Load Calculation Models

Mechanical engineers, modelers, and architects may use software for both:

• Computing loads and sizing components
• Annual energy estimates.

The software used for each task can be completely different, exactly the same, or product variations. Most mechanical engineers use software to size:

• Terminal airflow
• Outside airflow
• Fans
• Coils
• DX equipment
• Furnaces
• Chillers
• Pumps
• Cooling towers, etc.

These sizes are based on computed loads for each zone, which are based on sensible and latent heat gains or losses from:  

• Above-grade surfaces such as walls and roofs
• Below-grade surfaces such as foundations and underground walls
• Windows and skylights
• People, lights, plug loads, and equipment
• Infiltration and outside air ventilation

Many heat gains and losses have radiant or delayed components before turning into system loads. Estimating these delays is the primary reason for many load sizing and modeling algorithms.

Many of the same algorithms used to determine the loads and the required sizing for HVAC components can also be used to estimate their annual energy consumption. Since many of the same algorithms are used for both, it is not surprising that the inputs needed are generally very similar. This is also why several software applications either perform both or variations of the software are marketed toward each need.

For modelers, the distinction between determining peak loads for component sizing and providing annual energy estimates can be even more subtle. Almost all annual building energy simulation software includes the capability to size components automatically as part of the simulation process. Often called “autosizing,” this capability is especially useful in early design studies, any automatically generated alternative or baseline simulation cases for parametric studies, or determining the building's overall performance.

Load Calculation and Energy Estimating Algorithms    

ASHRAE recommends a heat balance-based algorithm for load sizing and energy estimates, but many people still rely on older algorithms. The heat balance method, described in Chapter 18 of the ASHRAE Handbook of Fundamentals (2021—revised) and in the ASHRAE Load Calculation Manual, uses an iterative approach to determining the heat balance for each building surface, including all the heat flows into and out of the surface.

Several algorithms are used for both load calculations and for energy estimates. Annual simulation software that utilize these algorithms would be especially useful when also used for load calculations.

Abbreviation	Algorithm	Load Calculation	Energy Estimate	Basis	Modern
HB	Heat Balance	X	X		X
RHB	Residential Heat Balance	X		HB	X
CLTD/CLF	Cooling load temperature difference/cooling factor	X	X
RLF	Residential Load Factor	X		RHB	X
RTS	Radiant Time Series	X	X	HB	X
TETD/TA	Total equivalent temperature difference/time averaging	X
WF	Weighting Factor		X

Basis - indicates the abbreviation for the algorithm that was used as the foundation

Modern - indicates an algorithm that is encouraged to be used today

These are documented in current and previous versions of ASHRAE Handbook of Fundamentals Chapters 17, 18, 19. Many other algorithms have also been developed for these purposes and this table is just a sampling of some more common ones.

These algorithms are mostly related to converting space heat gains and losses into heating and cooling loads on an HVAC system. Space heat gains and losses come in both sensible and latent. Sensible heat gains are added to the space based on conduction, convection or radiation. Latent heat gains are related to the addition of moisture to the space. For sensible heat gains, the radiant portion is first absorbed into a space's walls, ceiling, floor, furniture, and other objects. Then, as those surfaces and objects heat up, the air in the space is warmed via convection, but this process takes time. In addition, heat flowing through surfaces also takes time, especially if the surfaces contain significant mass. These time delays between when heat gains are added and when they become present in the air of the space as loads are the main reason for these algorithms. The space cooling and heating loads are the rate at which sensible and latent heat must be removed or added to the space to maintain a constant temperature and humidity.

Load calculation for displacement ventilation systems is a special case. Standard load calculation methods assume that air in the room is fully mixed and the temperature is the same from the floor to the ceiling. In a room with displacement ventilation, temperature can become stratified and the result is that a portion of sensible heat gain ends up directly in the return air rather than in the occupied portion of the space. The ASHRAE 2023 Handbook Applications covers design procedures for fully stratified air distribution in Chapter 58.

Simulation for Sizing vs. Annual   

Almost all building energy modeling software can automatically perform zone, system, and plant equipment sizing calculations. Often, the modeler has the choice:

• manually set equipment sizes
• automatically sized equipment based on climatic design conditions
• automatically sized equipment based on a weather file

And some software may offer additional options by blending or extending these choices.

Design Conditions

The choice of the climatic design conditions and the weather file also is left to the user. ASHRAE Handbook of Fundamental Chapter 14 on Climatic Design Conditions provides information for over 9000 locations primarily focused on the United States and Canada. An example of the information provided is shown for Atlanta at this link.

