Energy Cost Budget Method FAQ's

General Questions

What is the Energy Cost Budget Method and when can it be used?

The Energy Cost Budget Method is an alternative compliance path to the prescriptive requirements in ASHRAE 90.1. You calculate energy costs for both your actual design (design energy cost) and a budget building that meets minimum prescriptive requirements (energy cost budget), then compare them. If your design costs less to operate, you're compliant. The method works for any building with mechanical systems—if you're designing something without HVAC, you're stuck with prescriptive compliance.

Reference: Section 11.1.1

Can the Energy Cost Budget Method be used for partial building permits?

Yes, but with restrictions. When the permit covers less than the whole building, only parameters related to the permitted work can vary between the proposed and budget designs. Everything else—existing conditions and future components—must be identical in both models. Future building components still need to meet prescriptive requirements from Sections 5.5, 6.5, 7.5, and either 9.5 or 9.6. You can't game the system by claiming credit for work that's not part of this permit.

Reference: Section 11.1.2

What's the envelope limitation for new buildings or additions?

You cannot submit Energy Cost Budget Method results for building permit approval before the building envelope design has been submitted and approved. This prevents designers from promising an efficient envelope to justify an inefficient HVAC system, then value-engineering the envelope later. The envelope gets locked in first, then you can trade off other systems against the budget.

Reference: Section 11.1.3

What are the mandatory compliance requirements beyond just meeting the energy cost budget?

Five requirements apply. First, you must comply with all mandatory provisions in Sections 5.2.1, 6.2.1, 7.2.1, 8.2.1, 9.2.1, and 10.2.1. Second, design energy cost cannot exceed the energy cost budget. Third, installed component efficiencies must meet or exceed what you claimed in the design energy cost calculation. Fourth, you must meet verification, testing, and commissioning requirements from Section 4.2.5. Fifth, any systems or controls without criteria in Sections 5 through 10 need verification or testing per Section 4.2.5. Trading off to compliance doesn't exempt you from building what you said you would.

Reference: Section 11.2

Are the energy cost calculations predictions of actual building performance?

No, explicitly not. The informative note is clear about this. The calculations determine code compliance, period. Actual energy consumption will differ due to variations in occupancy, operations and maintenance, weather, energy uses not covered by the standard, changes in utility rates between design and occupancy, and calculation tool precision. Don't use these numbers for payback calculations or energy budgeting. They're for comparing one design approach against a standard reference, not forecasting actual bills.

Reference: Informative Note to Section 11.2

Can energy used for vehicle charging be excluded from the models?

Yes. The exception to Section 11.5.1 explicitly excludes energy used to recharge or refuel vehicles used for off-building site transportation. This makes sense because vehicle charging isn't a building system and varies based on factors completely outside the building's design. A building with electric fleet vehicles shouldn't be penalized in the energy cost comparison.

Reference: Exception to Section 11.5.1

Simulation Program Requirements

What are the minimum capabilities required for the simulation program?

The program must handle 8,760 hours per year with hourly variations in occupancy, lighting power, equipment power, thermostat setpoints, and HVAC operation, all defined separately for each day of the week and holidays. It needs to model thermal mass effects, ten or more thermal zones, part-load performance curves for mechanical equipment, capacity and efficiency correction curves for heating and cooling equipment, air-side and fluid economizers with integrated control, and the budget building design characteristics unless otherwise specified. It must either directly determine energy costs or produce hourly energy use reports by energy source for separate cost calculations. The program also needs to perform design load calculations for equipment sizing and flow rates.

Reference: Section 11.4.1.1, 11.4.1.2, 11.4.1.3

What testing requirements apply to the simulation program?

The program must be tested according to ASHRAE Standard 140, except Sections 7 and 8. Test results and modeler reports must be posted on a publicly available website, including results from other programs in Standard 140 Annexes B8 and B16. When results exceed maximum values, fall below minimum reference values, or are missing, the modeler report from Standard 140 Annex A2 Attachment A2.7 must be completed. There are no pass/fail criteria—the requirement is disclosure, not perfection. You get to see how your simulation program compares to others on standardized tests.

Reference: Section 11.4.1.4

What climatic data must be used in simulations?

