ASHRAE 90.1 2019 Appendix G FAQ's
General Questions
What is Appendix G and when is it required?
Appendix G establishes the Performance Rating Method for ASHRAE Standard 90.1. It provides the framework for demonstrating that a building design exceeds minimum code requirements through whole-building energy simulation. You'll use this when pursuing LEED certification, utility incentive programs, or any time you need to quantify performance above baseline code compliance. The method compares annual energy use between your actual design (the proposed building) and a code-minimum reference model (the baseline building). Success requires the proposed design to demonstrate measurable improvement over the baseline.
Reference: Section G3.1, Table G3.1
Must the entire building be included in the modeling scope?
Yes, with limited exceptions. For additions or alterations to existing buildings, you may exclude portions that remain unmodified if three conditions are met: the excluded areas are served by completely separate HVAC systems, the excluded portions comply with Sections 5 through 10, and temperature setpoints and schedules are consistent across the boundary. When portions are excluded from the proposed design, they must be excluded from the baseline as well. The intent is to ensure valid comparison between equivalent building models.
Reference: Table G3.1, Item 2
How do the proposed design and baseline building differ structurally?
The proposed design represents the actual building as documented in construction drawings, including all specified systems, efficiencies, and control strategies. The baseline building maintains identical geometry, floor area, and thermal zoning but substitutes prescriptive envelope assemblies, lighting power densities from lookup tables, and HVAC systems determined by building classification and climate zone per Tables G3.1.1-3 and G3.1.1-4. The baseline effectively represents a compliant but unoptimized version of the same building, providing a consistent reference point for comparison.
Reference: Table G3.1, Items 1 and 5-12
What happens when the simulation program cannot model a specified component?
Section G3.1, Item 13 permits substitution of a thermodynamically similar component model that reasonably approximates the expected performance of the component that cannot be explicitly modeled. This substitution requires documentation of the approach and assumptions, typically through spreadsheets or supplementary calculations showing how power demand and operating schedules were generated. The same approach must be applied in both the proposed and baseline models. This provision acknowledges software limitations while requiring transparent documentation of workarounds.
Reference: Table G3.1, Item 13
Can occupancy assumptions or schedules be modified to improve performance metrics?
No. Table G3.1, Item 12 explicitly states that occupancy and occupancy schedules shall not be changed between the proposed and baseline designs. Schedule modifications are permitted only when necessary to model automatic (not manual) control strategies, and only with rating authority approval. The prohibition exists because occupancy-based savings claims are difficult to verify and enforcement would be essentially impossible.
Reference: Table G3.1, Item 12
Building Envelope
Why must the baseline building be simulated at multiple orientations?
Unless exempted, Section G3.1, Item 5(a) requires simulation at the actual orientation plus rotations of 90, 180, and 270 degrees, with results averaged. This eliminates performance benefits attributable solely to favorable solar orientation rather than to building design quality. Exemptions apply when site constraints dictate orientation (subject to rating authority approval) or when fenestration area varies by less than 5% across all orientations, indicating that rotation would produce negligible differences.
Reference: Table G3.1, Item 5(a)
How is baseline fenestration area determined for building types not listed in Table G3.1.1-1?
For building area types absent from Table G3.1.1-1, baseline vertical fenestration equals the proposed design fenestration area or 40% of gross above-grade wall area, whichever is smaller. This cap prevents gaming the system through excessive glazing in unlisted building types. The fenestration must be distributed across building facades in the same proportions as the proposed design. For existing buildings, baseline fenestration equals existing fenestration area prior to the proposed work.
Reference: Table G3.1, Item 5(c)
What are the modeling requirements for dynamic glazing systems?
Automatically controlled dynamic glazing may be modeled with its full operational characteristics in the proposed design. Manually controlled dynamic glazing must use the average of minimum and maximum SHGC and VT values, per Section G3.1, Item 5(a)(5). This reflects the standard's general skepticism about relying on manual occupant actions for energy performance. The distinction matters significantly for electrochromic or thermochromic glazing where the performance range is wide.
Reference: Table G3.1, Item 5(a)(5)
Under what conditions can tested air leakage rates be used in the proposed design?
The baseline air leakage rate is fixed at 5.1 L/s·m² at 75 Pa (or 3.0 L/s·m² with verification per Section 5.9.1.2). The proposed design may use tested values only when whole-building air leakage testing per Section 5.4.3.1.1 is specified during design and completed after construction. This creates accountability. You cannot claim tighter envelope performance without committing to post-construction verification. The standard recognizes that design intent and as-built performance frequently diverge.
Reference: Table G3.1, Item 5(b) and Item 5(h)
How are uninsulated assemblies such as balconies or floor edges handled in the proposed design?
Section G3.1, Item 5(a)(1) requires these assemblies to be modeled separately, either as distinct elements within the simulation or by calculating individual U-factors and area-weighting them with adjacent assemblies of the same type. Assemblies covering less than 5% of total area for that assembly type may be aggregated with similar assemblies of the same orientation and thermal properties. This recognizes that thermal bridging matters but permits reasonable simplification for minor elements.
Reference: Table G3.1, Item 5(a), Exception 1
What baseline assembly types are specified for opaque envelope components?
The baseline uses specific assembly constructions from Normative Appendix A: insulation entirely above deck for roofs (A2.2), steel-framed for above-grade walls (A3.3), concrete block for below-grade walls (A4), steel-joist for floors (A5.3), unheated slab F-factors for slab-on-grade floors (A6), and opaque door types matching proposed construction (A7). All assemblies must conform to U-factors in Tables G3.4-1 through G3.4-8 for the applicable climate zone.
Reference: Table G3.1, Item 5(b)
How are manual and automatic shading devices treated differently?
Manual fenestration shading devices such as blinds or shades must be modeled identically in both the proposed and baseline designs, or not modeled in either. Automatically controlled fenestration shades or blinds must be modeled in the proposed design. Permanent shading devices such as fins, overhangs, and light shelves must be modeled in both designs. However, the baseline assumes all vertical fenestration is flush with the exterior wall with no shading projections, and manual window shading devices are not required in the baseline.
