Swimming Pools and the ENERGY STAR Score in the United States and Canada

---

Overview

The ENERGY STAR score provides a fair assessment of the energy performance of a property relative to its peers, taking into account the climate, weather, and business activities at the property. Stand alone swimming pools are not eligible to earn the ENERGY STAR score. However, because swimming pools are a common, energy-intensive amenity at other commercial building types (i.e., hotels and schools), the ENERGY STAR score does make adjustments to accommodate for the presence of swimming pools. The goal of the ENERGY STAR score is to rate the energy performance of the primary use of the building, not the swimming pool.

• Technical Approach. An engineered model is developed to estimate the energy use for the swimming pool. This estimated energy use is subtracted from the building’s actual energy use, yielding an estimate of energy use of the building without a pool. This allows the building to be evaluated as though it does not have a pool.
• Property Types. Heated swimming pools located inside or outside of a building can be entered for all property types and will be incorporated into the ENERGY STAR score for eligible property types. There are no calculations or adjustments for pools that are not heated, because heated pools use significantly more energy than pools that are not heated, and are more likely to have a noticeable effect on energy use for the whole property.
• Adjustments. The swimming pool model is based on engineered assumptions regarding basic energy requirements for swimming pools and includes: Heating Energy. To account for maintaining a constant temperature while accounting for heat loss due to convection, evaporation, and radiation.
• Pumping Energy. To account for energy associated with circulating the pool water.
• Release Date. The The model is updated periodically as industry standards for design and operation are updated and as better engineering data becomes available:
  • Most Recent Update: August 2023.
  • Original Release: January 2004

This document presents details on how the ENERGY STAR score accounts for swimming pools. More information on the overall approach to develop ENERGY STAR scores is covered in our Technical Reference for the ENERGY STAR Score (PDF, 709 KB).

The subsequent sections of this document offer specific details on the development of the pool model:

• Overview
• Theoretical background
• Indoor pools
• Outdoor pools
• Example calculations
• Appendix
• References

Theoretical background

The engineered model to predict pool energy use is based on the fundamental rules of physics involved in heated pools and their interaction with the surrounding space. The total energy consumed by a heated pool is the sum of pool heating energy consumption and pool pump electrical energy consumption. Heat loss from a pool includes evaporation loss, convection loss, long wave radiation loss to cold sky, and conduction through the lateral surfaces to the ground. For outdoor pools, heat loss is offset by heat gains due to solar irradiation. Pool pump electrical consumption can be estimated as a function of head loss, pool size, pump efficiency, and pump circulation time. The heating energy consumption represents a far larger contribution to total energy use than the pump energy consumption.

Pool energy consumption can be expressed using the equation below. Specific calculations for each term are detailed in the Appendix.

The model uses the following assumptions:

• Indoor Pool Heating. For indoor pools, only evaporation and convection are considered significant contributors to heat loss.
• Outdoor Pool Heating. For outdoor pools, evaporation, convection, and radiation are considered significant contributors to heat loss.
• Conduction Losses. Conduction loss through the lateral and bottom surfaces are small and hence is ignored.
• Temperature. Pools operate at a fixed temperature throughout the year.
• Make-up Water Heating. Make-up water heating load is ignored.
• Convection. A fixed convection heat transfer coefficient is used.
• Source Energy. Calculations assume that natural gas is used to heat the pool water and that electricity is used for pumping. Conversions from site to source energy are applied accordingly based on country (U.S. or Canada). ^{Footnote 1}
• Equation Inputs. Fixed values are used for most of the input variables in order to minimize user inputs. The values are based on engineering judgments and parametric sensitivity analysis.

