Airside HVAC assumptions for early-stage models

For any HVAC system type, there are many inputs needed to describe system performance. At the time that a simple box model is created, most of those details will not have been determined, so many assumptions are necessary. As with other types of inputs, there are default values for airside HVAC performance available in many simulation tools. Other useful sources of information are energy codes and design guidelines. This section highlights some of the more important inputs that deserve attention.

Heating and cooling efficiency

Energy codes include tables of minimum efficiency requirements for packaged equipment, chillers, and boilers, which often vary based on the cooling and heating capacity. Energy efficiency design guidelines can provide suggestions for better-than-code efficiency. ASHRAE Standard 189.1 is one source that includes tables of equipment efficiencies that exceed the minimum requirements of Standard 90.1. These references can provide reasonable efficiency values to use in a simple box model to represent high-efficiency alternatives. As an example, the table below lists packaged air conditioner and heat pump efficiency requirements from Standard 90.1 and Standard 189.1.

Select Equipment Efficiencies from ASHRAE Standards 90.1-2019 and 189.1-2020

Type	Cooling Capacity	90.1-2019	189.1-2020
Air-cooled air conditioner cooling efficiency	< 65k Btu/hr	13.0 SEER	15.0 SEER
	65k - 135k Btu/hr	11.0 EER	12.0 EER
	135k - 240k Btu/hr	10.8 EER	12.0 EER
	240k - 760k Btu/hr	9.8 EER	10.6 EER
	> 760k Btu/hr	9.5 EER	10.2 EER
Air-source heat pump heating efficiency	< 65k Btu/hr	8.0 HSPF	9.0 HSPF
	65k - 135k Btu/hr	3.30 COP*	3.40 COP*
	135k - 240k Btu/hr	3.20 COP*	3.20 COP*
	>240k Btu/hr	3.20 COP*	3.20 COP*
* Heating COP at 47F outdoor drybulb

Cooling and heating equipment part-load performance

HVAC systems usually operate at less than full load for most hours of the year. The efficiency of some system types degrades at low load. Other system types can improve performance at moderate or low load. Therefore, a reasonable model of part-load performance will ensure that results are useful. Most simulation tools include default data for their models used to represent system performance for different types of equipment, and it is important to understand how those inputs affect performance. If the simple box model is being used to evaluate different options for HVAC system selection, then it is especially important to verify that those model inputs are a reasonable representation of the proposed equipment. Performance curve data for many different types of cooling and heating equipment are available for some simulation programs from COMNET and California’s Nonresidential Alternative Compliance Method documentation.

Fan power

Fan pressure requirements vary by HVAC system type and building size, so make sure that your inputs are reasonable for your building. ASHRAE Standard 90.1 sets a fan power limit, which is different for variable air volume and constant air volume systems (Section 6.5.3.1.1). It also sets a limit for fan coil systems. These values are reasonable choices for a simple box model. They will likely be conservative for smaller buildings with shorter duct runs and more accurate for larger buildings.

Baseline Fan Power, ASHRAE Standard 90.1-2019 Appendix G

Fan system type	Fan power (bhp)	Electric input power (W)
Constant volume	cfm * 0.00094	bhp * 746 / fan motor eff.
Variable air volume	cfm * 0.0013	bhp * 746 / fan motor eff.
Fan coil		cfm * 0.3

Airflow control for single-zone systems

Single zone systems, such as packaged single zone systems, are traditionally designed for constant airflow, but energy codes now require variable or staged airflow in many cases. Variable airflow control reduces fan energy consumption. For these system types, pay attention to how airflow is being modeled.

Airflow control for multiple-zone VAV systems

One of the benefits of a VAV system is that fan power drops as airflow drops. In a real system, the relationship between airflow and fan power depends on the physical configuration of the duct system and the method used to control fan speed. In some cases, the default inputs in simulation tools are optimistic and they estimate lower fan power at low flow than is realistic. When comparing performance of different types of HVAC systems, try to include realistic inputs for fan power as a function of airflow.

Economizer cooling

Some form of free cooling is usually required by energy codes in all but hot, humid climates. Where required, an airside economizer should be included in the HVAC system for a simple box model, or the modeled HVAC system should provide some other form of free cooling.

Supply air temperature control

For systems delivering air to multiple zones, such as a VAV reheat system or a DOAS, the control system sets a supply air temperature setpoint, which may vary hour by hour. For a VAV system, the choice of supply air temperature affects the cooling, reheat and fan energy demand at any point in time. Lower temperature means that less airflow is needed, but it may also require more cooling and reheat energy. Therefore, the choice of control algorithm can have a significant energy impact. If, for example, the supply air temperature in a VAV system is modeled as being constant 55F, then there are likely many hours in the mild seasons when there is significant reheat energy. More commonly, a modern control system would be adjusting the supply air temperature setpoint based on zone demand. Be sure to pay attention to the supply air temperature control method specified in your model.

Outdoor air ventilation rate

Outdoor air ventilation rate is discussed in the previous section Zone HVAC. For systems that serve multiple zones, the individual zone outdoor air rates combine (using formulas such as in ASHRAE Standard 62.1) to determine the system total and the ratio of outdoor air to recirculated air.

Heat recovery

In cold climates and hot and humid climates, heat recovery is often an effective energy efficiency strategy and will reduce heating and cooling energy. In some cases, heat recovery is required by energy codes. Consider including heat recovery for the HVAC system in a simple box model in those cases.

Operating schedule

One model input that has perhaps the biggest energy impact is the building operating schedule. HVAC operating schedules are part of a set of building operating schedules, which also include occupancy, lighting and plug load schedules. For a simple box model, it may be appropriate to use standard operating schedules based on building type, which may be offered as a default selection in simulation software or may be found in many references (see also: Sources for default internal gain and operational assumptions). If the simple box model is being used to estimate absolute energy performance, then consult with the design team to define the operating schedules to represent a best guess at anticipated building operation.

System sizing

Inputs for cooling and heating capacity and supply air flow have an impact on energy results, especially for HVAC systems with efficiency that varies as load varies. Actual sizing values will seldom be known at the time of a simple box model exercise, but most simulation programs have the ability to autosize HVAC equipment based on one or more methods. A common method is to use inputs for cooling and heating design days to perform design-day calculations before running an annual simulation. The program may also allow inputs for sizing ratios, which add a margin of extra capacity to the peak load results. Regardless of the method used for sizing, it is important to check model results to make sure that the systems are not either undersized or grossly oversized. If undersized, then the system may not be meeting heating or cooling requirements and energy use may be underestimated. If grossly oversized, then the HVAC system will be operating at low part-load ratios and the model may not give an accurate estimate of performance.