Reference: Table G3.1, Item 5(a)(4) and Item 5(d)
What are the skylight area limitations for the baseline building?
Skylight area in the baseline equals the proposed design or 3%, whichever is smaller. If the proposed design exceeds 3%, baseline skylight area is decreased by an identical percentage in all roof components containing skylights to reach 3%. Skylight orientation and tilt remain the same as the proposed design. Skylight U-factor and SHGC properties must match the appropriate requirements in Tables G3.4-1 through G3.4-8 for the applicable skylight percentage.
Reference: Table G3.1, Item 5(e)
HVAC Systems
How is the baseline HVAC system type determined?
Section G3.1.1(a) establishes a priority hierarchy: first, building type with largest conditioned floor area; second, number of floors including below-grade but excluding parking-only floors; third, gross conditioned floor area; fourth, climate zone per Tables G3.1.1-3 and G3.1.1-4. For systems 1, 2, 3, 4, and 9 through 13, each thermal block receives its own system. For systems 5 through 8, each floor receives a separate system, though floors with identical thermal blocks may be grouped. The lookup is deterministic once building parameters are established.
Reference: Section G3.1.1(a)
What constitutes a thermal block for modeling purposes?
When HVAC zones are defined in design documents, each zone is typically modeled as a separate thermal block, subject to aggregation criteria in Section G3.1, Item 7. Zones may be combined if they share the same space use classification (or peak internal loads differing by less than 31.2 W/m²), face the same orientation within 45 degrees, are served by the same HVAC system type, and have schedules differing by 40 or fewer equivalent full-load hours per week. When zones are not yet designed, Section G3.1, Item 8 requires separate blocks for interior (>4.6 m from exterior walls) and perimeter spaces, with additional separation by glazed wall orientation, ground contact, and roof exposure.
Reference: Table G3.1, Items 7, 8, and 9
Must heating systems be modeled in spaces where none will be installed?
Generally yes, per Section G3.1, Item 1(b). All conditioned spaces must be simulated as both heated and cooled, even if no systems are specified, because the intent is to model the building's energy requirements under standard operating assumptions. The exception permits heating-only modeling for storage rooms, stairwells, vestibules, mechanical/electrical rooms, and restrooms that do not exhaust or transfer air from mechanically cooled zones. This exception recognizes that certain ancillary spaces legitimately operate without cooling.
Reference: Table G3.1, Item 1(b)
How should variable refrigerant flow systems be modeled in the proposed design?
VRF systems must be modeled using manufacturers' full-load and part-load performance data per Section G3.1, Item 10(b). The simplified efficiency equations in Section G3.1.2.1 apply only to baseline systems. Accurate VRF modeling requires manufacturer-provided performance curves across the operating range, including variations in outdoor temperature and part-load ratios. If adequate performance data are unavailable, the modeled savings will not reflect actual performance, which becomes the manufacturer's problem as much as yours.
Reference: Table G3.1, Item 10(b); Section G3.1.2.1
What special provisions apply to laboratory buildings?
Section G3.1.1(d) requires laboratory spaces in buildings with total laboratory exhaust exceeding 7,100 L/s to use a single System 5 or 7 serving only those spaces. The laboratory exhaust fan must be modeled as constant power, reflecting constant-volume stack discharge with outdoor air bypass. This recognizes that laboratory ventilation requirements typically dominate the energy profile and warrant separate system modeling.
Reference: Section G3.1.1(d)
How are computer rooms treated in baseline system selection?
Computer rooms with total peak cooling load exceeding 880 kW, or exceeding 175 kW when the baseline system type would otherwise be 7 or 8, must use System 11 per Section G3.1.1(g). Other computer rooms use System 3 or 4. Additionally, Section G3.1, Item 10 prohibits reheat for dehumidification in baseline computer room systems, though the proposed design may include it. HVAC fans in systems primarily serving computer rooms must run continuously during both occupied and unoccupied hours.
Reference: Section G3.1.1(g); Table G3.1, Items 4 and 10
What are the HVAC fan schedule requirements?
HVAC fans providing outdoor air for ventilation must run continuously during occupied hours and cycle to meet heating and cooling loads during unoccupied hours. Exceptions include: systems in buildings with no installed heating/cooling (which cycle during all hours), spaces with health and safety-mandated minimum ventilation during unoccupied hours, and systems primarily serving computer rooms (which run continuously during all hours). Fan schedules may differ between proposed and baseline only when Section G3.1.1(c) applies.
Reference: Table G3.1, Item 4
What fuel source is specified for baseline fossil fuel systems?
Fossil fuel systems must be modeled using natural gas as their fuel source. The exception permits propane as the baseline fuel when natural gas is not available for the proposed building site, as determined by the rating authority. This determination typically requires documentation that natural gas infrastructure does not serve the building location.
Reference: Table G3.1, Item 10
When must the baseline include humidification systems?
If the proposed design includes humidification, the baseline building design must use adiabatic humidification. However, if the proposed building humidification system complies with Section 6.5.2.4, then the baseline building design uses nonadiabatic humidification instead. This creates an incentive for efficient humidification strategies while ensuring the baseline represents realistic system configurations.
Reference: Table G3.1, Item 10
Lighting
What automatic lighting controls are required in the baseline building design?
The baseline includes automatic shutoff controls in buildings exceeding 500 m² and occupancy sensors in employee lunch and break rooms, conference/meeting rooms, and classrooms (excluding shop classrooms, laboratory classrooms, and preschool through grade 12 classrooms). The proposed design must include at minimum the mandatory controls specified in Section 9.4.1. Additional controls beyond these minimums appear only in the proposed design. The baseline does not benefit from optimization beyond minimum code requirements.
Reference: Table G3.1, Item 6
How are daylighting controls modeled in the proposed design?