Hourly values of temperature, humidity, solar radiation, wind speed, and wind direction from representative climatic data for the building's location. When multiple sources are available or when data don't exist for the exact site, the designer selects weather data that best represent the construction site conditions. The selected data need approval from the authority having jurisdiction. Both the proposed design and budget building must use the same weather data—you can't optimize one model with favorable weather assumptions.

Reference: Section 11.4.2; Section 11.4.4

How are renewable, recovered, and purchased energy handled?

Site-recovered energy isn't considered purchased energy and gets subtracted from proposed design energy consumption before calculating design energy cost. On-site renewable energy also gets subtracted, provided the building owner owns the system, has signed a lease for at least 15 years, or has a contractual agreement to purchase the energy for at least 15 years. The reduction from on-site renewable energy cannot exceed 5% of the calculated energy cost budget. The budget building uses the backup energy source (or electricity if no backup is specified) when the proposed design uses on-site renewable or site-recovered energy. For on-site generation that isn't renewable, the budget building includes the same generation systems excluding site-recovered energy.

Reference: Section 11.4.3.1, 11.4.3.2

What consistency requirements apply to compliance calculations?

Both the design energy cost and energy cost budget must use the same simulation program, same weather data, and same purchased energy rates. You can't shop around for the combination that makes your design look best. The comparison only works if the playing field is level.

Reference: Section 11.4.4

What's required for exceptional calculation methods?

When the simulation program can't model a design, material, or device, you need an exceptional calculation method approved by the authority having jurisdiction. Each item that can't be modeled gets calculated separately with exceptional savings determined individually. Applications for approval must include theoretical and empirical information verifying accuracy, step-by-step documentation detailed enough to reproduce results, copies of all calculation spreadsheets, a sensitivity analysis showing how energy consumption varies when estimated input parameters range from half to double the assumed value, calculations performed on a time-step basis consistent with the simulation program, and the energy cost budget and design energy cost calculated both with and without the exceptional methods. It's thorough documentation or nothing.

Reference: Section 11.4.5

Budget Building HVAC Systems

How are budget building HVAC system types determined?

Start with Figure 11.5.2. First decision: is the proposed design condenser water-cooled or evaporatively cooled (enter at "Water") or air-cooled (enter at "Air/None")? Closed-circuit dry coolers count as air-cooled. District cooling is treated as water-cooled. No mechanical cooling or systems not requiring heat rejection use "Air." Ground-source or groundwater-source heat pumps get budget System 6 (water-source heat pump). Second decision: heat source. Electric resistance, heat pump, or fuel-fired. District heating is treated as fossil fuel. No heating capability is treated as fossil fuel. For mixed fuel heating, the secondary source (smallest total installed output capacity) models identically in the budget building, while the primary source determines the budget system type via Figure 11.5.2. Third decision: system category. Single-zone residential systems serve residential spaces. Single-zone nonresidential systems serve other spaces. Everything else goes under "All Other."

Reference: Section 11.5.2(j)

What happens with components and parameters not specifically listed in Section 11.5.2?

They model identically to the proposed design, except where Sections 6.4 and 6.5 have specific requirements—then the budget building component efficiency adjusts to the lowest efficiency level allowed for that component type. This is important. If your proposed design has a feature that's better than code minimum but the code doesn't specifically address it in the budget building rules, the budget building gets the same feature. You don't automatically get credit for everything that's not explicitly called out.

Reference: Section 11.5.2(a)

How are minimum equipment efficiencies determined for the budget building?

All HVAC and service water-heating equipment models at minimum efficiency levels (both part-load and full-load) from Sections 6.4 and 6.4. Chillers use Path A efficiencies from Table 6.8.1-3. For package equipment where efficiency ratings include supply fan energy, the rating adjusts to remove fan energy using specific equations in Section 11.5.2(c) based on the budget system type. When multiple zones combine into a single thermal block, efficiencies for budget Systems 6, 8, and 10 are based on 2.6 kW equipment capacity for residential spaces or the thermal block capacity divided by the number of zones for other spaces. Systems 3, 4, 9, and 11 efficiencies are based on cooling equipment capacity of a single floor when grouping identical floors. Fan energy models separately per Section 11.5.2(h).

Reference: Section 11.5.2(b), 11.5.2(c)

How are outdoor air ventilation rates handled in the budget building?