This data is often available as a library of data for simulation software, and the user has options, including:

• Heating conditions for 99.6% annual cumulative frequency of occurrence
• Heating conditions for 99.0% annual cumulative frequency of occurrence
• Cooling conditions for 2.0% annual cumulative frequency of occurrence
• Cooling conditions for 1.0% annual cumulative frequency of occurrence
• Cooling conditions for 0.4% annual cumulative frequency of occurrence

The annual cumulative frequency of occurrence indicates what fraction of the time that temperature has historically occurred. This can determine the likelihood of designing to those conditions being exceeded. For example, in Atlanta, the cooling dry-bulb temperatures are:

• 93.7 F at 0.4% annual cumulative frequency of occurrence
• 91.6 F at 1% annual cumulative frequency of occurrence
• 89.7 F at 2% annual cumulative frequency of occurrence

There is a direct relationship between the capacity for equipment, and thus the cost of the equipment, and the temperature used for sizing that equipment. Therefore, the more the occupants of the building can tolerate the slightly warmer temperatures inside on the hottest days of the year, the lower the cooling capacity needed. Significant equipment cost savings can be made using the 2% design condition rather than the 0.4% design condition. Discussing which design conditions to use is essential to have with the architect and building owner. None of these design conditions corresponds to Atlanta's 105.7 F 50-year extreme temperature. Designing a building based on that temperature would never make sense since it would only occur for one day every 50 years.

The ASHRAE design conditions are based on historical data and given that the world is undergoing climate changes, designing based on future weather conditions may be desired by some architects or building owners. These future design conditions are typically based on estimates of climate change and how peak conditions are generally related to average conditions.

For each of these design conditions annual cumulative frequency of occurrence, the table shows:

• Heating dry-bulb temperature
• Heating humidification dew point and mean coincident dry-bulb temperature
• Heating wind speed and mean coincident dry-bulb temperature
• Cooling dry-bulb temperature and mean coincident wet-bulb temperature
• Cooling wet-bulb conditions with mean coincident dry-bulb temperature
• Cooling wind speed and mean coincident dry-bulb temperature
• Cooling dew-point and humidity ratio and mean coincident dry-bulb temperature
• Cooling enthalpy and mean coincident dry-bulb temperature

Most often, the dry-bulb temperature is used as a design parameter, but depending on the location and building type, some of these other design conditions may also be considered. All of these options can be selected for simulation software that allows multiple conditions, and the one that sets the maximum equipment sizing will be used. Monthly design conditions are also included in this example design condition data for Atlanta. Because sun angles significantly affect the heating and cooling loads on buildings, it has become increasingly common also to use these monthly data. While uncommon, some buildings may have peak cooling loads at lower outdoor temperatures in the fall due to greater solar loads from the lower sun angles during those months. Individual zones with windows may have peak cooling load at lower outdoor temperatures, which affects terminal unit airflow sizing even if it may not affect overall air handler or chiller sizing.

Factors Impacting Sizing

Buildings or zones with very high ventilation rates, such as laboratories, hospitals, and kitchens, may find that the peak cooling load is driven by the hot, moist air introduced into the building. The cooling wet-bulb temperature or cooling dew-point data might set the building's peak load for these types of buildings. Also, the sizing of the zone airflow for some of these space types may be driven by minimum air change requirements rather than by thermal loads.

Using additional safety factors to further oversize equipment is generally not necessary if the sizing is properly performed. Unfortunately, properly sizing equipment involves engineering judgment when making assumptions about the building on topics that may be difficult to determine, such as actual building internal loads and infiltration.

For annual energy estimates, it is important to include thermostat setpoint changes, which primarily occur between occupied and unoccupied building operations. Having wider thermostat setpoints during unoccupied hours almost always saves energy and is common practice in nearly all types of buildings. For most modeling software, the transition between unoccupied to occupied setpoints will prompt large pickup loads to be simulated as the HVAC system tries to regain control. When overnight setpoints transition to occupied setpoints, the pickup loads should not be used for sizing purposes. In the actual building, setpoint transitions can be optimized or staged to minimize the impact of these. For models used for load calculations, using a constant occupied setpoint often makes sense even during unoccupied periods. However, care should be taken not to use those schedules for annual energy estimates.

One option annual simulation software can have is to size equipment based on a weather file. Because most weather files intend to represent typical weather, sizing equipment only using this type of weather file may result in undersized equipment, with a significant number of days each year when the equipment fails to meet temperature setpoints in the spaces during the actual operation of the building. Some weather files may actually include peak temperatures that exceed the design conditions, leading to oversizing. The differences are due to the data selection for a weather file being “typical” does not match the selection process for data used in establishing the design condition. Some extreme weather files have been developed for specific locations that are more representative of design conditions.

Timing of Loads and Sizing

For most buildings, not all thermal zones are likely to have peak loads occurring simultaneously. Instead, the timing of peak loads often depends on the facade of a building that the zone is near and especially the size and orientation of windows in the zone. For buildings that use individual packaged DX systems serving each zone, the timing differences between the peak loads do not matter. Each packaged system needs to be sized to match the peak for each zone, including the sensible and latent cooling and heating capacities and the fan size.

For buildings using centralized systems that serve multiple zones, the timing of peak for each zone can impact the sizing of central components such as fans, pumps, chillers, boilers, cooling towers, and heat recovery systems. Instead of simply adding up the peak loads for each zone, the coincident peak of those loads is the largest sum of those loads at a specific time. This is often significantly smaller than the sum of the individual zone peak loads occurring at different times. These coincident loads are sometimes called block loads since they represent the peak load for a block of zones. The sizing of zone level components such as nominal and minimum airflow rates and zone-level heating and cooling coils or systems should still use the peak loads for that zone.