Section G3.1, Item 6(h) permits two approaches: direct modeling within the simulation program, or schedule adjustments derived from separate daylighting analysis approved by the rating authority. Either method must separately account for primary sidelighted areas, secondary sidelighted areas, and toplighted areas, because lumping these zones together overstates the energy benefit. The baseline receives no daylighting controls unless specifically listed in Table G3.7, which currently does not include them.
Reference: Table G3.1, Item 6(h); Table G3.7
Can lighting power be reduced for high-efficacy task lighting?
Only if the same illuminance levels as the baseline are maintained and demonstrated through calculation, per the exception to Section G3.1, Item 6(e). The standard distinguishes between providing the same light with less power (legitimate efficiency improvement) and simply providing less light (not an efficiency measure). If task lighting allows reduction of ambient lighting while maintaining required illuminance, that reduction must be calculated and documented.
Reference: Table G3.1, Item 6(e), Exception
How are occupancy sensor reductions applied to lighting schedules?
Lighting controlled by occupancy sensors not required in the baseline is reduced each hour by the occupancy sensor reduction factors in Table G3.7 for the applicable space type, per Section G3.1, Item 6(i). This reduction applies only to lighting controlled by the sensors, not to all lighting in the space. For programmable controls in buildings under 500 m², a 10% schedule reduction may be applied. These reductions must be applied to the proposed design schedules; the baseline uses unadjusted schedules before applying its own required sensor credits.
Reference: Table G3.1, Item 6(i); Table G3.7
How is baseline lighting power determined?
Interior lighting power in the baseline building design is determined using the values in Table G3.7. Lighting must be modeled with automatic shutoff controls in buildings exceeding 500 m² and occupancy sensors in employee lunch and break rooms, conference/meeting rooms, and classrooms (excluding shop classrooms, laboratory classrooms, and preschool through grade 12 classrooms). Exterior lighting in areas identified as "Tradable Surfaces" in Table G3.6 uses baseline lighting power from that table. Other exterior lighting is modeled the same in both the baseline and proposed designs.
Reference: Table G3.1, Item 6; Table G3.6; Table G3.7
What lighting power values are used when lighting systems do not exist or are not designed?
When lighting neither exists nor is submitted with design documents, lighting must comply with but not exceed the requirements of Section 9. Lighting power is determined using the Building Area Method. For dwelling units, hotel/motel guest rooms, and other spaces where lighting connects via receptacles and is not shown on design documents, lighting power used in the simulation equals the lighting power allowance in Table 9.6.1 for the appropriate space type or as designed, whichever is greater. For dwelling units specifically, lighting power equals 6.5 W/m² or as designed, whichever is greater.
Reference: Table G3.1, Item 6(c) and 6(e)
Service Water Heating
When must condenser heat recovery be included in the baseline design?
Section G3.1, Item 11(d) requires baseline models to include condenser heat recovery systems meeting Section 6.5.6.2 requirements for large, 24-hour facilities that meet the prescriptive criteria, regardless of exceptions that might otherwise apply. This reflects the standard's position that heat recovery is economically compelling in these applications. If the simulation program cannot model such a system, it becomes a prescriptive requirement for the actual building, and no heat recovery is modeled in either the proposed or baseline design.
Reference: Table G3.1, Item 11(d)
Why are piping losses excluded from modeling?
Section G3.1, Items 11(f) and 11(i) explicitly state that piping losses shall not be modeled in either the proposed or baseline design. This represents a pragmatic acknowledgment that reliable piping loss calculations require detailed layout information often unavailable during design, and that modeling results would be highly sensitive to assumptions about installation quality. Recirculation pump energy must be modeled explicitly, but heat loss from the distribution system is not included.
Reference: Table G3.1, Item 11(f) and 11(i)
What service water heating reductions may be claimed in the proposed design?
Three categories of reduction are permitted per Section G3.1, Item 11(g) exceptions: documented water conservation measures reducing physical volume (low-flow fixtures with calculated flow reductions), measures reducing required mixed-water temperature or increasing entering water temperature (alternative sanitizing technology, heat recovery to makeup water), and measures reducing the hot fraction of mixed water (shower or laundry heat recovery). All claimed reductions require calculation and methodology approval by the rating authority. Baseline flow rates must be determined per Table G3.1, Item 1 requirements.
Reference: Table G3.1, Item 11(g), Exceptions 1-3
How are service water heating systems sized in the baseline?
When a complete service water heating system exists or is specified, Section G3.1, Item 11(a) requires one baseline system for each building area type, sized per Section 7.4.1 and meeting minimum efficiency requirements in Section 7.4.2. When no system exists but water heating loads are anticipated, the baseline is sized identically to the proposed design while meeting Section 7.4.2 efficiency minimums. This approach ensures the baseline represents a code-compliant installation serving the same loads as the proposed design.
Reference: Table G3.1, Item 11(a) and 11(b)
How are service water heating loads and temperatures determined?
Service water heating energy consumption must be calculated explicitly based on the volume required and the entering makeup water and leaving service water heating temperatures. Entering water temperatures are estimated based on location. Leaving temperatures are based on end-use requirements. Service water loads and use must be the same for both the proposed design and baseline building design and documented by calculation procedures in Section 7.4.1. It is acceptable to use either annual average ground temperatures or monthly average ground temperatures for water main supply temperatures.
Reference: Table G3.1, Items 11(e), 11(g), and 14(c)
What baseline equipment type is specified for service water heating?
The service water heating system in the baseline building design is specified in Table G3.1.1-2. Gas storage water heaters must be modeled using natural gas as their fuel. Where natural gas is not available for the proposed building site, as determined by the rating authority, gas storage water heaters are modeled using propane as their fuel instead.
Reference: Table G3.1, Item 11(h); Table G3.1.1-2
Receptacle and Process Loads
Why are receptacle loads held constant between proposed and baseline designs?
Section G3.1, Item 12 establishes that receptacle and process loads are assumed identical in both models, except when specifically approved by the rating authority for demonstrating performance exceeding Standard 90.1 requirements (but not when using the Performance Rating Method as an alternative compliance path per Section 4.2.1.1). This restriction recognizes that Appendix G targets building systems rather than plug loads, and prevents claims based on equipment selections outside the standard's scope.