Minimum outdoor air ventilation rates are the same for both proposed design and budget building. Exhaust air heat recovery models for the budget building per Section 6.5.6.1. Exceptions permit differences when modeling demand-controlled ventilation in the proposed design for spaces where it's not required by Section 6.4.3.8, or when the proposed design provides outdoor air exceeding Section 6.5.3.7 requirements (the budget building reflects the minimum required by Section 6.5.3.7).

Reference: Section 11.5.2(d)

How are economizers handled in the budget building?

Budget building systems listed in Table 11.5.2-1 have air economizers or fluid economizers, the same as in the proposed design, per Section 6.5.1. The high-limit shutoff follows Table 11.5.2-4—air economizers use Table 6.5.1.1.3, fluid (integrated) economizers shut off when operation will no longer reduce HVAC system energy. Notice that this differs from Appendix G's approach. The budget building economizer matches the proposed design's economizer choice, not a prescriptive lookup table.

Reference: Section 11.5.2(e); Table 11.5.2-4

How are preheat coils and supply airflow rates determined?

If the proposed design has a preheat coil, the budget building models one controlled the same way. For supply airflow, budget building design rates are 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 supply air setpoint or highest heating supply air setpoint. If the proposed design has return or relief fans, the budget building models them sized for supply fan quantity less minimum outdoor air, or 90% of supply fan quantity, whichever is larger. Exceptions: laboratory spaces use 9°C differential (or required ventilation/makeup air, whichever is greater), and if proposed design airflow is governed by latent loads exceeding sensible loads, use the same supply-air-to-room-air humidity ratio difference for the budget building.

Reference: Section 11.5.2(f), 11.5.2(g)

How is fan system efficiency determined for the budget building?

Fan system efficiency (input kW per L/s of supply air, including belt losses but excluding motor and drive losses) is the same as the proposed design or up to the limit in Section 6.5.3.1, whichever is smaller. If the limit is reached, each fan is proportionally reduced in input kW until the limit is met. Fan electrical power is then determined by adjusting the calculated fan kW by minimum motor efficiency from Section 10.4.1 for the appropriate motor size for each fan. You can't claim credit for a terrible fan system, but you don't get penalized for one that's merely okay.

Reference: Section 11.5.2(h)

How are equipment capacities sized in the budget building?

Budget building equipment capacities are sized proportionally to proposed design capacities based on sizing runs. The ratio between capacities used in annual simulations and capacities from sizing runs must be the same for both models. Unmet load hours cannot exceed 300 hours (of 8,760 simulated) for either design, and proposed design unmet hours cannot exceed budget building unmet hours. Alternatively, unmet hours exceeding these limits may be approved by the building official with sufficient justification that simulation accuracy isn't significantly compromised. This prevents undersizing equipment in the budget building to make the proposed design look better.

Reference: Section 11.5.2(i)

What special provisions apply to kitchen exhaust systems?

For kitchens with total exhaust hood airflow exceeding 2,400 L/s, the budget building uses demand ventilation on 75% of exhaust air. The system reduces exhaust and replacement air system airflow by 50% for half the kitchen occupied hours. If the proposed design uses demand ventilation, the same airflow schedule applies. Maximum exhaust flow rate for hoods must meet Section 6.5.7.2.2 requirements for the numbers and types of hoods and appliances in the proposed design. This credits demand ventilation in large commercial kitchens where it's a legitimate efficiency measure.

Reference: Section 11.5.2(k)

Budget Building System Descriptions

What are the characteristics of budget System 1 (VAV with parallel fan-powered boxes)?

This is a VAV system with chilled water cooling and electric resistance heating. Fans in parallel VAV fan-powered boxes are sized for 50% of peak design flow rate and modeled with 0.74 W per L/s fan power. Minimum volume setpoints for fan-powered boxes equal the minimum rate required for ventilation per Exception 1(b) to Section 6.5.2.1. Supply air temperature setpoint is constant at design condition. The VAV supply, return, or relief fan motor uses variable-speed drive and meets VAV fan part-load performance requirements from Section G3.1.3.15. If the proposed design has zone-level DDC, static pressure setpoint resets based on zone requirements per Section 6.5.3.2.3.

Reference: Table 11.5.2-1, System 1; Footnotes a, d

What are the characteristics of budget System 2 (VAV with reheat)?