Indoor pool

The Using standard engineering references, the Appendix presents a summary of equations that can be used to compute each element that contributes to energy use (e.g., convection). These standard equations require several assumed inputs for factors such pool temperature. The input parameters used by EPA for the equations are shown in Figure 1, along with an explanation of the values used. Some of values are known quantities, and others were estimated based on recommended operating practices and engineering estimates. For some of the input variables (e.g., pool water temperature, swimming pool area relative humidity), values can vary based on pool operation. A sensitivity analysis was performed to test multiple values for several variables. The impact of each variable on total pool energy consumption was examined, as well as the resulting ENERGY STAR scores for buildings in Portfolio Manager with indoor pools. A combination of values was chosen that resulted in a reasonable adjustment to the ENERGY STAR score.

Using the values in Figure 1, a simple form of each equation in the Appendix is generated; these are summarized in Figure 2. At the bottom of Figure 2 there is a final combined equation, which includes all contributions to energy consumption. This equation is a general equation for the annual source energy consumption of an indoor pool, based on three factors: the Pool Area, the Activity Factor, and the Source-Site Ratio.

In Portfolio Manager, users have a choice of three standard pool sizes (recreational, short course, or Olympic). Portfolio Manager will assume a certain Pool Area based on the selected size. Activity Factor is based on the property type, which does not require a separate input. The property type is designated by Portfolio Manager based as the property use type which accounts for more than 50% of the total floor area. The Activity Factor values are included in Figure 1. Accounting for the three available pool sizes and the three activity factors, Figure 3 presents the exact pool adjustments.

Note that the swimming pool energy adjustments in Figure 3 are presented in different units for the U.S. and Canada. The ENERGY STAR score for the U.S. is developed using units of kBtu for energy, while the ENERGY STAR score for Canada is developed using units of gigajoules (GJ) for energy. While the calculations within Portfolio Manager occur in different units, ultimately the results for the any property (U.S. or Canadian) can be displayed in Portfolio Manager in either kBtu or GJ.

Outdoor pool

Energy consumption in outdoor pools is more difficult to calculate than indoor pools, because there is more variability in the input parameters for the equations in the Appendix. In particular, the following parameters can vary significantly:

• V. Wind speed
• T_w. Pool water temperature
• T_a. Temperature of outdoor air
• Φ. Relative humidity for outdoor air
• Solar Radiation. which is dependent on assumptions for surface shading level
• t_o. The time a pool is in operation throughout the year

To understand the range of energy consumption in outdoor pools, a parametric sensitivity analysis was conducted, calculating estimates for energy consumption using several values for each of the input parameters included above. For the variables that vary by climate, six different locations were examined: Boston, Chicago, Denver, Miami, Phoenix and Portland (OR). Outdoor pools in the two warmest cities in the analysis (Miami and Phoenix) were assumed to be open April through October. Pools in the other four cities were assumed to be open June through August.

A wide range of energy consumption estimates was observed. Given this variability, the most accurate assessment of pool energy consumption would require several additional questions in Portfolio Manager. Because the intent of Portfolio Manager is to assess the energy performance of the building, not the pool, this approach was deemed to be overly complex for the application. Instead, it is recommended that you install sub-meters to track energy use at outdoor pools. This pool energy should be subtracted from the main meter and excluded from Portfolio Manager, enabling an assessment of the building only.

In some cases it may not be possible to sub-meter and exclude outdoor pool energy consumption. For these cases, Portfolio Manager will still permit the building to benchmark, and will apply a conservative estimate for outdoor pool energy consumption. The estimate is based on the minimum adjustment determined through the parametric sensitivity analysis, averaged across the locations included in the analysis as shown in Figure 4. Because this is a conservative estimate, the most accurate option is to sub-meter pool energy consumption, subtract it from total energy use, and enter only the main building energy consumption into Portfolio Manager.

Example calculation

As detailed in our Technical Reference for the ENERGY STAR Score (PDF, 709 KB), there are five steps to compute a score. The following is an example for a school in the U.S. with a swimming pool:

1 User enters building data into Portfolio Manager

• 12 months of energy use information for all energy types (annual values, entered in monthly meter entries)
• Physical building information (size, location, etc.) and use details describing building activity (hours, etc.)