Reference: Table G3.1, Item 12
What credit is available for receptacle controls beyond minimum requirements?
The exception to Section G3.1, Item 12 permits a 10% reduction in hourly receptacle schedules for spaces with receptacle controls installed where not required by Section 8.4.2. The credit is calculated as RPC = RC × 10%, where RC is the percentage of all controlled receptacles, and applied as EPSpro = EPSbas × (1 – RPC). This provides modest recognition of voluntary controls while maintaining schedule equivalence for uncontrolled receptacles. The baseline receives no receptacle controls beyond code minimums.
Reference: Table G3.1, Item 12, Exception
How are motors modeled in the baseline design?
Motors in the baseline must be modeled with efficiency ratings from Table G3.9.1. Other Section 10 systems and miscellaneous loads are modeled identically to the proposed design, including schedules and control sequences. Energy for cooking equipment, receptacles, computers, medical/laboratory equipment, and manufacturing/process equipment not specifically addressed in the standard must be identical between proposed and baseline. Receptacle schedules must be the same as the proposed design before the receptacle power credit is applied.
Reference: Table G3.1, Item 12; Table G3.9.1
Under what conditions may baseline equipment differ from the proposed design?
The exception to Section G3.1, Item 12 permits baseline equipment to differ from the proposed design only when quantifying performance beyond Standard 90.1 (not for alternative compliance), and only with rating authority approval based on documentation that the proposed equipment represents a significant, verifiable departure from current conventional practice. The burden is demonstrating through documentation that accepted conventional practice differs from the proposed installation. This is a high bar intentionally. Occupancy and schedules may not be changed under any circumstances.
Reference: Table G3.1, Item 12, Exception
How are receptacle and process load estimates determined?
Receptacle and process loads, such as those for office and other equipment, must be estimated based on building area type or space type category. These loads must always be included in simulations and are required when calculating both the proposed building performance and the baseline building performance per Section G1.2.1. The loads are assumed identical unless specifically approved by the rating authority when quantifying performance exceeding Standard 90.1, not when the Performance Rating Method is used as an alternative compliance path.
Reference: Table G3.1, Item 12
Modeling Requirements
What recourse exists when simulation software cannot model a proposed design feature?
Section G3.1, Item 13 permits substitution of a thermodynamically similar component model that approximates expected performance when explicit modeling is impossible. The substitution requires documentation of assumptions, typically through spreadsheets or supplementary calculations showing how power demand and operating schedules were generated. The same approach must be applied in both the proposed and baseline models. This provision acknowledges software limitations while requiring transparent documentation of workarounds.
Reference: Table G3.1, Item 13
May schedules differ between the proposed and baseline designs?
Temperature and humidity control setpoints, schedules, and throttling ranges must be identical, per Section G3.1, Item 4. Exceptions permit schedule differences for HVAC systems providing thermal comfort through means other than direct air temperature control (if equivalent comfort is demonstrated per ASHRAE Standard 55), for modeling nonstandard efficiency measures (automatic lighting controls, natural ventilation, demand-controlled ventilation, service water heating controls) when approved by the rating authority, and when Section G3.1.1(c) applies to baseline system selection. Manual controls never justify schedule differences.
Reference: Table G3.1, Item 4
How are elevators modeled in both designs?
Section G3.1, Item 16 requires modeling of elevator cab motors, ventilation fans, and lighting when elevators are included in the proposed design. Peak motor power is calculated from car weight, rated load, counterweight (from Table G3.9.2), car speed, and mechanical/motor efficiencies using the equation Pm = (weight + load – counterweight) × speed × 0.00981/(hmechanical × hmotor). The baseline uses the same motor schedule as the proposed design. Baseline cab ventilation fans are 0.69 W/L·s and lighting is 33.79 W/m², both operating continuously.
Reference: Table G3.1, Item 16; Table G3.9.2
What documentation is required for shading by adjacent structures?
Section G3.1, Item 14(a) requires that structures and significant vegetation or topographical features affecting solar radiation be adequately reflected in the simulation. Elements with effective height exceeding their distance from the proposed building, and width facing the building exceeding one-third the building width, must be accounted for. Both the proposed and baseline designs must include identical shading representations. This ensures that site-specific shading is not counted as a building performance characteristic.
Reference: Table G3.1, Item 14(a)
How are distribution transformers addressed in the modeling?
Low-voltage dry-type distribution transformers are modeled only when proposed design transformers exceed the efficiency requirements of Table 8.4.4. When modeled in the proposed design, the baseline must also model transformers using Table 8.4.4 efficiency values, with the same ratio of capacity to peak electrical load as the proposed design. If the proposed transformers merely meet minimum efficiency, neither model includes them explicitly.
Reference: Table G3.1, Item 15; Table 8.4.4
How is refrigeration equipment modeled in both designs?
Where refrigeration equipment in the proposed design is rated per AHRI 1200, the rated energy use must be modeled. Otherwise, the proposed design uses actual equipment capacities and efficiencies. Where refrigeration equipment is specified in the proposed design and listed in Tables G3.10.1 and G3.10.2, the baseline is modeled as specified in those tables using actual equipment capacities. If the refrigeration equipment is not listed in Tables G3.10.1 and G3.10.2, the baseline is modeled the same as the proposed design.
Reference: Table G3.1, Item 17; Tables G3.10.1 and G3.10.2
Special Conditions
What provisions apply to buildings with multiple area types?
Section G3.1.1(b) requires additional baseline system types for nonpredominant conditions (residential versus nonresidential) when those conditions apply to more than 1,900 m² of conditioned floor area. This prevents inappropriate system assignments for mixed-use buildings. For fenestration, each building area type uses the values from Table G3.1.1-1 appropriate to that type. For service water heating, separate baseline systems are required for each building area type.
Reference: Section G3.1.1(b); Table G3.1, Items 5(c) and 11(a)
How are zones with significantly different loads handled in baseline systems 5 through 13?