VAV system with chilled water cooling and hot-water fossil fuel boiler heating. Minimum volume setpoints for VAV reheat boxes are the larger of: the minimum primary outdoor airflow required to meet Standard 62.1 Simplified Procedure ventilation requirements for the zone, or airflow required by codes/accreditation standards (including pressure relationships or minimum air change rates). Supply air temperature for cooling resets 2.8°C higher under minimum cooling load conditions. VAV fan requirements are the same as System 1—variable-speed drive, part-load performance per Section G3.1.3.15, and static pressure reset if the proposed design has zone DDC.

Reference: Table 11.5.2-1, System 2; Footnotes b, d

What are the characteristics of budget Systems 3 and 4 (Packaged VAV systems)?

System 3 uses direct expansion cooling with electric resistance heating. System 4 uses direct expansion cooling with hot-water fossil fuel boiler heating. Both are packaged VAV systems. The cooling fuel type matches the proposed design. VAV fan requirements mirror Systems 1 and 2. System 4 follows the VAV with reheat minimum volume setpoint and supply air temperature reset requirements. System 3 follows the parallel fan-powered box requirements. These are the packaged rooftop equivalents of the split Systems 1 and 2.

Reference: Table 11.5.2-1, Systems 3 and 4; Footnotes b, c, d

What are the characteristics of budget System 5 (Two-pipe fan coil)?

Two-pipe fan coil with single- or two-speed fan, chilled water cooling, and electric resistance heating. Fans operate as one or two speed as required by Section 6.5.3.2, regardless of what the proposed design uses. Fan operation is controlled the same as the proposed design—either whenever the space is occupied or cycled based on heating and cooling calls. This is a simple terminal unit system, appropriate for residential and similar applications.

Reference: Table 11.5.2-1, System 5; Footnotes e, i, j

What are the characteristics of budget System 6 (Water-source heat pump)?

Single- or two-speed fan, direct expansion cooling with electric heat pump heating, plus boiler. The cooling fuel type matches the proposed design. Water-source heat pumps connect to a common heat pump water loop controlled to maintain 16°C to 32°C. Heat rejection uses a closed-circuit axial-fan evaporative fluid cooler with fan-speed control per Section 6.5.5.2. Heat addition uses a boiler with the same fuel as the proposed design (natural draft), or fossil fuel if the proposed design has no boilers. Single boiler for plant load ≤176 kW, two equally sized boilers for >176 kW, staged as required. Loop flow is variable with shutoff at each heat pump when its compressor cycles off per Section 6.5.4.5. If the proposed design has no pumps, budget design pump power is 349 kW/1000 L/s (23 m head, 65% combined impeller and motor efficiency). Loop pumps ride the curve or use variable-speed drives when required by Section 6.5.4.2.

Reference: Table 11.5.2-1, System 6; Footnotes c, g, h, i, j

What are the characteristics of budget System 7 (Four-pipe fan-coil)?

Four-pipe fan-coil with single- or two-speed fan, chilled water cooling, and hot-water fossil fuel boiler heating. This is the heating-capable version of System 5, allowing simultaneous heating and cooling availability. Fan speed and operation requirements are the same as System 5.

Reference: Table 11.5.2-1, System 7; Footnotes e, f, i, j

What are the characteristics of budget System 8 (Packaged terminal heat pump)?

Single-speed fan, direct expansion cooling with electric heat pump heating. The cooling fuel type matches the proposed design. Electric air-source heat pumps model with electric auxiliary heat, controlled by a multistage space thermostat and outdoor air thermostat wired to energize auxiliary heat only on the last thermostat stage and when outdoor air temperature is below 4°C. Fan operation is controlled the same as the proposed design. This is a PTHP system, common in hotels and multifamily residential.

Reference: Table 11.5.2-1, System 8; Footnotes c, h, i

What are the characteristics of budget System 9 (Packaged rooftop heat pump)?

Single- or two-speed fan, direct expansion cooling with electric heat pump heating. Same auxiliary heat control as System 8—multistage thermostat and outdoor air thermostat limiting auxiliary heat to the last stage when outdoor temperature is below 4°C. Fans operate as one or two speed per Section 6.5.3.2. This is the packaged rooftop version of System 8.

Reference: Table 11.5.2-1, System 9; Footnotes c, h, i, j

What are the characteristics of budget System 10 (Packaged terminal air conditioner)?

Single-speed fan, direct expansion cooling, hot-water fossil fuel boiler heating. This is a PTAC system with separate heating source rather than a heat pump. Fan operation matches the proposed design.

