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homes table 3


Inclusion of unfinished spaces in project scope

Unfinished spaces must be included in the energy model if they are part of the project scope of work. In core and shell projects, a large portion of the space may be unfinished; in new construction and major renovation projects these spaces must not make up more than 40% of the total space. In addition, all projects other than core and shell that have incomplete spaces must submit a letter of commitment, signed by the owner, confirming that the remaining incomplete spaces will satisfy the requirements of each prerequisite and credit achieved by this project if and when completed by the owner.

Modeling HVAC, lighting, hot water systems for unfinished spaces

Refer to ASHRAE 90.1–2010, Tables G3.1.6(c), G3.1.10(c) and (d), and G3.1.11(c), for unfinished space modeling requirements. If a lighting, HVAC, or service hot water system has not yet been designed, the system required in the baseline building for that unfinished space must also be modeled in the proposed building. Refer to Table G3.1.8 on how to model thermal zones for such space.

Example 1. A two-story office building has a ground-floor retail area that is entirely unfinished. The building contains a chase for future ductwork and a location on the roof for the mechanical equipment for the future tenant, but no system exists or has been specified.

In this case, the proposed HVAC system for that space must be modeled using the same HVAC system type, capacity ratios, efficiencies, and controls as those modeled for the baseline building.

Example 2. The same hypothetical two-story office building now includes chilled and hot water connections for the future unfinished retail space. A portion of the HVAC system has been designed.

The proposed building may be modeled as a system that uses the chilled and heating hot water (e.g., 4-pipe fan coil unit). However, because the air-handling units and terminal distribution have not yet been designed, the cooling and heating capacities, design fan volume, minimum volume, fan power, fan controls, etc., must be modeled identically in both the baseline and proposed model, and equal to the requirements of the baseline model.

Additions to existing buildings

Project teams wishing to certify an addition to an existing building must follow the Appendix G requirements in Table G3.1.2, the most important of which is (b). If the existing building will be excluded, then the HVAC system serving the addition to the building must be entirely separate from the systems serving the existing building. Refer to the table for all requirements.

Building envelope

Baseline building envelope

Construction type and maximum U-factors for baseline walls, roofs, and floors are specified by Table G3.1-5 Baseline (b). The constructions for walls, roofs, and floors are specified by the standard and do not depend on the proposed design. For example, if a building will have concrete masonry walls, the baseline model will still have steel-framed walls.

Existing building envelope

For an existing building that was conditioned before major renovation and will be conditioned postrenovation, the baseline building envelope should reflect the existing conditions, before the scope of work (Table G3.1-5 Baseline (f)).

However, for an existing building (or spaces in the building) that was previously unconditioned and is being renovated to include conditioning, the baseline building envelope (or the envelope for any previously unconditioned spaces in the building) must be modeled as if the building is new construction (i.e., according to Table 5.5).

Proposed model U-values

The proposed model must reflect the building as designed or built. To the extent possible, construction assemblies need to match the dimension and U-value inputs in the model.

Apply Appendix A to the proposed envelope. Provide the assembly U-value, rather than a point U-value, by determining the overall construction assembly U-value that takes into account for thermal bridging as shown in Appendix A.

Ensure that window U-values are input as the assembly U-value, which takes into account the U-value of the framing system. The center-of-glass value is not acceptable.

Baseline model U-values, semi-exterior surfaces

For the baseline envelope properties, use the semiheated requirements to model surfaces that adjoin unconditioned spaces to conditioned spaces (e.g., a wall separating a semiheated warehouse from a conditioned office) or semiheated space to conditioned space (e.g., the slab separating an unconditioned parking garage from the conditioned ground floor of the building). Figure 5.1 in 90.1–2010 illustrates this requirement.


Baseline HVAC system selection

The HVAC system for the baseline model must be selected based on requirements in ASHRAE 90.1–2010, Section G3.1.1. The system selected will depend on the proposed building type, size, and heat source. Building type must be based on predominant conditions (i.e., those that account for the majority or plurality of the building area), and no space types can be excluded from the model. Building size is determined from conditioned area. Once the floor area of the predominant condition is known, consult Table G3.1.1A to determine the predominant baseline HVAC system.