Section G3.1.1(c) requires separate single-zone systems conforming to System 3 or 4 for HVAC zones with occupancy, internal gains, or schedules differing significantly from other zones served by the system. Significant differences are defined as total peak internal gains differing by 31.2 W/m² or more from the average, or schedules differing by more than 40 equivalent full-load hours per week. Natatoriums and continually occupied security areas are cited as examples. This exception does not apply to computer rooms, which have separate provisions.
Reference: Section G3.1.1(c)
What special modeling applies to heating-only zones in the proposed design?
Section G3.1.1(e) specifies that thermal zones designed with heating-only systems in the proposed design, when serving storage rooms, stairwells, vestibules, electrical/mechanical rooms, and restrooms not exhausting or transferring air from mechanically cooled zones, must use System 9 or 10 in the baseline. Section G3.1.1(f) requires that if the baseline system type is 9 or 10, additional system types must be used for all zones that are mechanically cooled in the proposed design. These provisions ensure appropriate system-type matching.
Reference: Section G3.1.1(e) and G3.1.1(f)
How are hospitals treated in baseline system selection?
Section G3.1.1(h) requires hospitals to use System 5 or 7 in all climate zones, depending on building type. This overrides the standard lookup table hierarchy. The provision recognizes that hospital ventilation requirements, infection control considerations, and operational patterns differ sufficiently from typical commercial buildings to warrant dedicated system specification regardless of climate zone.
Reference: Section G3.1.1(h)
How are multifamily residential buildings zoned for modeling?
Residential spaces must be modeled using at least one thermal block per dwelling unit, except that units facing the same orientations may be combined into one thermal block. Corner units and units with roof or floor loads may only be combined with units sharing these features. This requirement applies equally to both the proposed design and baseline building design.
Reference: Table G3.1, Item 9
What requirements apply to ground and water main temperatures?
It is acceptable to use either annual average ground temperatures or monthly average ground temperatures for calculating heat loss through below-grade walls and basement floors. Similarly, for service water heating calculations, either annual water main supply temperature or monthly average water main supply temperatures may be used. If annual or monthly water main supply temperatures are not available from the local water utility, annual average ground temperatures may be used instead.
Reference: Table G3.1, Items 14(b) and 14(c)
Purchased Energy Systems
How are systems using purchased heat modeled in the baseline?
When the proposed design uses purchased hot water or steam, the baseline must also use purchased heat rather than on-site boilers, electric heat, or furnaces. Both models use the same heating source, and costs are based on actual utility rates. This prevents unfair comparisons where the baseline gets penalized for modeling equipment the proposed design doesn't actually need to operate.
Reference: Section G3.1.1.1
How are systems using purchased chilled water modeled in the baseline?
Similar to purchased heat, when the proposed design uses purchased chilled water, the baseline uses it too. Costs are based on actual utility rates, and on-site chillers or direct expansion equipment are not modeled in the baseline. Again, the idea is to compare equivalent system architectures, not to model equipment that doesn't exist in the actual building's energy profile.
Reference: Section G3.1.1.2
What baseline system modifications apply when using only purchased heat?
Tables G3.1.1-3 and G3.1.1-4 still determine the baseline system type, but purchased heat substitutes for whatever heating type the table specifies. The proposed and baseline use the same heating source. So if the table says "fossil fuel boiler" but you're buying steam from a district system, the baseline uses purchased steam instead.
Reference: Section G3.1.1.3.1
What baseline system modifications apply when using only purchased chilled water?
Several modifications occur. First, purchased chilled water replaces the cooling types in Table G3.1.1-4. Second, specific system substitutions apply: Systems 1 and 2 become constant-volume fan-coil units with fossil fuel boilers, Systems 3 and 4 become constant-volume single-zone air handlers with fossil fuel furnaces, System 7 replaces System 5, and System 8 replaces System 6. These changes reflect typical configurations when chilled water is purchased rather than generated on-site.
Reference: Section G3.1.1.3.2
What baseline system modifications apply when using both purchased chilled water and purchased heat?
When both are purchased, Tables G3.1.1-3 and G3.1.1-4 still determine the system type, but both purchased heat and purchased chilled water substitute for the heating and cooling types listed. System 1 becomes constant-volume fan-coil units, System 3 becomes constant-volume single-zone air handlers, and System 7 replaces System 5. These are simpler configurations than when one energy source is purchased and the other generated on-site.
Reference: Section G3.1.1.3.3
Are on-site distribution pumps modeled when using purchased energy?
Yes, all on-site distribution pumps must be modeled in both the proposed and baseline designs. The utility may deliver energy to the building boundary, but moving that energy through the building is still the building's responsibility and energy cost. Excluding these pumps would ignore a real energy consumer.
Reference: Section G3.1.1.3.4
Infiltration and Air Leakage
How is the building envelope air leakage rate converted for simulation programs?
The air leakage rate at 75 Pa (I75Pa) must be converted using specific formulas depending on how the simulation program describes air leakage. For methods using floor area: IFLR = 0.112 × I75Pa × S/AFLR. For methods using above-grade wall area: IAGW = 0.112 × I75Pa × S/AAGW. These conversions account for reference wind speed and normalize the measured leakage to the appropriate basis for the simulation.
Reference: Section G3.1.1.4
How is measured air leakage calculated from blower door test results?
When using actual test data per ASTM E 779, calculate I75Pa = Q/S, where Q is the volume of air (L/s) flowing through the building envelope at 75 Pa differential, and S is the total area of the building envelope including lowest floor, all walls (below-grade and above-grade), roof, and all fenestration. This gives you the normalized air leakage rate that feeds into the conversion formulas.
Reference: Section G3.1.1.4
Can multizone airflow models be used instead of the standard infiltration method?
Yes, but with conditions. Where calculations are independent of the energy simulation program, the method must comply with Section G2.5. The conversion method from I75Pa to simulation program units must be fully documented and submitted to the rating authority for approval. This exception exists because sophisticated infiltration modeling can be more accurate, but it requires verification that you're not just making up numbers.