Reference: Table 11.5.2-1, System 10; Footnotes f, i

What are the characteristics of budget System 11 (Packaged rooftop air conditioner)?

Single- or two-speed fan, direct expansion cooling, fossil fuel furnace heating. This is a standard packaged rooftop unit. Fans operate as one or two speed per Section 6.5.3.2. Fan operation matches the proposed design. This is probably the most common light commercial HVAC system type.

Reference: Table 11.5.2-1, System 11; Footnotes i, j

Chilled Water and Hot Water System Details

How are chilled water systems modeled when not using purchased chilled water?

The budget building chiller plant uses chillers with number per Table 11.5.2-2 (based on total plant load) and type per Table 11.5.2-3 (based on individual chiller load). For <1,055 kW: one chiller. For 1,055-2,110 kW: two equally sized chillers. For ≥2,110 kW: two minimum with chillers added so none exceeds 2,813 kW, all equally sized. Chiller types: ≤352 kW scroll electric or single-effect absorption direct-fired; >352 kW to <1,055 kW screw electric or double-effect absorption direct-fired; ≥1,055 kW centrifugal electric or double-effect absorption direct-fired. Mixed fuel sources in the proposed design result in budget building chillers with the same fuel types and proportional capacities. Chilled water is 6.7°C supply, 13°C return. Supply temperature resets per Section 6.5.4.4. Piping losses aren't modeled.

Reference: Table 11.5.2-1, Footnote e; Tables 11.5.2-2 and 11.5.2-3

How are chilled water pumps and controls modeled?

Pump system power for each pumping system matches the proposed design. If the proposed design has no chilled-water pumps, budget building pump power is 349 kW/1000 L/s (23 m head, 65% combined impeller and motor efficiency). The system is modeled as primary-only variable flow with flow maintained at design rate through each chiller using a bypass. Chilled-water pumps ride the curve or use variable-speed drives when required by Section 6.5.4.2. Each chiller has separate condenser water and chilled-water pumps interlocked to operate with the associated chiller.

Reference: Table 11.5.2-1, Footnote e

How are cooling towers and condenser water systems modeled?

Heat rejection uses open-circuit axial-fan cooling tower with variable-speed fan control (if required by Section 6.5.5) meeting Table 6.8.1-7 performance requirements. Condenser water design supply temperature is calculated using tower approach to 0.4% evaporation design wet-bulb with 5.6°C design temperature rise: Approach + 5.6°C Range = 10.02 - (0.24 × WB), where WB is 0.4% evaporation design wet-bulb in °C (valid for 12.8°C to 32.2°C). Except during economizer operation, the tower maintains leaving water temperature per Table 11.5.2-5 where weather permits, floating up to design leaving water temperature. If the proposed design has no condenser water pumps, budget design pump power is 301 kW/1000 L/s (18 m head, 60% combined impeller and motor efficiency).

Reference: Table 11.5.2-1, Footnote e; Table 11.5.2-5

How are hot water boiler systems modeled when not using purchased heat?

The boiler plant uses the same fuel as the proposed design and is natural draft. Single boiler for plant load ≤176 kW, two equally sized boilers for >176 kW, staged as required by load. Hot-water is 82°C supply, 54°C return. Supply temperature resets per Section 6.5.4.4. Piping losses aren't modeled. If the proposed design has no hot-water pumps, budget design pump power is 301 kW/1000 L/s (18 m head, 60% combined impeller and motor efficiency). The system is primary-only with continuous variable flow. Hot-water pumps ride the curve or use variable-speed drives when required by Section 6.5.4.2.

Reference: Table 11.5.2-1, Footnote f

Service Water Heating and Other Systems

How are service water-heating systems modeled in the budget building?

The budget building service water-heating system type is identical to the proposed design. Performance meets Sections 7.4 and 7.5 requirements. Exceptions: if the system type isn't listed in Table 7.8, it's determined from Table G3.1.1-2. Where Section 7.5 applies, the boiler splits into separate space-heating boiler and hot-water heater with efficiency requirements set to the least efficient allowed. For 24-hour facilities meeting Section 6.5.6.2 prescriptive criteria for condenser heat recovery, a system meeting those requirements is included in the budget building regardless of Section 6.5.6.2 exceptions. If the system can't be modeled, it becomes a prescriptive requirement with no heat recovery in either design. Energy consumption is calculated explicitly based on volume required and entering/leaving temperatures. Entering water temperatures are estimated based on location; leaving temperatures are based on end-use requirements. Piping losses aren't modeled in either design.

