Selecting appropriate HVAC systems

In many simple box modeling analyses, the same HVAC system will be used for all cases, with the exception that the system capacity might vary between models based on peak cooling and heating loads. In other cases, the choice of HVAC system type will be a subject for study and models are developed with different system types.

In the case when the same HVAC system is used for all models, then the system’s performance has only an indirect impact on the results when comparing design alternatives. But the choice does affect the magnitude of the difference, so some attention should be given to choosing a reasonable system type. The general idea is to choose an HVAC system that is a good candidate considering the building type and climate and that meets code-required levels of performance. Choosing an appropriate system type is especially important if the goal of the analysis is to estimate the absolute energy performance such as EUI.

How to choose an appropriate system

There are several reasonable approaches to choosing an appropriate HVAC system type.

1. Use the planned system type, if known.
2. Choose a system type based on rules for energy code performance compliance calculations, which specify system types based on building type and size. See system type assignments for ASHRAE Standard 90.1 Appendix G or California’s Nonresidential Alternative Calculation Method Reference Manual in the tables below.
3. Use energy efficient design guidelines, such as the Advanced Energy Design Guides published by ASHRAE, for guidance in choosing a system type.
4. Use the default system type provided by simulation software (if the software includes a default system assignment feature), ideally one that is based on building type and size.

Factors to consider when choosing an appropriate HVAC system type for a simple box model

• Building floor area
• Number of stories
• Occupancy type
• Owner preferences
• Desired heating source: electricity or fossil fuels
• Space constraints, such as available of roof space
• Local control requirements
• Climate

Common HVAC system types for simple box modeling

There are many possible HVAC system configurations, but these are some common reasonable choices to use in a simple box model at an early design stage.

Packaged single zone

Appropriate for one or two story buildings, typically 25,000 ft² or less. May have either a gas furnace or electric heat pump as the heating source. For locations requiring dehumidification hot-gas reheat may be included as part of the packaged DX cooling. May use constant or variable air flow control.

Multiple-zone variable air volume with reheat

Generally appropriate for larger commercial buildings, especially those of 3 or more stories. For medium sized buildings, packaged DX cooling units are common. For larger buildings, chilled water cooling is common. Terminal units will typically be either standard VAV boxes with hot water reheat coils or parallel fan-powered VAV boxes with electric resistance reheat coils.

Single-zone fan coils

These fan coils are provided with chilled water and hot water from a central plant and are most applicable for hotels, schools and high-rise multifamily buildings. They are typically accompanied by a dedicated outdoor air system.

Single zone water-source heat pumps

A water source heat pump is similar in application to a fan coil and is commonly used in the same building types. Each heat pump is connected to a condenser water loop which provides heat when the unit is in heating mode and removes heat when the unit is in cooling mode.

Dedicated outdoor air system

Systems such as fan coils and water-source heat pumps are often provided with outdoor ventilation air via a separate air handler called a dedicated outdoor air system (DOAS), which tempers the air. The cooling source may be chilled water or DX.

Chilled water plant

For systems with chilled water cooling, smaller buildings will typically use air-cooled chillers, and larger buildings often have water-cooled chillers. The typical building size threshold between air-cooled and water-cooled chillers is in the range of 75,000 ft² to 150,000 ft², roughly equal to between 150 and 300 tons of chiller capacity.

Hot water plant

Traditionally, most hot water plants use natural gas fired boilers. For all-electric projects, heat pumps or heat recovery chillers should be considered as the heating source in the simple box model.

Condenser water loop for water source heat pumps

The common condenser water loop includes a cooling tower for rejecting excess heat in the cooling season and a boiler for adding heat in the heating season. Ground-coupling is also an option.

Implications of HVAC system choice for simple box modeling

• The performance of the HVAC system affects the sensitivity of results for other design options. A model with a high efficiency HVAC system will show less impact of, for example, window area than a model with a lower efficiency HVAC system.
• Some HVAC systems take advantage of mild outdoor air temperature for free cooling through features such as an airside economizer. Depending on climate, the presence of an economizer can have a significant impact on the cooling energy estimate. Systems with the capability to include an airside economizer, such as a packaged single zone system or a VAV system, are a good choice in mild climates. Economizers offer least benefit in warm and humid climates.  
• An electric heat pump heating system will typically show lower site energy consumption than a gas furnace or boiler serving the same heating load, but depending on electricity and gas utility rates it may have higher energy cost. So the choice of performance metric, such as site energy, source energy or energy cost, may favor one or the other.

Code baseline system types

Baseline HVAC System Types for the Performance Rating Method of ASHRAE Standard 90.1-2019 Appendix G and for California Title-24-2019 Performance Compliance

	Floor Area	Floors	ASHRAE 90.1-2019	California Title 24- 2019
Residential, hotel/motel	Any	>3 floors	PTAC or PTHP
	Any	Up  to 7 floors		SZAC
	Any	8 or more floors		FPFC
Public assembly	<120,000 ft2		PSZ-AC or PSZ-HP
	≥120,000 ft2		SZ-CV-HW or SZ-CV-ER
Retail	Any	2 floors or fewer	PSZ-AC or PSZ-HP	SZVAV
Other non residential	<25,000 ft2	Up to 3 floors	PSZ-AC or PSZ-HP	SZVAV
	< 25,000 ft2	4 or 5 floors	PVAV or PVAV-PFPB	PVAV
	25,000 to 150,000 ft2	Up to 5 floors	PVAV or PVAV-PFPB	PVAV
	>150,000 ft2	Any	VAV or VAV-PFPB	VAV
	Any	Over 5 floors	VAV or VAV-PFPB	VAV

ASHRAE Standard 90.1 System Types

PTAC	Packaged terminal air conditioner, constant volume, direct expansion, fossil fuel boiler
PTHP	Packaged terminal heat pump, constant volume, direct expansion, electric heat pump
PSZ-AC	Packaged rooftop air conditioner, constant volume, direct expansion, fossil fuel furnace
PSZ-HP	Packaged rooftop air conditioner, constant volume, direct expansion, electric heat pump
SZ-CV-HW	Single-zone system, constant volume, chilled water, hot-water fossil fuel boiler
SZ-CV-ER	Single-zone system, constant volume, chilled water, electric resistance
PVAV	Packaged rooftop VAV with reheat, direct expansion, hot-water fossil fuel boiler
PVAV-PFPB	Packaged rooftop VAV with parallel fan-powered boxes, direct expansion, electric reheat
VAV	VAV with reheat, chilled water, hot-water fossil fuel boiler
VAV-PFPB	VAV with parallel fan-powered boxes, direct expansion, electric reheat

Title 24 System Types

PTAC	Packaged terminal air conditioner, constant volume, direct expansion, fossil fuel boiler
PTHP	Packaged terminal heat pump, constant volume, direct expansion, electric heat pump
PSZ-AC	Packaged rooftop air conditioner, constant volume, direct expansion, fossil fuel furnace
PSZ-HP	Packaged rooftop air conditioner, constant volume, direct expansion, electric heat pump
SZ-CV-HW	Single-zone system, constant volume, chilled water, hot-water fossil fuel boiler
SZ-CV-ER	Single-zone system, constant volume, chilled water, electric resistance
PVAV	Packaged rooftop VAV with reheat, direct expansion, hot-water fossil fuel boiler
PVAV-PFPB	Packaged rooftop VAV with parallel fan-powered boxes, direct expansion, electric reheat
VAV	VAV with reheat, chilled water, hot-water fossil fuel boiler
VAV-PFPB	VAV with parallel fan-powered boxes, direct expansion, electric reheat