ASHRAE 90.1 Appendix G Baseline System Selection Tool
This interactive tool simplifies the complex process of selecting the correct Baseline HVAC System for LEED Energy Modeling, consistent with ASHRAE 90.1-2010 Appendix G requirements.
This document outlines the required inputs, selection criteria, system definitions, and modeling methodologies necessary to establish the ASHRAE 90.1-2010 Appendix G Performance Rating Method (PRM) Baseline HVAC System.
To determine the correct baseline HVAC system (System 1 through System 10), the simulation modeler must gather the following project-specific data, as defined in ASHRAE Standard 90.1-2010, Appendix G, Table G3.1.1A.
ASHRAE 90.1 Baseline Selector
Performance Rating Method (PRM) Determination Tool based on Appendix G (Table G3.1.1A & G3.1.1B).
Project Parameters
System 5 - Packaged VAV w/ Reheat
Packaged Rooftop VAV with Reheat
System Definition
Key Requirements
Based on Zone 4A (Table G3.1.2.6A)
Modeling Specifications (G3.1.2 & G3.1.3)
Criteria for Primary Baseline HVAC System Mapping
The baseline system type is derived from the building type, size, and heating source. Use the guide below or the calculator above to find yours:
Residential
(Dormitory, Hotel, Motel, Multifamily)
Small Non-Residential
(≤3 Floors AND <2,300 m²)
Medium Non-Residential
(4-5 Floors OR 2,300-14,000 m²)
Large Non-Residential
(>5 Floors OR >14,000 m²)
System-Specific Modeling Requirements
Excerpts for common central systems, highlighting key differences:
| Parameter | System 1 (PTAC) | System 7 (VAV w/ Reheat) | System 8 (VAV w/ PFP) |
|---|---|---|---|
| Cooling Type | Direct Expansion (DX) | Chilled Water | Chilled Water |
| Heating Type | Hot-water fossil fuel boiler | Hot-water fossil fuel boiler | Electric Resistance |
| Fan Control | Constant Volume | Variable Air Volume (VAV) | Variable Air Volume (VAV) |
| Hot Water | 82°C supply / 54°C return (Reset by Outdoor Temp) | 82°C supply / 54°C return (Reset by Outdoor Temp) | Not Applicable |
| Chilled Water | Not Applicable | 6.7°C supply / 13°C return (Reset by Outdoor Temp) | 6.7°C supply / 13°C return (Reset by Outdoor Temp) |
4. Minimum Equipment Efficiency Reference
Minimum equipment efficiencies for all systems (Proposed and Baseline) are determined by the tables located in ASHRAE Standard 90.1, Section 6.8.1, based on the equipment type and size.
| Equipment Type | Efficiency Table (6.8.1) | Metrics |
|---|---|---|
| Unitary AC & Condensing Units | Table 6.8.1A | SCOP, ICOP, COPc |
| Heat Pumps (Unitary/Applied) | Table 6.8.1B | SCOPH, COPH, SCOPc |
| Chillers | Table 6.8.1C | COP, IPLV, NPLV |
| PTAC / PTHP | Table 6.8.1D | COPc, SCOPH |
| Boilers | Table 6.8.1F | AFUE, Et, Ec |
5. Exhaust Air Energy Recovery (HERS)
Energy recovery is required for each fan system when the system’s supply air flow rate exceeds the minimum value listed below, dependent on Climate Zone and %OA.
5.1 Criteria for Inclusion (Simplified Table 6.5.6.1)
| Climate Zone | % OA (30-40%) | % OA (≥ 80%) |
|---|---|---|
| 7, 8 | ≥ 1180 L/s | > 0 L/s |
| 4A, 5A, 6A | ≥ 2596 L/s | > 0 L/s |
| 3B, 3C, 4B, 4C, 5B | ≥ 2360 L/s (at 70-80% OA) | ≥ 2360 L/s |
5.2 Required Performance
- 50% Effectiveness: System must have at least 50% sensible or enthalpy recovery effectiveness.
- Bypass Control: Must include bypass or control to permit air economizer operation when free cooling is available.
5.3 Exemptions
- Systems not cooled/heated to < 16°C (60°F).
- Systems exhausting toxic/corrosive fumes or kitchen grease.
- Climate Zones 1 & 2 (Heating Recovery).
- Systems where >60% heating is from site-recovered energy.
💡 The "Code-Minimum" Analogy
The process of running the Baseline System Selection Wizard is like assembling a default, code-minimum building from pre-approved modular blocks. You identify the project's fundamental characteristics (climate, size, function), and the code hands you a specific, predetermined "blueprint" (System 1-10) along with a mandatory checklist (Appendix G properties) to ensure your virtual baseline meets the legal minimum standard for energy efficiency.
Detailed Baseline System Requirements (Appendix G)
HVAC systems in the baseline building design shall conform with the general provisions in Section G3.1.2.
G3.1.2.1 Equipment Efficiencies
All HVAC equipment in the baseline building design shall be modeled at the minimum efficiency levels, both part load and full load, in accordance with Section 6.4. Where efficiency ratings, such as EER and COP, include fan energy, the descriptor shall be broken down into its components so that supply fan energy can be modeled separately.
G3.1.2.2 Equipment Capacities & Sizing
The equipment capacities (i.e. system coil capacities) for the baseline building design shall be based on sizing runs for each orientation and shall be oversized by 15% for cooling and 25% for heating.
G3.1.2.5 Fan System Operation
Supply and return fans shall operate continuously whenever spaces are occupied and shall be cycled to meet heating and cooling loads during unoccupied hours.
G3.1.2.6 Ventilation
Minimum ventilation system outdoor air intake flow shall be the same for the proposed and baseline building designs.
G3.1.2.7 Economizers
Outdoor air economizers shall NOT be included in baseline HVAC Systems 1, 2, 9, and 10. Outdoor air economizers shall be included in baseline HVAC Systems 3 through 8 based on climate as specified in Table G3.1.2.6A (e.g., generally required in zones 2B through 8, but not in 1A-4A).