Many simulation software applications allow different assumptions to be used when performing heating and cooling sizing calculations such as schedule multipliers and nominal internal gains for plug loads and occupancy. This allows the modeler to consider cases in which the peak heating or cooling loads may occur with different numbers of occupants in a building or plug loads. The owner may specify the internal gains and other conditions for sizing equipment that are different than during typical operation. Care must be taken since the point of equipment sizing is to estimate the heating or cooling capacities that can maintain a specific set point for occupant comfort. For example, assuming no occupants are present in a zone when the peak heating load is computed is inconsistent with the need to maintain comfort since, without people, the temperature probably doesn’t matter (except perhaps for freeze protection). On the other hand, a big box retail store might have a sale on one of the hottest days of the year, which means its occupancy level could be much higher on a cooling design day.

Sizing During Design Process

Load calculations may be performed multiple times during the design process. During schematic design, when only the broad parameters of the building are fully understood, the load calculations will help the design team understand the rough size of equipment needed and help in selecting the type of system that might be considered for the design. Often, these sizes will be used to understand the cost impacts for the overall design as well as for different system configurations. Later, during design development, much more is understood about the details of the building, and load calculations are used to select the sizes of all the HVAC components. It is essential to know that the level of detail needed for the rough sizing during schematic design differs from the level of detail necessary for selecting actual equipment sizes during design development.

It would be surprising if the equipment sizing for a building performed by two software applications ever matched, such as between dedicated sizing software and an annual simulation program. Even assuming that the algorithms are the same or closely related (such as heat balance and radiant time series), many assumptions may be different. As a modeler, it is good to note the differences and try to understand the pattern. If the simulation software is regularly 5% smaller on all cooling coils compared to the sizing results from dedicated sizing software, then the differences in assumptions are probably the cause. Changes to this pattern may indicate assumptions that should be updated by either the modeler or design engineer. For example, if all cooling coils are 5% smaller than the annual model, then one cooling coil is 10% higher, which may indicate different assumptions for the zone associated with that cooling coil. It might be worth discussing with the rest of the design team to make sure that consistent assumptions are being used. Another case worth discussing with the design team is when the annual simulation loads reach the capacity of the equipment, especially for cooling equipment. This may indicate equipment that is undersized, and it is worth alerting the design team and being prepared to discuss the outdoor conditions when those equipment loads occur.

Output Reporting

Typical outputs from software include:

• Peak loads
• Breakdown of peak load contributions
• Equipment sizing results

The peak loads include the results of the loads for each thermal zone as well as the simultaneous peak loads of groups of zones that are served by a common air handler, by centralized plant, or for the entire building. These loads include the immediate gains from occupants, plug loads, and convection gains, plus the delayed impacts of the radiant components of internal gains and solar loads and gains through the building envelope. These loads are often expressed as loads per area or per person and often include the airflow and outdoor airflow that occur at the same time. The conditions of the zone, including the temperature and humidity ratio when the peak load is experienced, can help provide an understanding of how the equipment is sized for the peak time.

The components of the peak heating and peak cooling loads for each thermal zone as well as during simultaneous peaks of multiple zones can be a good indicator for the modeler of how different aspects of the model are impacting the sizing. These components often include heat from:

• Occupants
• Lighting and equipment
• Infiltration
• Exterior walls, roofs, and other surfaces
• Underground floors and walls
• Fenestration conduction and solar
• Interior partitions, etc.

They are usually expressed as sensible and latent components and may even indicate the portion of each that is instantaneous versus delayed.

Finally, equipment sizing information is provided by most software that includes:

• Airflow and minimum airflow fraction for air terminal unit
• Heat and cooling capacity of terminals or equipment serving the zone
• Baseboard capacity
• Air handling unit air flow and heating and cooling capacity
• Water flow rates for heating and cooling coils
• Fan air flow volume and minimum turn down
• Water loop capacity and pump flow rates
• Chiller, boiler, cooling tower and other central equipment capacities and flow rates

These capacities and flow rates are often reported when sized automatically by the software and when manually sized and entered by the modeler.

References

Chapter 14 of the ASHRAE Handbook of Fundamental “Climatic Design Conditions” (2021 - revised)

Chapter 17 of the ASHRAE Handbook of Fundamentals “Residential Cooling and Heating Load Calculations” (2021 - revised)

Chapter 18 of the ASHRAE Handbook of Fundamentals “Nonresidential Cooling and Heating Load Calculations”(2021 - revised)

Chapter 19 of the ASHRAE Handbook of Fundamentals “Energy Estimating and Modeling Methods” (2021 - revised)

Load Calculation Applications Manual. Jeff Spitler. ASHRAE. 2014.

ASHRAE Toolkit for Load Calculation - Pedersen, C.O., R.J Liesen, R.K. Strand, D.E. Fisher, L. Dong, P.G. Ellis, 2001.