ASHRAE Journal - October 2023 - 34

TECHNICAL FEATURE
of an HVAC system would be one that analyzes the
interactions of all components of the HVAC system;
accounts for part-load performance, duct and piping
design, system type, and system controls, normalizes
for building loads, and does not allow for trade-offs
between building systems with different equipment
lives (Figure 1). The total system performance ratio
(TSPR) metric (developed by Pacific Northwest National
Laboratory [PNNL]) addresses all these issues as it
measures the amount of energy required to deliver
each unit of heating and cooling to the building over the
course of a typical year.1
With TSPR, systems using less overall energy each year
to meet the building's annual thermal and ventilation
loads are rated as more efficient. The TSPR metric does
not reward improvements in other building systems
(such as reduction in loads from a better performing
envelope), and performance is demonstrated solely
for the efficiency of HVAC systems in the building. The
TSPR metric and the related calculation procedures are
further discussed below.
The TSPR metric and the Mechanical System
Performance Rating Method were first implemented in
Appendix D of the Washington State Energy Code2 and
more recently in Appendix L of Standard 90.1-2022.3
Mechanical System Performance Rating Method
This section defines Standard 90.1-2022's Mechanical
System Performance Rating Method and the total
system performance ratio (TSPR) metric and provides
an overview of the compliance approach. It provides
additional details on the process used to calculate
the mechanical performance factors (MPFs) and how
these details factor into the compliance determination
calculation.
TSPR: Definition and Calculation
Total system performance ratio (TSPR) is a ratio of
the annual heating and cooling provided for a building
to the energy consumed for heating, cooling and
ventilating a building. The calculation is performed
using whole-building simulation, similar to wholebuilding
performance energy modeling, but using a
simplified methodology as defined by the Mechanical
System Performance Rating Method.
TSPR Input Metric. TSPR is calculated as the ratio
of the sum of a building's annual heating and cooling
34
ASHRAE JOURNAL ashrae.org O CTO B ER 2023
FIGURE 1 Compliance paths and need for a system performance approach
Whole Building
Level
Whole
Building
Envelope
System
Lighting
System
HVAC
System
SHW
System
Heating
System
Cooling
System
Fan
System
Etc.
Compliance Gap
load to some metric that represents the annual energy
consumed by the building HVAC systems. The input
metric representing energy consumption for Standard
90.1-2022 is energy cost using national average energy
prices. This approach produces a consistent result that
does not change based on region or serving utility. For
specific applications, different metrics can be used
based on local jurisdiction policy, and alternative metric
MPFs for carbon emission, site energy and source energy
are included in Informative Tables L5-3, L5-4 and L5-5,
respectively, of Standard 90.1-2022.
The TSPR Concept: A larger TSPR indicates lower
HVAC energy use to meet building loads; therefore,
a system with a larger TSPR can be considered more
efficient than one with a smaller TSPR. The annual
heating and cooling loads include envelope loads;
solar gains through fenestration; internal loads due to
lights, equipment and occupants; as well as ventilation
and infiltration loads. This metric provides a single
evaluation criterion that addresses all components of
the HVAC systems used to move heat and air into, out of,
and within a building.
The annual HVAC system energy use of all system
components (including auxiliary components)
is included in TSPR for a complete HVAC system
evaluation. Hence, the total HVAC energy use includes
fuel-fired and electric heating coils (including reheat
coils), direct expansion cooling coils, boilers, chillers,
heat rejection, energy recovery and distribution system
fans and pumps. The impact of HVAC system controls,
such as temperature and pressure resets and variable
speed control, is accounted for through the energy use
of that particular component. For example, the energy
consumption of a fan in a variable air volume (VAV)
system with static pressure reset will typically be lower
than that for the same system without static pressure
Component
Level
Chiller
Cooling
Coil
Pump
Cooling
Tower
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ASHRAE Journal - October 2023

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