Reference: Exception to G3.1.1.4
Equipment Sizing and Performance
How are baseline HVAC equipment efficiencies determined?
All baseline equipment is modeled at minimum efficiency levels (both part-load and full-load) from Tables G3.5.1 through G3.5.6. When multiple zones or residential spaces combine into a single thermal block, efficiencies for System Types 1, 2, 3, 4, 9, and 10 are based on the thermal block's equipment capacity divided by the number of zones or spaces. For System Types 5 or 6, efficiencies are based on a single floor's cooling capacity when identical floors are grouped. This prevents gaming the system by aggregating zones to hit more favorable efficiency tiers.
Reference: Section G3.1.2.1
What oversizing factors apply to baseline system coil capacities?
System coil capacities are based on sizing runs for each orientation per the baseline orientation requirements, then oversized by 15% for cooling and 25% for heating. The ratio between capacities used in annual simulations and those from sizing runs must be exactly 1.15 for cooling and 1.25 for heating. Plant capacities use coincident loads rather than the sum of non-coincident zone peaks. This standardized oversizing reflects typical engineering practice without rewarding excessive overdesign.
Reference: Section G3.1.2.2
What schedules are used for sizing runs?
For cooling sizing runs, all internal load schedules (infiltration, occupants, lighting, equipment) use the highest hourly value from annual simulation runs, applied to the entire design day. For heating sizing runs, occupant, lighting, and equipment schedules use the lowest hourly value, while infiltration uses the highest hourly value. The exception: for residential cooling sizing, use the most-used weekday hourly schedule from annual simulation instead. These assumptions ensure equipment is sized for realistic peak conditions.
Reference: Section G3.1.2.2.1
What are the acceptable limits for unmet load hours?
Unmet load hours cannot exceed 300 hours (out of 8,760 simulated). Alternatively, higher unmet loads may be accepted with rating authority approval if sufficient justification demonstrates that simulation accuracy isn't significantly compromised. Excessive unmet loads usually indicate undersized equipment or control problems in the model, both of which undermine the validity of the comparison.
Reference: Section G3.1.2.3
How do baseline fan systems operate during occupied and unoccupied hours?
Supply and return fans operate continuously during occupied hours and cycle to meet loads during unoccupied hours. Exceptions: zones with health and safety-mandated minimum ventilation during unoccupied hours keep fans running continuously. For Systems 6 and 8, only the terminal-unit fan and reheat coil energize during unoccupied heating, not the entire air-handling system. This reflects standard practice for VAV systems with fan-powered boxes.
Reference: Section G3.1.2.4
Ventilation Requirements
Are minimum ventilation rates the same in proposed and baseline designs?
Generally yes, minimum ventilation system outdoor air intake flow is the same for both. Exceptions permit differences for demand-controlled ventilation in qualifying systems, for improved zone air distribution effectiveness per Standard 62.1, when the proposed design exceeds code or rating authority requirements (baseline uses the greater of code or rating authority), and for laboratory spaces prohibited from recirculating return air (baseline uses 100% outdoor air). These exceptions recognize legitimate efficiency strategies while preventing ventilation from becoming a gaming variable.
Reference: Section G3.1.2.5
When can demand-controlled ventilation create differences between proposed and baseline ventilation rates?
DCV is permitted in systems with outdoor air capacity ≤1,400 L/s serving areas with average design capacity of 100 people per 93 m² or less. This exception recognizes that DCV in appropriate applications is a legitimate efficiency measure, but it's limited to spaces with variable occupancy where the energy savings are meaningful and the occupancy density justifies the control complexity.
Reference: Exception to G3.1.2.5, Item 1
How does zone air distribution effectiveness affect ventilation rates?
When designing per Standard 62.1 Section 6.2 Ventilation Rate Procedure, zones with Ez > 1.0 can use reduced ventilation in the proposed design. The baseline calculates ventilation using the proposed design's Ventilation Rate Procedure but changes Ez to 1.0 in zones where it exceeded 1.0. The complete calculations must be submitted to the rating authority. This credits better air distribution without allowing unrealistic baseline assumptions.
Reference: Exception to G3.1.2.5, Item 2
Economizers and Free Cooling
Which baseline systems include air economizers?
Economizers are excluded from Systems 1, 2, 9, and 10. Integrated air economizer control is included in Systems 3 through 8 and 11, 12, and 13 based on climate per Table G3.1.2.6. Climate zones 0A, 0B, 1A, 1B, 2A, 3A, and 4A show "NR" (not required) for economizers; all other zones include them. The climate dependency reflects where outdoor air economizing actually saves energy rather than adds to cooling loads.
Reference: Section G3.1.2.6; Table G3.1.2.6
What exceptions permit excluding economizers from the baseline?
Economizers are excluded for: systems with gas-phase air cleaning per Standard 62.1 Section 6.1.2 (only if the proposed design doesn't match building design), systems where outdoor air affects supermarket open refrigerated casework (only if proposed design doesn't use an economizer), and systems serving computer rooms per Section G3.1.2.6.1. These exceptions recognize applications where economizers create operational problems that outweigh energy benefits.
Reference: Exception to G3.1.2.6
How are economizers handled for computer room systems?
Computer rooms using System 3 or 4 don't get economizers in the baseline. Computer rooms using System 11 must include an integrated fluid economizer meeting Section 6.5.1.2 requirements in the baseline. This distinction reflects that air-side economizing in computer rooms often causes humidity control problems, while waterside economizing with System 11 provides free cooling without those issues.
Reference: Section G3.1.2.6.1
What economizer high-limit shutoff is specified for the baseline?
The baseline uses a dry-bulb fixed switch with setpoint temperatures from Table G3.1.2.7: 24°C for climate zones 2B, 3B, 3C, 4B, 4C, 5B, 5C, 6B, 7, and 8; and 21°C for zones 5A and 6A. This standardizes the baseline economizer control strategy rather than allowing optimization that would make the baseline unrealistically efficient.
