ASHRAE Journal - October 2023 - 36

TECHNICAL FEATURE
target attempts to strike a balance between the least
efficient prescriptively allowed configuration and a
highly efficient system configuration by selecting a
" good system, " which is used to calculate the desired
performance for each building type and climate zone.
" Good systems " are set at the applicable minimum
efficiency levels listed in Tables 6.8.1-2 through 6.8.1-21
of Standard 90.1-2022.
A " reference system " is also defined, which follows
the Standard 90.1 Appendix G baseline configuration,
which meets the Standard 90.1-2004 code
requirements for efficiency levels, controls, fan power,
economizers, energy recovery, etc., and provides a
stable baseline for comparison.
Mechanical Performance Factors: To demonstrate
compliance, the proposed system TSPR must be
greater than a reference system TSPR divided by a
mechanical performance factor (MPF). Section 6 of
Standard 90.1-2022 includes a table of MPFs for various
building types in each climate zone. The reference
systems are based on those defined in the Standard
90.1-2022 Appendix G baseline, which are set roughly
at the efficiency levels of Standard 90.1-2004. The
MPFs are analogous to building performance factors
in Appendix G and establish the improvement over
the stable 2004 baseline required to meet the current
code. MPFs were developed based on a target level of
performance considering current code requirements
and are intended to be updated with each code cycle.
Figure 2 shows how MPFs are initially generated with
prototype buildings. Then the real-time run of the
proposed building is used with both proposed and
reference systems. The reference system in the proposed
building TSPR is multiplied by the MPF to generate a
target for the proposed building. The proposed system
passes if its TSPR is greater than the calculated target.
Designers may want to use TSPR to avoid some
prescriptive requirements in these situations:
* For some systems, an outdoor air economizer might
not be desirable even though prescriptively required.
Using TSPR the designer could show that the energy
savings of an economizer could be offset by lower fan
energy and higher cooling efficiency.
* In some buildings, the fan power limits might be
difficult to achieve. Where higher fan power is desired,
the higher fan energy use could be offset through
better heating and cooling system efficiency.
36
ASHRAE JOURNAL ashrae.org O CTO B ER 2023
Analysis Tool
Although a web-based tool to implement the
Mechanical System Performance Rating Method has
been developed by PNNL, any simulation tool meeting
the requirements laid out in the Mechanical System
Performance Rating Method in Normative Appendix
L of Standard 90.1-2022 and approved by the code
official can be used to determine compliance with this
approach.
The following paragraphs describe the simulation
tool requirements, as outlined in Standard 90.1-2022
Appendix L; software architecture of the tool; the
various simplifications implemented within the tool
to support the Mechanical System Performance Rating
Method; and the compliance report generated by the
tool.
Simulation Tool Requirements: The simulation
tool must operate within the parameters defined
in Table L2.2.3 of Appendix L, where each system
component will have either fixed or user-defined
characteristics within a specified range. The intent of
these simplifications is to maintain consistency and
reduce the level of effort associated with developing an
energy model by both limiting the parameters that can
be entered by the user and using standard modeling
defaults for parameters not available for the user to edit.
Some of the key requirements for the tool include:
* Ability to limit user inputs to those required for
defining the proposed building HVAC system as defined
by the ruleset in accordance with Appendix L;
* Implement defaults for schedules, loads and
additional strategies such as fan curves, pump curves
and demand-controlled ventilation in accordance with
Appendix L.
* Automatically generate the standard reference
design based on the user-defined proposed building.
* Run annual simulations for the reference and
proposed buildings to determine the annual HVAC
energy use and annual heating and cooling loads.
* Determine compliance by calculating the proposed
building TSPR and reference building TSPR and
comparing its ratio against the required MPF for that
climate zone and building type.†
* Generate a compliance report that outlines the
compliance outcome and additional details including
†An area-weighted average MPF would be calculated for a mixeduse
building.
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ASHRAE Journal - October 2023

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