Load reduction modeling

Peak heating and cooling loads are affected by building envelope heat gain or loss as well as internal heat gain from sources such as lighting, office equipment and occupants

Schematic design decisions can significantly affect HVAC loads and system sizing. Conducting a BEM load reduction analysis during this stage can provide valuable insights, enabling the design team to identify and implement savings opportunities that might be unattainable in later design stages when substantial changes become less feasible.

A primary goal of load reduction modeling is to take a big-picture view and develop an understanding of the sources of heating and cooling loads. Most BEM software can provide output reports that show a breakdown of contributors to peak heating and cooling load. That information can help the design team understand, for example, the magnitude of peak cooling load contribution from envelope components such as conduction through walls, windows and roof and solar heat gain through windows. Results will also show contribution from sources of internal heat gain such as lighting, plug loads and people. With insights from these results, the design team can identify potential load reduction strategies.

An advantage to using BEM tools to evaluate peak load reduction strategies is that in addition to providing peak load results they also provide annual heating and cooling loads and associated energy consumption. In other words, BEM provides both HVAC sizing impacts and operating cost impacts, which the design team can use to identify appropriate load reduction strategies.

ASHRAE Standard 209

ASHRAE Standard 209-2018 includes a modeling cycle dedicated to load reduction modeling. The purpose of modeling cycle #3, which is required to be performed before the end of schematic design, is to identify strategies to reduce both annual energy use and peak HVAC loads. The first step is to create a baseline model and identify the distribution of energy by end use and the peak heating and cooling loads. Then BEM is used to evaluate at least three load reduction strategies.

Impact of load reduction

Buildings with lower peak heating and cooling loads need less airflow and smaller heating and cooling equipment capacity, providing many potential benefits.

• Smaller equipment rooms
• Smaller vertical shafts
• Less ceiling plenum height
• Less roof space required for HVAC equipment
• Less structural load from HVAC equipment
• Lower embodied carbon due to HVAC system components

In addition, strategies that reduce peak loads are also likely to reduce peak and annual energy consumption, providing a reduction in energy costs and potentially also operational carbon emissions.

Alternatives

The effect of load reduction strategies described here fit into four categories: external heat gain or loss, internal heat gain, load shifting (or heat storage) and thermal comfort. BEM is useful for evaluating the impact of each of these strategies as well as their interactions.

External heat gain or loss measures

See the following topics for information about strategies to reduce external heat gain and heat loss.

Building massing and orientation

• Compare massing options
• Analyze the building's orientation

Programming layout

• Analysis of how HVAC loads differ for different programming layouts

Fenestration design: Area, orientation, glazing performance, shading.

• Fenestration and daylighting options
• Window-to-wall ratio parametric studies
• Analysis of thermal performance of different glazing performance specifications
• Shading feature design analysis

Opaque envelope design: Insulation, thermal mass, surface properties.

• Constructions and thermal mass options  
• Analysis of thermal performance of different wall construction materials

Infiltration reduction

• Infiltration

Internal heat gain measures

For a good overview of the sources of internal heat gain, including lighting, equipment and occupants, see Basics of internal gains.

Lighting: installed lighting power, lighting controls

• Test strategies to reduce lighting power density
• Analyze the impact of lighting control strategies

Equipment: Energystar equipment and appliances, plug load controls.

Additional information about internal heat gain sources can be found on these pages

• Sources for default internal gain assumptions
• Define internal loads (occupants, lighting, equipment)

Load shifting measures

Some design strategies reduce peak cooling loads through heat storage, creating a delay between the moment of a heat gain and corresponding air temperature rise. When heat from either external or internal sources enters a space, a portion of the heat enters the air directly and raises its temperature while the other portion is absorbed by surfaces within the space in the form of longwave infrared radiation. The portion that is absorbed raises the temperature of those objects, which then heat the air with some time delay. In many cases this delay also reduces the magnitude of the total peak load.

A common space-load shifting strategy is the use of interior constructions with high thermal mass, such as concrete, that is exposed to the space. Another strategy is to use phase-change materials, such as wallboard that incorporates material that absorbs or releases heat as it changes phase.

Thermal comfort measures

Design strategies that expand the range of indoor air temperature while also maintaining occupant thermal comfort can provide peak load savings.  

• Ceiling fans to provide air movement
• Radiant heating and cooling systems
• Highly insulated windows, to improve mean radiant temperature
• Programming layouts that locate occupants away from exterior surfaces. See Analysis of energy and comfort impacts of different programming layouts

Guidance on modeling approach

A load reduction model should aim for a reasonably accurate representation of both building envelope systems and internal heat gains, while also ensuring that inputs related to HVAC system sizing are appropriate. In order to provide timely design feedback, the modeler will need to use judgment regarding appropriate simplifications and assumptions.

Analysis approach

• Parametric analysis. Consider a parametric analysis (or sensitivity analysis) approach that explores both individual strategies and combinations of strategies to evaluate interactive effects.
• Extreme cases. Consider including alternatives that represent maximum technical potential, or with extreme high and low performance, to help understand the boundaries of the impact.  

Envelope modeling

• Opaque constructions. Use detailed constructions, account for thermal bridging, and pay attention to solar reflectance inputs as described here: Analysis of thermal performance of different wall construction materials
• Fenestration. Use an appropriate window specification method as described here: Analysis of thermal performance of different glazing performance specifications.

Internal heat gain modeling

• Daylighting. Consider including automatic daylighting control, otherwise the results will not capture the energy benefit of daylight to offset electric lighting.
• Schedules. Ensure that schedules for lighting, plug loads and occupants are reasonable.

HVAC inputs

• Thermostat setpoints. Check that cooling and heating thermostat setpoints are reasonable.
• Warm up/cool down controls. Modeled peak heating loads, and sometimes also cooling loads, may occur at the first scheduled hour of operation as the space temperature setpoints step from their unoccupied to occupied values. Consider schedules that change temperature setpoints over more than one simulation timestep.
• HVAC system type. Choose a reasonable system type so that energy impacts of alternatives are appropriate. See Selecting appropriate HVAC systems.
• HVAC sizing. Pay attention to inputs for indoor design conditions that are used by the BEM tool in its autosizing calculations.
• Ventilation. Check that the outdoor air ventilation rate is reasonable because it can have a significant impact on system heating and cooling loads, especially in extreme climates. See Ventilation rates - calculations and inputs for mechanical ventilation.

Weather data

• Design day. Most BEM tools allow input of design-day data for use in load calculations. Check that the inputs are appropriate for the building location.
• Future weather. Consider running the analysis with future weather data in addition to historical weather data to check for potential peak load impacts of changing climate. See weather data selection.

Model quality review

• Review outputs: Review and analysis to verify model quality
• Compare to benchmarks: Compare inputs and outputs to common "rules of thumb"

Guidance on presenting results

Considerations:

• Present the breakdown of peak cooling loads by source.
• Plot hourly profiles of peak heating and peak cooling load days
• Consider using hourly results to present the frequency of loads within different ranges, as shown in the example below

Example BEM results showing breakdown of source of peak cooling load

Example results showing cooling load frequency, plotting the hours per year when cooling load falls into each cooling load range