Reference: Table 11.5.1, Item 11

How are receptacle, motor, and process loads modeled?

These loads are estimated based on building area type or space type category and are assumed identical in proposed and budget building designs. All end-use load components within and associated with the building must be modeled unless specifically excluded, including exhaust fans, parking garage ventilation, exterior building lighting, swimming pool heaters and pumps, elevators and escalators, and cooking equipment. When systems covered by Sections 8 and 10 have been designed and submitted, they're determined per those sections. When not submitted, they comply with but don't exceed those sections' requirements. This prevents gaming the system by claiming exceptionally low plug loads.

Reference: Table 11.5.1, Item 12

How is refrigeration equipment modeled in the budget building?

When proposed design refrigeration equipment is rated per AHRI 1200, the rated energy use is modeled. Otherwise, actual equipment capacities and efficiencies are used. If refrigeration equipment is specified in the proposed design and listed in Table 6.8.1-13, the budget building models it per that table using actual equipment capacities. If equipment isn't listed in Table 6.8.1-13, the budget building models it the same as the proposed design. This is similar to Appendix G's approach.

Reference: Table 11.5.1, Item 13

What modeling exceptions are permitted for the proposed design?

Components and systems may be excluded from the proposed design simulation model if: (1) component energy use doesn't affect the energy use of systems and components being traded off, and (2) the applicable prescriptive requirements from Sections 5.5, 6.5, 7.5, and either 9.5 or 9.6 applying to the excluded components are met. No modeling exceptions exist for the budget building—it models everything. This lets you exclude peripheral systems that meet prescriptive requirements and don't interact with the systems you're trading off.

Reference: Table 11.5.1, Item 14

Lighting and Envelope

How is lighting power determined for the proposed design?

Where complete lighting systems exist, use actual lighting power for each thermal block. Where lighting has been designed, determine lighting power per Sections 9.1.3 and 9.1.4. Where no lighting exists or is specified, use the Building Area Method for the appropriate building area type. Lighting system power includes all components shown on plans—lamps, ballasts, task fixtures, furniture-mounted fixtures. For dwelling units, hotel/motel guest rooms, and other spaces with plug-in fixtures not shown on design documents, assume identical lighting power for proposed and budget designs. Lighting schedules reflect mandatory automatic controls from Section 9.4.1. Automatic daylighting controls may model directly or through schedule adjustments from separate analysis approved by the authority having jurisdiction, separately accounting for primary sidelighted, secondary sidelighted, and toplighted areas.

Reference: Table 11.5.1, Item 6

How are automatic lighting controls credited in the proposed design?

Manual-ON or partial-auto-ON occupancy sensors model by reducing the lighting schedule each hour by the occupancy sensor reduction factors in Table G3.7 for the applicable space type, multiplied by 0.25. Automatic lighting controls listed in Table 9.6.3 model using the sum of applicable control factors (CF). Apply control factors only to the wattage portion of fixtures in the space controlled by said lighting control. Divide each hour of the lighting schedule by (1 + CF), where CF indicates the sum of all applicable control factors for that space per Section 9.6.3 and Table 9.6.3. This differs from Appendix G's approach for crediting controls.

Reference: Table 11.5.1, Item 6(g)

How is lighting power determined for the budget building?

Where complete lighting systems exist, the budget building uses the same lighting power as the proposed design. Where lighting has been designed, interior lighting power allowance is determined using either Building Area Method or Space-by-Space Method with space use classification matching the proposed design. Lighting power equals the maximum allowed for the corresponding method and category in Section 9.2. Additional interior lighting power for non-mandatory controls allowed under Section 9.6.3 isn't included in the budget building. Dwelling units use 6.5 W/m². Where lighting neither exists nor is submitted with design documents, the budget building uses the same lighting power as the proposed design. Mandatory automatic controls from Section 9.4.1 model the same as the proposed design.

Reference: Table 11.5.1, Item 6

How is the building envelope modeled in the budget building?