Section G3.1.1 also specifies whether HVAC systems must be modeled with a system per floor or a system per thermal block. Systems 1–4 are modeled with one system per thermal block and systems 5–10 with one system per floor, using systems 9 and 10 where applicable.

When multiple floors have identical thermal blocks, those floors may be combined in the energy model.

Note that a floor with a roof and a floor without a roof do not have identical thermal blocks and cannot be combined. A multistory building with identical thermal blocks would need to be modeled with no fewer than three floors: a ground floor, a middle floor with appropriate multiplier, and a top floor.

There are six exceptions to the baseline HVAC system determination. These exceptions are mandatory and must be taken if they are applicable to the project.

G3.1.1 exception (a). Check for nonpredominant conditions, such as nonresidential in a primarily residential building, or where a portion of a building is supplied by electric heat but the rest is from fossil fuels. The area of nonpredominant conditions can be deducted from the total area when determining the baseline HVAC system. If nonpredominant conditions apply to more than 20,000 ft2 (1860 m2), use exception (a) and select an additional baseline HVAC system type to serve those spaces.

Example. A 210,000 ft2 (19 510 m2) multifamily high-rise has 23,000 ft2 (2140 m2) of ground-floor retail space. The residential units are served by heat pump units with supplemental electric heat, and the retail areas are served by a split DX unit with fossil fuel furnace. The required baseline HVAC system for the residential spaces would be System 2–PTHP, but for the retail areas it would be System 3–Packaged DX with fossil fuel furnace, since the nonresidential spaces meet the 20,000 ft2 (1860 m2) exception (a) in G3.1.1.

G3.1.1 exception (b). If using systems 5, 6, 7, 8, 9 or 10, individual zones with atypical thermal loads or occupancy profiles must be modeled with individual single-zone systems of type 3 or 4, according to exception (b). Examples for this include computer server rooms, natatoriums, and school gymnasiums.

If this exception is not properly incorporated into the baseline model, the model results may show an unusually high number of unmet load hours or significantly oversize the baseline case systems. A good practice is to check the baseline output reports and verify that the thermal loads for each thermal block do not vary by more than 10 Btuh/ft2 (31.5 W/m2) from the average of the other thermal zones on the floor, and adjust the baseline model as necessary to include this exception.

Baseline HVAC system selection (laboratory spaces)

G3.1.1 exception (c). If laboratory spaces in the building have a total laboratory exhaust rate greater than 5,000 cfm (2360 L/s), a single system of type 5 or 7 must be modeled to serve only those spaces. Section G3.1.2.11 requires exhaust air energy recovery in accordance with Section, which is likely to include laboratories.

Baseline HVAC system selection (kitchens)

G3.1.1 exception (d). If kitchens in the building have a total exhaust hood airflow rate greater than 5,000 cfm (2360 L/s), system type 5 or 7 must be modeled and must include demand-controlled ventilation.

Baseline HVAC system selection: heated-only storage or circulation spaces

G3.1.1 exception (e). Heating-only systems serving rooms not exhausting or transferring air from mechanically cooled spaces, such as storage rooms, stairwells, or mechanical rooms, should be modeled as system 9 or 10.

G3.1.1 exception (f). When the predominant system is type 9 or 10, any fully conditioned spaces (such as an small, fully conditioned office in a heated-only warehouse) should be modeled using the appropriate system type for the size, number of floors, occupancy type, and heating type for the nonpredominant area of the building.

Baseline HVAC system fuel type

Any project with a combination of fossil fuel and electric heat serving the same space must use the fossil fuel baseline HVAC system (systems 1, 3, 5, and 7) unless it meets one of the exceptions to G3.1.1.

Example. A building has been designed with electric water-source heat pumps for the space loads. A 100% outdoor air gas-fired rooftop unit provides ventilation. The spaces are served by both electric heating from the heat pumps and ventilation air from the gas-fired unit; therefore, the spaces are considered hybrid heating and must model the baseline HVAC system type as “Fossil Fuel, Fossil/Electric Hybrid, and Purchased Heat” (from Table G3.1.1a).

