Compare the performance of different HVAC system options
Fundamental HVAC system design decisions are often finalized during schematic design, including the selection of the HVAC system type and the location of major components. BEM can aid selection decisions with vital information about the comparative energy performance of various HVAC system alternatives.
In real life, the performance of HVAC systems fluctuates in response to varying operating conditions, particularly changes in outdoor temperature and heating or cooling loads. The diverse responses of different systems to these dynamic conditions create a challenge in determining optimal performance based solely on their rated efficiencies or full-load efficiency. Simulation models address this challenge by accounting for variations in system capacity and efficiency under changing conditions. They furnish estimates of energy consumption and demand by factoring in project-specific loads and weather conditions. BEM analysis often shows that the most suitable HVAC system selection varies from one project to another based on their unique contextual considerations.
Impact of HVAC System Selection
There are several important impacts that can be evaluated when using BEM to compare HVAC system selection alternatives:
- Energy consumption
- Peak energy demand
- Energy cost
- Carbon emissions
- Water consumption
- Load shifting capability
HVAC system selection has, of course, many additional impacts that might influence a decision. These are some additional impacts that are not directly evaluated with BEM:
- Architectural impacts, as outlined here: HVAC system options
- Personal control capability for thermal comfort
- Indoor air quality impact
- Acoustical impact
- Embodied carbon of HVAC system components
- Construction cost
- Maintenance requirements
- Reliability
- Longevity
- Compatibility with natural ventilation
- Flexibility for future occupancy changes
- Refrigerant global warming potential (GWP) and ozone depletion potential (ODP)
HVAC Selection Alternatives
At the schematic design phase, HVAC system alternatives may be provided by the design team, or it may be the modeler’s role to make appropriate suggestions. Several BEMcyclopedia pages provide guidance on understanding HVAC system options.
- HVAC system options. Describes fundamental features that distinguish HVAC system types. Identifies important early-design decisions such as the method of delivering heating and cooling to the zones and the type of heating and cooling equipment.
- Selecting appropriate HVAC systems. Lists common system types to consider for early design modeling
- HVAC system types. Provides detailed descriptions of several common HVAC systems.
- Fundamentals of HVAC. Describes basic alternatives for ventilation method, cooling source and heating source.
Energy efficiency options for HVAC systems should also be considered during schematic design. While some options can be evaluated during later design phases, many efficiency measures are best considered early due to potential impact on architectural design or the design of other systems. The following are some examples of features that can affect architectural features or the space required for HVAC components.
- Low-pressure-loss air distribution
- Energy recovery
- Thermal energy storage
- Mixed mode ventilation
- Displacement ventilation
- Ground heat exchange
For more efficiency options, see also the list of energy conservation measures here: energy conservation measures.
Guidance on modeling approach
When evaluating HVAC system options, the modeler is faced with the challenge of making a fair and accurate comparison when limited information is available. One complicating factor is that each system type can likely be specified with varying efficiency levels and with a range of optional features and controls. This section offers three considerations regarding modeling approach.
- Develop the loads model to an appropriate level of detail
- Identify reasonable and fair inputs for each HVAC system option
- Review model quality
See also conceptual design guidance on modeling approach.
Develop loads model to appropriate level of detail
At the schematic design phase, the appropriate level of detail in the model will vary depending on the state of the design. Preliminary floorplans and elevations may be available. These will provide information about space layout and occupancy type as well as window area and orientation. Other information may not be available, such as envelope construction details, lighting design, thermal zoning and operating schedules.
Determining the appropriate inputs and level of detail in the model requires some judgment. When comparing HVAC options during schematic design, the following are reasonable goals.
- Thermal zoning reasonably represents actual diversity
- Reasonably accurate window area and orientation
- Envelope component performance about right
- Internal gains about right
- Operation schedules about right
For guidance on identifying appropriate inputs, see also preparing model inputs.
Identify reasonable and fair inputs for each HVAC system option
At this point, a schematic design may be available for one or more HVAC options, but it is also possible that no HVAC design yet exists. In either case, the challenge is to develop models for each HVAC option that provide a fair and reasonable comparison. The general goal is to identify inputs that represent good design practice for each type, avoiding a comparison of the best case for one option to a poor example of another option. Consider the following approach.
- Understand how the systems work. See HVAC system types for information about some common system types.
- Understand how the models work. Review BEM software documentation, especially with respect to HVAC control inputs and part-load efficiency modeling methods.
- Gather typical performance data. For guidance, see preparing model inputs.
- Perform sensitivity runs and test uncertain inputs. Gain an understanding of which inputs have a significant impact on results.
Review model quality
General model quality review recommendations are provided here: review and analysis to verify model quality.
For an HVAC system comparison, some additional model reviews will help provide confidence in results. HVAC models consist of many inputs, and an examination of hourly or time-step outputs is very useful to verify that models are reasonable and behave as expected. The following are a few examples.
- System cooling and heating loads. Check that the pattern of loads is reasonable, checking for unexpected extreme values or for loads occurring at unexpected times, such as heating load in summer or cooling in winter.
- System control. Check that flows and temperatures are controlled as expected. For airside systems, these checks might include variables such as zone air temperature (for a sample of zones), supply air flow, supply air temperature, or outdoor air flow. For waterside systems, check supply and return temperatures and water flow.
- System efficiency. Check hourly cooling and heating efficiency by calculating efficiencies based on hourly coil loads and electricity or fuel demand.
Guidance on presenting results
BEM results that are typically useful are described in this page: analyzing model outputs.
Be sure to include performance metrics that are important to the owner and the design team. Keep in mind that the comparative ranking of HVAC system types may vary depending on the performance metrics that are considered. For example, the system providing the lowest annual energy cost might not be the system that provides the lowest CO2 emissions.
Consider highlighting differences in the impact on architectural design, if any, between the HVAC system options.
In some cases, the type and quantity of refrigerants will vary among options. Information regarding potential global warming or ozone depletion impact may be an important decision factor.
Relevant ASHRAE Standard 209 cycles
ASHRAE Standard 209 modeling cycle #4
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