Reference: Section G3.1.2.7; Table G3.1.2.7
Baseline System Airflow and Fan Power
How are baseline design airflow rates determined for most systems?
For all systems except 9 and 10, design supply airflow is based on an 11°C supply-air-to-room temperature differential, minimum outdoor airflow, or airflow required by codes/accreditation standards, whichever is greater. For multiple zone setpoints, use the setpoint resulting in the lowest cooling or highest heating supply air setpoint. Return or relief fans, if specified in the proposed design, are sized for supply fan quantity less minimum outdoor air, or 90% of supply fan quantity, whichever is larger.
Reference: Section G3.1.2.8.1
What exceptions apply to baseline airflow calculations?
Laboratory spaces use a 9°C supply-air-to-room temperature differential (rather than 11°C), or required ventilation/makeup air, whichever is greater. If the proposed design airflow based on latent loads exceeds that based on sensible loads, the same supply-air-to-room-air humidity ratio difference used for the proposed design applies to the baseline. These exceptions recognize that labs have different thermal profiles and that latent loads sometimes govern in humid climates.
Reference: Exception to G3.1.2.8.1
How are baseline design airflow rates determined for Systems 9 and 10?
Airflow is based on the temperature difference between 41°C supply air setpoint and the design space heating temperature setpoint, minimum outdoor airflow, or airflow required by codes/accreditation standards, whichever is greater. If the proposed design includes fans sized and controlled for non-mechanical cooling, the baseline includes a separate fan sized and controlled identically. This reflects heating-only systems with optional ventilation cooling.
Reference: Section G3.1.2.8.2
How is baseline system fan power calculated?
Different formulas apply by system type. Systems 1 and 2: Pfan = (L/s) × 1.4158e-4. Systems 3-8 and 11-13: Pfan = input kW / fan motor efficiency, where input kW comes from Table G3.1.2.9 and motor efficiency from Table G3.9.1 for the next motor size greater than input kW. Systems 9 and 10 supply fan: Pfan = CFMs × 0.3. Systems 9 and 10 non-mechanical cooling fan: Pfan = CFMnmc × 0.054. The calculated power distributes to supply, return, exhaust, and relief fans in the same proportion as the proposed design.
Reference: Section G3.1.2.9; Section G3.1.2.9.1
How is baseline fan motor input power determined for VAV systems?
Table G3.1.2.9 provides equations. For constant-volume Systems 3, 4, 12, and 13: kWi = L/s × 0.0015 + A. For variable-volume Systems 5-8: kWi = L/s × 0.0021 + A. For System 11: kWi = L/s × 0.001 + A. In all cases, A is calculated per Section 6.5.3.1.1 using pressure-drop adjustments from the proposed design and the baseline system design flow rate. Pressure-drop adjustments for evaporative coolers or heat recovery devices not required in the baseline are excluded.
Reference: Table G3.1.2.9
Exhaust Air Energy Recovery
When is exhaust air energy recovery required in the baseline?
Individual fan systems with design supply air capacity ≥2,400 L/s AND minimum design outdoor air supply ≥70% must include energy recovery with at least 50% enthalpy recovery ratio. The system must include bypass or control to permit air economizer operation where applicable. This requirement targets systems where the outdoor air fraction makes heat recovery economically compelling.
Reference: Section G3.1.2.10
What exceptions permit excluding energy recovery from the baseline?
Seven exceptions apply: systems serving spaces not cooled and heated to less than 16°C; systems exhausting toxic, flammable, corrosive fumes, paint, or dust (only if proposed design doesn't use recovery); Type 1 commercial kitchen hoods per NFPA 96 (only if proposed design doesn't use recovery); heating systems in climate zones 0-3; cooling systems in zones 3C, 4C, 5B, 5C, 6B, 7, and 8; where the largest exhaust source is less than 75% of design outdoor airflow (only if proposed doesn't use recovery); and systems requiring dehumidification with series energy recovery (only if proposed doesn't use series recovery). These recognize applications where heat recovery is impractical or climate makes it ineffective.
Reference: Exception to G3.1.2.10
System-Specific Baseline Requirements
How are heat pumps modeled in baseline Systems 2 and 4?
Electric air-source heat pumps include electric auxiliary heat and an outdoor air thermostat. Auxiliary heat energizes only when outdoor temperature is below 4°C. The heat pump continues operating while auxiliary heat is energized, not as a lockout. This reflects standard heat pump control strategies where supplemental heat assists rather than replaces the heat pump.
Reference: Section G3.1.3.1
How are boilers configured in baseline Systems 1, 5, 7, 11, and 12?
The boiler plant is natural draft except for purchased heat applications. Plants serving ≤1,400 m² are modeled with a single boiler; plants serving >1,400 m² use two equally sized boilers staged as required by load. Hot-water design supply temperature is 82°C with 54°C return. Supply temperature resets based on outdoor dry-bulb: 82°C at -7°C and below, 66°C at 10°C and above, ramping linearly between. The reset exception: systems served by purchased heat don't reset.
Reference: Sections G3.1.3.2, G3.1.3.3, G3.1.3.4
How are hot-water pumps modeled in the baseline?
Baseline hot-water pump power is 300 W·s/L (220 W·s/L for purchased heat systems). The pumping system is primary-only with continuous variable flow and 25% minimum design flow. Systems serving ≥11,000 m² use variable-speed drives; systems serving <11,000 m² ride the pump curve. This represents reasonable baseline pumping configurations without crediting advanced strategies like primary-secondary arrangements.
Reference: Section G3.1.3.5
Are piping losses modeled in the baseline?
No. Piping losses are not modeled in either the proposed or baseline design for hot-water, chilled-water, or steam piping. This applies to Systems 1, 5, 7, 8, 11, 12, and 13. The exclusion is pragmatic because reliable piping loss calculations require installation details rarely available during design, and results would be highly assumption-dependent.
Reference: Section G3.1.3.6
How are chillers configured in baseline Systems 7, 8, 11, 12, and 13?