The budget building has identical conditioned floor area, exterior dimensions, and orientations as the proposed design. Opaque assemblies (roofs, floors, doors, walls) have the same heat capacity as the proposed design but with minimum U-factor required in Section 5.5 for new buildings/additions or Section 5.1.3 for alterations. Exterior roof surfaces model with solar reflectance and thermal emittance required by Section 5.5.3.1.1(a). All other roofs, including those exempted from Section 5.5.3.1.1, model the same as the proposed design. No shading projections are modeled—fenestration is flush with the wall or roof. If fenestration area for new buildings/additions exceeds Section 5.5.4.2 maximum, the area is reduced proportionally along each exposure until the limit is met.

Reference: Table 11.5.1, Item 5

How are fenestration properties determined in the budget building?

Fenestration U-factor equals criteria from Tables 5.5-0 through 5.5-8 for the appropriate climate. SHGC equals criteria from the same tables for the appropriate climate. For table portions without SHGC requirements, SHGC equals that determined per Section C3.6(c). VT equals that determined per Section C3.6(c). If vertical fenestration area facing west or east exceeds Section 5.5.4.5 limits, the energy cost budget is generated by simulating the budget building design at actual orientation and again after rotating 90, 180, and 270 degrees, then averaging results. Building envelope alterations reflect Section 5.1.3 limitations on area, U-factor, and SHGC.

Reference: Table 11.5.1, Item 5(c)

When are skylights included in the budget building?

Skylights are included in each thermal block when required by Section 5.5.4.2.3. Exception: when trade-offs are made between an addition and existing building per the exception to Section 4.2.1.2, the building envelope assumptions for the existing building in the budget building design reflect existing conditions prior to any revisions in this permit. You don't get to claim credit against a hypothetical upgraded existing building envelope.

Reference: Table 11.5.1, Item 5(d)

Submittal Requirements

What documentation must be submitted for permit application?

Thirteen items are required: (1) energy cost budget for the budget building and design energy cost for the proposed design; (2) simulation program and version; (3) project overview including stories (above and below grade), typical floor size, building uses, gross area of each use, and whether each use is conditioned; (4) list of energy-related features included in the design and on which compliance is based, documenting all features that differ between models; (5) list showing proposed design compliance with all mandatory provisions from Sections 5.4, 6.4, 7.4, 8.4, 9.4, and 10.4; (6) building elevations and floor plans; (7) diagram showing thermal blocks used in the simulation.

Reference: Section 11.7.2(a) through (g)

What additional technical documentation is required?

Continuing from item 7: (8) explanation of significant modeling assumptions; (9) backup calculations and material supporting data inputs—U-factors for assemblies, NFRC ratings for fenestration, end uses identified in Table 11.5.1; (10) input and output reports from the simulation program, including energy usage breakdown by lights, internal equipment loads, service water-heating equipment, space-heating equipment, space cooling and heat-rejection equipment, fans, and other HVAC equipment (pumps, etc.). Output reports must show unmet load hours for both designs; (11) purchased energy rates used in simulations; (12) explanation of any error messages in the simulation output; (13) for exceptional calculation methods, documentation of predicted energy savings by energy type, energy cost savings, narrative explaining the method, and theoretical or empirical information supporting accuracy.

Reference: Section 11.7.2(h) through (m)

What final items complete the submittal requirements?

Items 14 and 15: (14) the reduction in design energy cost associated with on-site renewable energy; (15) the software version and link to the website containing ASHRAE Standard 140 results for the version used per Section 11.4.1.4. Completion requirements must comply with Sections 5.7.3, 6.7.3, 7.7.3, 8.7.3, 9.7.3, and 10.7.3. This is comprehensive documentation—you can't hand-wave any significant aspect of the analysis.

Reference: Section 11.7.2(n) through (o); Section 11.7.3

Schedules and Thermal Blocks

What schedule types are required and how must they be applied?

Schedule types listed in Section 11.4.1.1(b) are required input—hourly variations in occupancy, lighting power, miscellaneous equipment power, thermostat setpoints, and HVAC system operation, defined separately for each day of the week and holidays. Schedules must be typical of the proposed design as determined by the designer and approved by the authority having jurisdiction. Required schedules are identical for proposed design and budget building design. Temperature and humidity control setpoints, schedules, and throttling range are the same for both designs. HVAC fan schedules for fans providing outdoor air for ventilation run continuously when spaces are occupied and cycle to meet loads during unoccupied hours, with exceptions for spaces without installed systems, spaces with health and safety-mandated minimum ventilation during unoccupied hours, and dedicated outdoor air supply fans (which stay off during unoccupied hours). HVAC fans remain on during occupied and unoccupied hours in systems primarily serving computer rooms.