In the case of electric heating equipment designed with a fossil fuel preheat coil, or a backup fossil fuel boiler, the intent is that the equipment will be used; thus it is considered hybrid heating, and the team must use the fossil fuel baseline heating system.

Baseline fan power

The baseline fan power is calculated according to Section G3.1.2.10, which indicates that the system fan power is based on the supply airflow and distributed to supply, return, exhaust, and relief fans. If the proposed system has additional return, exhaust, and/or relief fans, the team may not adjust the baseline model to account for the additional fan power. Section G3.1.2.10 also includes Table G3.1.2.9, whose value A is calculated according to Section using pressure drop adjustments. Pressure drop adjustments may not be taken for system types 1, 2, 9, or 10.

The calculations are straightforward, but a common issue involves pressure credits. Table G3.1.2.9 allows pressure drop adjustments for evaporative coolers or heat recovery devices only when they are required in the baseline building system. Also, the pressure drop adjustment is applicable only to the design airflow through each device.

For example, if only the ventilation air is filtered with a MERV 13 filter, then only the ventilation airflow rate may apply the 0.9 in. w.c. (224.2 Pa) adjustment, not the entire supply airflow rate.

Pressure credit may be taken only for those systems present in the proposed building.

The credit for fan power allowance cannot be based on plenum return. The credit can be applied only when the return is fully ducted; systems that have a combination of ducted and nonducted may not use this pressure credit.

For return or exhaust airflow control devices (which maintain a specific pressurization relative to other spaces), a project team claiming this credit in spaces other than a laboratory, hospital, or similar space type must provide evidence of this control device. The credit may be applied only for the amount of airflow passing through the control device.

A project team using the modeling software to automatically determine the baseline building fan power must ensure that the correct allowance has been calculated. Publicly available fan power calculators can be used to verify and determine the correct fan power.

Proposed HVAC system sizing

Table G3.1.1(a) requires that the proposed building be consistent with the design documents, including envelope, lighting, HVAC, and service hot water systems. Additionally, all end-use load components within and associated with the building must be modeled.

Table G3.1.10(b) requires that the HVAC model be consistent with the design documents. All modeled HVAC system parameters (e.g., fan volumes, fan powers, efficiencies, heating and cooling capacities) must be consistent with the mechanical schedules and drawings. The simulation should never be allowed to automatically size the HVAC system for the proposed case model when there is a complete design.

Heat pumps (operation)

Section G3.1.3.1 describes the operation of baseline building heat pumps. The heat pump and auxiliary heat should operate together at low-temperature conditions, with the compressor as the lead machine. The outside air cutoff temperature for the compressor must be no greater than the temperature associated with the low-temperature heating efficiency requirements of Table 6.8.1B (17ºF) (–8.3ºC). The HSPF rating for packaged heat pump units smaller than 65,000 Btuh (19 kW) and packaged terminal heat pumps accounts for electric auxiliary operation and includes test conditions at 17 degrees F (–8.3ºC). The heat pump efficiency curves in the model should reflect the heat pump ratings that account for simultaneous operation of the electric resistance and heat pump elements below 40ºF (4.4ºC).

Unitary heating and cooling efficiencies

Use the correct Table 6.8.1 to determine equipment efficiencies:

· Table 6.8.1A for system types 3, 5 and 6

· Table 6.8.1B (with electric resistance heating section) for system Type 4

· Table 6.8.1D for system types 1 and 2

These efficiencies are based on the capacity of each system individually, not a sum of all units. It is important to correctly adjust efficiencies of each piece of equipment to separate fan power at AHRI rating conditions, per Section G3.1.2.1. Most simulation software programs can perform this step automatically.

Humidity controls

Humidification must be modeled identically in the baseline and the proposed models, since it is not addressed in Appendix G. Use the exceptional calculation method if claiming savings.

If the proposed design includes dehumidification controls, they must be modeled as designed. Dehumidification controls may be modeled in the baseline only if one of the exceptions to Section applies. Exception (d) for process dehumidification does not apply to computer rooms.

Table G3.1.4 requires that identical schedules be used in both models, and this includes humidity setpoints. A problem may arise if the proposed building has a dedicated outdoor air system (DOAS) that maintains proper humidity. PTAC or small DX systems in the baseline design may not be able to maintain both temperature and humidity simultaneously in the same way that the proposed system can. The project team may then incur a penalty for higher humidity levels in the baseline building.

