CASE STUDY: Natatoriums
Learning Objectives
After reading this case study, you should be able to:
- Understand why natatoriums are some of the most energy-intensive buildings to design, model, and operate.
- Understand how evaporation drives the high energy intensity nature of natatoriums.
- Recognize why natatoriums are difficult to model accurately using conventional energy modeling software.
- Evaluate how operational practices can significantly influence real-world decarbonization outcomes.
Project Overview
ASHRAE defines a natatorium as a building containing one or more aquatic venues, such as pools, spas, or water features, where the public is exposed to water intended for recreational or therapeutic use. Systems West Engineers provided MEP design and energy modeling services for recently completed renovations at two municipal natatorium facilities for the City of Eugene: Echo Hollow Pool and Sheldon Pool. Each project included a pool expansion and electrification strategies intended to reduce the carbon intensity of the heating systems. Central to both designs was the installation of a heat pump chiller (HPC) to offset natural gas consumption previously driven by each facility’s pre-renovation steam boiler system.
Each facility has now operated for approximately two to four years following the renovation. During this period, observed energy consumption has not aligned with the original modeled performance developed during design. This case study evaluated current system operation and control setpoints, measured performance data, and compared those conditions to the assumptions used in the original energy models. The goal here was to identify the primary sources of discrepancy and understand the potential operational adjustments that could help achieve energy savings and provide performance that better matches with original modeled predictions.
Efficiency Challenges
Natatoriums are among the most demanding building types to design because they must simultaneously protect occupant health, preserve long-term structural integrity, maintain swimmer and spectator comfort, and conserve energy and water resources. The two main drivers of these challenges involve controlling chloramine production and evaporation loads associated with the aquatic environment.
Chloramine Control
One of the key health and durability concerns in natatoriums is chloramine formation. Chloramines form when chlorine combines with organic contaminants introduced by swimmers and bathers. This can become the dominant indoor air quality problem in poorly designed or poorly operated facilities. Exposure can irritate eyes and skin, aggravate the respiratory system, and increase health risks for swimmers and staff, while the same corrosive environment can also damage building materials and metal components over time. For that reason, natatorium design must be approached as an integrated problem involving pool chemistry, ventilation effectiveness, humidity control, and building durability.
Properly controlling all of these variables consumes energy, causing natatoriums to be extreme energy outliers. Typical recreation centers without natatoriums often operate between 50 and 70 kBtu/sf-year, while recreation facilities containing natatoriums can experience whole-building EUIs five to ten times greater. Natatorium-only spaces can exceed 1,000 kBtu/sf-year. In other words, adding a natatorium can increase facility energy use several times above that of a conventional recreation building. This occurs because natatoriums are required to:
- Operate continuously
- Maintain negative pressure
- Control for humidity
- Heat the pool water
- Dehumidify the air
- Reheat that air to maintain comfort
Evaporation Loads
Evaporation from the pool surface is the primary driver behind these considerations because it affects nearly every major end use within the facility. As evaporation increases, pool heating, dehumidification requirements, and air reheat loads increase exponentially.
Furthermore, evaporation rates and heating loads for natatoriums are highly sensitive to environmental and operational conditions. Key drivers include:
- Air velocity across the pool surface – Even relatively low air speeds (~125 fpm) can increase evaporation rates by approximately 40%. This becomes especially significant in spaces with excessive airflow across the water surface such as outdoor pools.
- Pool water temperature – A 4°F increase in pool temperature can increase evaporation by roughly 33%.
- Air temperature and relative humidity (RH) – ASHRAE guidance recommends maintaining natatorium air temperatures approximately 2–4°F above pool water temperature and RH between 50% and 60% in order to suppress evaporation rates and associated latent heat losses.
Decarbonization Challenges
Because natatoriums consume so much energy, designers often pursue decarbonization opportunities supported by energy modeling. However, the same characteristics that make natatoriums energy intensive also make them difficult to decarbonize and model accurately.
Heating loads remain high around the clock, warm-up expectations can drive peak equipment sizing, and low outdoor air temperatures can reduce heat pump performance at the exact time the facility requires the most heating. At the same time, typical energy modeling software does not adequately account for the vapor pressure physics of pool surfaces, and modelers are forced to generate custom evaporation profiles externally. These profiles are also unusually sensitive to assumptions regarding pool covers, internal heat gains, heat recovery effectiveness, occupancy activity, temperature setpoints, and evaporation profiles.
Energy models remain valuable tools for identifying opportunities to reduce energy use and carbon emissions, but natatoriums can be particularly difficult to represent accurately unless operations-informed assumptions and carefully developed external load profiles are incorporated into the analysis.
Our Approach
To address the differences between observed operation and the original energy model predictions, Systems West revisited and recalibrated the energy models using measured operational data from Echo Hollow and Sheldon pools.
The first step was determining whether the discrepancy was being driven primarily by plant inefficiencies or by higher-than-estimated pool thermal loads.
The primary finding was that the heat pump chillers were operating correctly and functioning as the primary heating source during winter operation. The figure below shows that the HPC covers the majority of the average building thermal load. As the total load approaches the HPC capacity (~1,300 MBH), the system performs largely as intended, with the heat pump handling the base heating load. However, when outdoor air temperatures drop below approximately 47°F, heat pump capacity decreases and the steam system is required to satisfy the remaining load.
Overall, this indicates that the electrification strategy is successfully reducing fossil fuel usage and that the heat pump systems are providing the intended decarbonization benefit. However, the total building heating load was significantly higher than originally modeled. Further investigation identified several operational and modeling conditions that contributed to the discrepancy:
- Pool temperature setpoints are operating 1–4°F higher than original design assumptions.
- Natatorium space temperatures are operating 4–6°F lower than design assumptions.
- Natatorium space relative humidity setpoints are operating 5–10% lower than design assumptions.
- Pools are uncovered approximately 12 additional hours per week beyond modeled assumptions.
- The Sheldon Pool solar hot water system is offline post-renovation.
- Pool activity factors in the original energy models which underestimates evaporation loads associated with swimmer activity.
After updating the energy models to reflect these operational conditions, annual energy usage aligned within approximately 2–10% of measured consumption at each facility. This indicates that the primary source of discrepancy was not equipment underperformance, but rather pool heating and evaporation loads that were substantially higher than originally estimated.
As the City evaluates recommissioning opportunities at both facilities, operational setpoints are being reviewed to better align with the original design intent. One of the clearest findings from this study is that maximizing pool cover usage can significantly reduce evaporation and the associated heating and dehumidification energy use. Longer term, the existing solar hot water system at Sheldon Pool will also be investigated to determine how it can be restored to operation and integrated into the overall heating strategy.
Lessons Learned
- Evaporation is the primary energy driver in natatoriums.
- Small operational changes involving water temperature, humidity setpoints, air velocity, and pool cover schedules can increase natatorium energy use by 30–40% or more.
- Natatorium energy models often underrepresent evaporation-driven loads, making calibrated, operations-informed modeling essential.
- Decarbonization outcomes are highly sensitive to real-world operations and control assumptions.
- Properly accounting for swimmer activity is important to avoid underestimating evaporation and associated pool heating loads.
Case Study Contributors: Joe Iaccarino, Dorrie Matthews, Steve Schual, Nate Jenkins
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