Reference: Table 11.5.1, Item 4

How are thermal blocks defined when HVAC zones are already designed?

Where HVAC zones are defined on HVAC design drawings, each zone models as a separate thermal block. Exceptions permit combining different zones into a single thermal block or identical thermal blocks with multipliers if all these conditions are met: (1) space-use classification is the same throughout, or all zones have peak internal loads differing by less than 31 W/m² from the average; (2) all zones adjacent to glazed exterior walls face the same orientation or orientations vary by less than 45 degrees; (3) all zones are served by the same HVAC system or same kind of system; (4) all zones have schedules differing by 40 or fewer equivalent full-load hours per week.

Reference: Table 11.5.1, Item 7

How are thermal blocks defined when HVAC zones are not yet designed?

Thermal blocks are defined based on similar internal load densities, occupancy, lighting, thermal and space temperature schedules, combined with specific requirements. Separate thermal blocks for interior spaces (>4.6 m from exterior or semiexterior wall) and perimeter spaces (≤4.6 m from exterior or semiexterior wall). No separate zone needed for areas adjacent to semiexterior walls separating semiheated from conditioned space. Separate thermal blocks for spaces adjacent to glazed exterior or glazed semiexterior walls—separate zone for each orientation, except orientations differing by less than 45 degrees may be considered the same. Floor area within 4.6 m of glazed perimeter walls with more than one orientation divides proportionately between zones. Separate thermal blocks for spaces with floors contacting ground or exposed to ambient. Separate thermal blocks for spaces with exterior ceiling or roof assemblies.

Reference: Table 11.5.1, Item 8

How are multifamily residential buildings modeled?

Residential spaces model using at least one HVAC zone per dwelling unit, except units with the same orientations may combine into one thermal block. Corner units and units with roof or floor loads only combine with units sharing these features. This prevents unrealistic aggregation of dissimilar units while allowing reasonable simplification of identical units.

Reference: Table 11.5.1, Item 9

Additions, Alterations, and Space Classification

How are additions and alterations handled in the models?

It's acceptable to demonstrate compliance using building models that exclude parts of the existing building if all these conditions are met: (1) work performed under the current permit in excluded parts meets Sections 5 through 10 requirements; (2) excluded parts are served by HVAC systems entirely separate from those serving included parts; (3) design space temperature and HVAC system operating setpoints and schedules on either side of the boundary between included and excluded parts are identical; (4) if declining block or similar utility rate is used in the analysis and excluded and included parts are on the same meter, the rate reflects the utility block or rate for the building plus the addition. The budget building excludes the same parts as the proposed design.

Reference: Table 11.5.1, Item 2

How are building and space use classifications determined?

Building area type or space type classifications are chosen per Section 9.5.1 or 9.6.1. The user or designer specifies space use classifications using either building area type or space type categories but doesn't combine the two types within a single permit application. More than one building area type category may be used for mixed-use facilities. Exception: where space types neither exist nor are designated in design documents, use type is specified per Section 9.5.1. The budget building uses the same space use classifications as the proposed design.

Reference: Table 11.5.1, Item 3

When the Energy Cost Budget Method is applied to buildings where energy-related features haven't been designed (like a lighting system), those yet-to-be-designed features are described in the proposed design to minimally comply with applicable mandatory and prescriptive requirements from Sections 5 through 10. Where space classification for a building isn't known, the building is categorized as an office building. You can use the Energy Cost Budget Method early in design, but you have to make assumptions about undesigned systems, and those assumptions can't be better than code minimum.

Reference: Table 11.5.1, Item 1(c)

What consistency is required between proposed design and design documents?

The simulation model of the proposed design must be consistent with design documents, including proper accounting of fenestration and opaque envelope types and areas, interior lighting power and controls, HVAC system types, sizes, and controls, and service water-heating systems and controls. All conditioned spaces in the proposed design are simulated as being both heated and cooled, even if no cooling or heating system is being installed. Temperature and humidity control setpoints and schedules, as well as temperature control throttling range, are the same for proposed design and budget building design. You can't model one thing and build another.

Reference: Table 11.5.1, Item 1(a) and (b)

PreviousASHRAE 90.1 2019 Appendix G FAQ's NextPerformance-Based Design Briefs

Last updated 3 days ago

Was this helpful?