Electric chillers are used regardless of the actual cooling energy source (even for direct-fired absorption or absorption from purchased steam). The number and type follow Table G3.1.3.7: for <528 kW, one water-cooled screw chiller; for 528-1,055 kW, two equally sized water-cooled screw chillers; for ≥1,055 kW, two or more water-cooled centrifugal chillers with no chiller larger than 2,813 kW, all sized equally. Exception: purchased chilled water follows Section G3.1.1.3 instead.
Reference: Section G3.1.3.7; Table G3.1.3.7
What chilled-water temperatures and reset schedules apply to the baseline?
Design supply temperature is 6.7°C with 13°C return. Supply temperature resets based on outdoor dry-bulb: 7°C at 27°C and above, 12°C at 16°C and below, ramping linearly between. Exceptions: for computer room systems, reset higher based on the system requiring most cooling until one valve is nearly wide open (maximum 12°C reset); purchased chilled water systems don't reset.
Reference: Sections G3.1.3.8, G3.1.3.9
How are chilled-water pumps configured in the baseline?
Systems use primary/secondary configuration with constant-flow primary and variable-flow secondary. For ≥1,055 kW capacity, secondary pumps have variable-speed drives with 25% minimum flow. For <1,055 kW capacity, secondary pumps ride the curve. Primary pump power is 140 W·s/L; secondary pump power is 210 W·s/L at design. For System 11 with integrated fluid economizer, add 48 W·s/L to primary pump power for economizer flow. Exception: purchased chilled water uses variable-speed building distribution pump with 25% minimum flow and 250 W·s/L pump power.
Reference: Section G3.1.3.10
How are heat rejection devices modeled in the baseline?
The baseline uses axial-fan open-circuit cooling towers with variable-speed fan control and 3.23 L/s·kW efficiency at Table 6.8.1-7 conditions. Condenser-water design supply temperature is calculated using tower approach to 0.4% evaporation design wet-bulb: Approach + 5.6°C Range = 10.02 - (0.24 × WB), valid for wet-bulb 12.8°C to 32.2°C. The tower floats leaving water temperature per Table G3.1.3.11 up to design temperature. Condenser-water pump power is 300 W·s/L, constant volume. For System 11 with fluid economizer, add 48 W·s/L for economizer flow. Each chiller has separate condenser-water and chilled-water pumps interlocked with the chiller.
Reference: Section G3.1.3.11; Table G3.1.3.11
What supply air temperature reset applies to baseline Systems 5 through 8 and 11?
Cooling supply air temperature resets 2.3°C higher under minimum cooling load conditions. This modest reset reflects basic control optimization without crediting sophisticated reset strategies.
Reference: Section G3.1.3.12
What VAV minimum flow setpoints apply to baseline Systems 5 and 7?
Minimum volume setpoints for VAV reheat boxes are 30% of zone peak airflow, minimum outdoor airflow, or airflow required by codes/accreditation standards, whichever is larger. Exception: laboratory spaces reduce exhaust and makeup air during unoccupied periods to the largest of 50% zone peak airflow, minimum outdoor airflow, or required code/accreditation airflow.
Reference: Section G3.1.3.13
How are fan-powered boxes controlled in baseline Systems 6 and 8?
Parallel VAV fan-powered box fans run as first-stage heating before the reheat coil energizes. Fans are sized for 50% of peak design primary air (from the VAV air-handling unit) and modeled with 0.74 W per L/s fan power. Minimum volume setpoints equal 30% of peak primary airflow or minimum outdoor air ventilation requirement, whichever is larger. Supply air temperature setpoint is constant at design condition.
Reference: Section G3.1.3.14
How is VAV fan part-load performance modeled in baseline Systems 5 through 8 and 11?
VAV supply fans have variable-speed drives with part-load performance modeled using either Method 1 (tabular data from Table G3.1.3.15) or Method 2 (polynomial equation: Pfan = 0.0013 + 0.1470 × PLRfan + 0.9506 × PLRfan² - 0.0998 × PLRfan³). Both methods represent typical VFD fan performance curves. The choice depends on what the simulation program accepts.
Reference: Section G3.1.3.15; Table G3.1.3.15
What schedules apply to computer room equipment?
Computer room equipment is modeled as a constant fraction of peak design load per monthly schedule: Months 1, 5, 9 at 25%; Months 2, 6, 10 at 50%; Months 3, 7, 11 at 75%; Months 4, 8, 12 at 100%. This creates an annual load profile without requiring detailed operational data, reflecting the reality that computer rooms rarely operate at constant full load year-round.
Reference: Section G3.1.3.16
How are System 11 supply air temperature and fan volume controlled?
Minimum volume setpoint is 50% of maximum design airflow, minimum outdoor airflow, or code/accreditation-required airflow, whichever is largest. Fan volume resets from 100% airflow at 100% cooling load to minimum airflow at 50% cooling load. Supply air temperature resets from minimum supply temperature at 50% cooling load and above to space temperature at 0% cooling load. In heating mode, supply temperature modulates to maintain space temperature while fan volume fixes at minimum airflow.
Reference: Section G3.1.3.17
How is dehumidification modeled in baseline Systems 3 through 8 and 11, 12, and 13?
If the proposed design has humidistatic controls, the baseline uses mechanical cooling for dehumidification with reheat available to avoid overcooling. When the baseline system doesn't comply with Section 6.5.2.3 exceptions, only 25% of system reheat energy is included in baseline performance. The reheat type matches the system heating type. This limits baseline credit for inefficient simultaneous heating and cooling while recognizing that some reheat is necessary for proper dehumidification.
Reference: Section G3.1.3.18
How are preheat coils modeled in baseline Systems 5 through 8?
The baseline includes a preheat coil controlled to a fixed setpoint 11°C less than the design room heating temperature setpoint. This represents typical freeze protection and cold-deck temperature control without crediting reset strategies or eliminating the preheat coil entirely.
Reference: Section G3.1.3.19
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