerations about the needs of the community in the next 50-100 years. The 2-story CCP includes 26 treatment rooms, a 36-bed medical surgery nursing unit, and airborne infection isolation rooms.


Because the hospital was so focused on preparing for future development, the design called for constant reassessment and forward-thinking about the needs of the hospital community in the next 50-100 years.
  • Delivery Method: Design-Assist
  • Cost: $215 Million
  • Size: 273,000 SF

The University of Washington’s Montlake Tower is a medical tower housing diagnostic imaging, surgery, and patient beds, including a 32-bed adult acute care unit, a neonatal intensive care unit providing care for 47 newborns, and an inpatient oncology unit. The eight-story superstructure has three inpatient floors and a dedicated interstitial space for mechanical, electrical, and plumbing systems. A second design phase completed shell space in the Montlake Tower and renovations in the Pacific and Muilenburg Towers. This work involved five universal and two hybrid operating rooms, two intensive care nursing units, a medical/surgical nursing unit, and support areas.

  • Delivery Method: GCCM
  • Cost: $150 Million (estimate)
  • Size: 267,000 SF

The patient-centered principles of Virginia Mason's lean production system were used to design and construct the new Floyd & Delores Jones Pavilion in Seattle. This 19-story medical campus addition included emergency and critical care units, a new lobby entrance, and a pedestrian bridge. Double-tall floors on multiple levels provided a connection to the existing 17-story hospital tower. The design emphasized flexibility, allowing any floor to function as treatment, diagnostic, surgery, or patient space.

Scripps CCB (Done) (2013-6973)


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P2S provided Mechanical, Electrical, and Plumbing engineering design for the two-story Critical Care Building (CCB) with a new Central Utility Plant and emergency airlift helipad. The CCB consists of a new emergency department, procedure rooms, and med-surge patient beds. Because the hospital was so focused on preparing for future development, the design called for constant reassessment and forward-thinking about the needs of the hospital community in the next 50-100 years.

Scripps CCB (Done) (2013-6973)


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P2S provided Mechanical, Electrical, and Plumbing engineering design for the two-story Critical Care Building (CCB) with a new Central Utility Plant and emergency airlift helipad. The CCB consists of a new emergency department, procedure rooms, and med-surge patient beds. Because the hospital was so focused on preparing for future development, the design called for constant reassessment and forward-thinking about the needs of the hospital community in the next 50-100 years.