HVAC system options - conceptual design

Four options for delivering heating, cooling and ventilation to a space. Each has different architectural impacts.

Most often in conceptual design BEM studies, identical HVAC systems are used in all models, while the impact of options for other design features such as building massing or fenestration is evaluated. But architectural decisions made during conceptual design can affect the feasibility of some HVAC system types, which means that those architectural decisions can indirectly affect the potential for HVAC system performance. Therefore, it can be a good idea to use BEM during conceptual design as part of an integrated design exercise to evaluate the relative performance of HVAC system options and to use the results to inform architectural design decisions.

Here are a few scenarios when BEM evaluation of HVAC system options can be helpful during conceptual design:

• The project has aggressive energy or emissions performance targets, and it is important to gain an early understanding of the energy implications of HVAC system selection.
• The owner wants to understand the energy and emissions implications of all-electric systems compared to systems that directly consume fossil fuels.
• The architect wants to avoid making decisions that preclude high performing HVAC system options.

Impact of HVAC System Options

At the conceptual design phase, probably the two most important issues related to HVAC system options are the architectural impacts and energy performance impact.

There are many potential architectural impacts:

• Roof space required for equipment
• Mechanical room size
• Air shaft space
• Space above ceiling for ducts and/or VAV boxes
• Ceiling height and floor-to-floor height
• Space for air diffusers: ceiling, wall or floor
• Raised access floor for air distribution
• Exposed ceilings and/or floors for radiant systems
• Outdoor space for cooling towers
• Louvers for outdoor air and exhaust air
• Space for thermal storage tanks

Energy consumption and peak energy demand can vary significantly between different HVAC system types serving identical buildings.

• Systems may vary in terms of their efficiency in generating heating and cooling. At a simple level, many types of equipment are available in both standard and high efficiency versions. Perhaps more importantly, systems may differ in how they respond to varying demand for heating and cooling; they have different part-load efficiency characteristics. For example, some air conditioners use constant-speed compressors that cycle on and off at partial load, while other air conditioners use variable-speed compressors that are more efficient under partial load conditions.  
• Systems also differ in the energy consumed for distributing conditioned air or water through the building. Variable-flow systems are typically more efficient than constant-flow systems. And hydronic systems (water-based systems) typically use less energy for pumps than air-based systems use for fans.  
• A difference that becomes more important as there is more demand for grid-interactive building control is that some system types can shift energy consumption from one time of day to another. Thermal storage systems are one example. These systems might use hot water or chilled water storage tanks to store heating and cooling at one time of day and supply it to the building at another time of day. Other systems produce ice for thermal storage.

HVAC System Alternatives

As noted above, a BEM study performed during conceptual design often uses the same HVAC system in each model in order to provide a consistent basis for comparing performance of other design features. In that case, the topic selecting appropriate HVAC systems is a place to find guidance on reasonable HVAC system choices.

In other situations, a goal of the BEM analysis may be to compare the performance of different HVAC systems. The architect and mechanical engineer may have some specific systems in mind, or it may be up to the energy modeler to suggest options. In either case, it is important to define the characteristics of each system option appropriately to achieve a fair and accurate comparison. For more on that topic, see guidance on modeling approach in the next section.

There are many potential HVAC system options to evaluate in a BEM study, and appropriate choices will depend on many factors such as occupancy type, building size, number of stories and climate. One approach that may be appropriate during conceptual design is to identify and compare performance of HVAC system types that can have a significant impact on architectural design. Here are some features to consider when selecting HVAC system alternatives for evaluation:

Method of delivering heating and cooling to the zones:

• Overhead air (space above ceiling)
• Underfloor air (raised floor)
• Displacement ventilation (wall diffusers)
• Radiant ceiling and/or floor (tubes in slab or radiant panels)

Cooling type:

• Chilled water (space for chillers)
• Packaged DX (typically rooftop, with air shaft)
• Multi-split DX (rooftop condensers and indoor fan coils)
• Water-source heat pumps (indoor water-to-air heat pumps connected to condenser water loop)

Heat rejection type:

• Air-cooled (outdoor condensers)
• Water-cooled (outdoor cooling tower)
• Ground-coupled (boreholes or other types of ground heat exchangers)

Heating type:

• Hot water (gas boiler, air-to-water heat pump)
• Air (gas furnace, air-to-air heat pump)

Thermal storage:

• Chilled water
• Ice
• Hot water

Other HVAC system type considerations:

• Some systems, such as radiant systems, have an upper limit on the amount of heating or cooling they can deliver, and conceptual design analysis can explore building envelope and internal load measures that would be necessary for those HVAC systems to be feasible
• Improvements to building envelope constructions might reduce or eliminate the need for separate systems to provide heating or cooling to perimeter zones, and BEM analysis can be used to explore this opportunity.
• Fan energy depends on air pressure loss in the air distribution system. Pressure loss and the corresponding fan energy can be minimized by increasing the size of ducts and air handlers and reducing the length of ducts. BEM can be used to evaluate the energy savings potential of measures to reduce air pressure loss, and that information can be used to inform architectural design and potentially allow extra space for air distribution.
• For buildings with spaces that have different operating schedules or different space conditioning requirements, BEM can be used during conceptual design to explore the benefit of grouping those space uses and serving them with separate HVAC systems.
• Free-cooling opportunities vary among system types; some accommodate use of an airside economizer, others require a waterside economizer or have limited free cooling capability. BEM can be used to evaluate the impact of free-cooling options.
• Heat recovery opportunities also vary depending on system type and configuration. The impact will vary based on outdoor air ventilation requirements and climate, and BEM can be used during conceptual design to evaluate the importance of heat recovery.
• For projects considering 100 percent outdoor air systems for the purpose of indoor air quality, BEM can be used to estimate the energy impact of that approach.

Guidance on Modeling Approach

Examples of energy model thermal zoning that may be appropriate for conceptual design models

For a conceptual design study, simplified models can provide useful information about the relative performance of different types of HVAC systems, and a simple box model approach is often appropriate.  To help ensure that the results are valid, 1) make appropriate simplifications to the building geometry and thermal zoning, and 2) choose model inputs to represent good design practice for each HVAC system option and do not rely solely on default inputs. In both cases some judgment is required, but the goal is to develop models with reasonably accurate heating and cooling loads and with HVAC models that reasonably represent the performance of each system type. An analysis that compares, for example, a poorly controlled overhead air delivery system to an optimized radiant system will not provide useful information.

For guidance on appropriate simplifications to the model, see the sections on thermal zoning and thermal zoning simplification in the page defining the building geometry. A goal for early-stage BEM is to develop models that are as simple as possible but not too simple.  

For guidance on HVAC model inputs see airside HVAC assumptions for early-stage models and waterside HVAC assumptions for early-stage models. Those pages highlight inputs that can have a significant impact on results and deserve attention, such as supply air temperature control method and part-load performance of cooling and heating equipment.

Additional considerations:

• Use of software autosizing features for HVAC components is appropriate in early-stage models, but it is a good idea to check the sizing result to make sure they are reasonable. Additionally, some building and space types such as healthcare and laboratories may have HVAC design dictated by air-change rates that exceed the airflow needed to meet the space loads so, in these cases, autosized values may not be appropriate.
• Simplifications of HVAC system configurations are often appropriate. For example, modeling several multiple-zone air handlers that serve similar types of spaces as a single larger air handler reduces complexity and saves modeling time. Some judgment is required, because such simplification may not be appropriate for all system types.
• Appropriate operating schedules are especially important if a goal of the BEM analysis is to predict likely energy consumption.
• Changing HVAC systems types in energy modeling software is often a more complex process than varying other model inputs such as envelope construction types or lighting power, so allow adequate time.

Guidance on Presenting Results

BEM results that are typically found to be useful are described in this page: Analyzing Model Outputs. For comparison of HVAC system types, in addition to results for energy consumption, energy demand and carbon emissions, consider also presenting autosized capacity for equipment, such as chillers, boilers, and fans. The size of equipment affects both construction cost and space requirements.

Here are some other important considerations for presenting results:

• Provide answers and insights
• Present results visually
• Provide an input summary