In this situation, model a DOAS in the baseline design using the same volume of outdoor air as for the proposed design, but with the same efficiency and efficiency curves as the baseline HVAC systems. Additionally, the baseline fan power allowance would be separated between the DOAS and the baseline system using the same ratio as the proposed system.


Ventilation rate inputs

Table G3.1.10(b) requires that the proposed building ventilation rate be consistent with the rate indicated on the mechanical schedule. Section G3.1.2.6 requires that the ventilation rate be identical between the proposed and baseline buildings and states that reduced ventilation “is not considered an opportunity for energy savings under the Performance Rating Method”; ventilation is energy neutral, per the User’s Manual. However, there are exceptions to this requirement.

Ventilation (above minimum required)

Exception (c) penalizes projects for providing more ventilation air to the space than is required by ASHRAE 62.1–2010 or a local code, whichever requires more ventilation air. If the proposed project provides outdoor air in excess of the amount required, the baseline must be modeled with the required ventilation rates, which will be lower than the proposed ventilation rate. This creates an “energy penalty” for the additional fan and conditioning energy.

For various reasons, however, it is common practice to specify slightly more ventilation air than required. A project team that has specified up to 5% more total ventilation air than required may model identical ventilation rates.

If exhaust requirements dictate the amount of ventilation air that must be provided to the building, as indicated in Section 5.9.2 of ASHRAE 62.1–2010, provide an explanation, documentation, and calculations as necessary to show that exhaust requirements exceed the minimum ventilation flows, and model the ventilation rate identically in both buildings.

Demand-control ventilation and nighttime ventilation requirements

Exception (a) allows credit for demand-control ventilation when it is not required by Sections 6.3.2(p) or If demand-control ventilation is being modeled for credit, Table G3.1.4 (baseline) indicates that schedules may be modified and allowed to differ to take it into account, provided the schedules are approved by the rating authority. In this instance, project teams must submit both proposed and baseline ventilation schedules.

ASHRAE 90.1, Section, requires shutoff dampers that automatically shut during unoccupied periods when the HVAC system cycles on and off to meet loads except when ventilation reduces energy costs (e.g., night purge), or when ventilation must be supplied to meet local requirements (such as minimum flow requirements for hospital or chemical storage rooms during unoccupied periods).

Therefore, the demand-control ventilation schedules presented for both the baseline and proposed cases should show zero outside airflow during unoccupied periods unless the supplemental documentation supports that ventilation during unoccupied periods reduces energy cost or is required by local code, in which case the baseline and proposed ventilation rates during unoccupied periods must be modeled with identical flow rates. Additionally, the baseline ventilation flow must be modeling using minimum required rates.

Ventilation (zone air distribution effectiveness)

Exception (b) allows for lower ventilation rates in the proposed building for efficient ventilation system designs that have high zone air distribution effectiveness (Ez >1.0), as determined by ASHRAE 62.1–2010.

In this case, the baseline ventilation levels can be based on the proposed calculations, only with reduced zone air distribution effectiveness (Ez = 1.0). This makes the baseline outdoor airflow rates higher than the proposed outdoor airflow rates, so ventilation calculations must be submitted to claim the exception for a higher Ez in the proposed case.

If a lower ventilation flow rate is an aspect of the design, the project team must provide ventilation rate procedure calculations for both the proposed and baseline designs, with the proposed design using the actual Ez value and the baseline design using an Ez value of 1.0 in each zone where the Ez value is greater than 1.0, but equal to the proposed building for all other zones where the Ez value is not greater than 1.0.

If ASHRAE 62.1, Section 6.2, Ventilation Rate Procedure, is not used for the ventilation design, then this exception may not be used.

Credit may not be taken, via ventilation flows, for any other ventilation design, such as a 100% outdoor air unit. Additionally, credit may not be taken for increased system ventilation efficiency, Ev, of a proposed ventilation system compared with a baseline ventilation system; Appendix G does not allow this. The only exception would be a different Ev value due to an Ez greater than 1.0, as described above.

Natural ventilation

The ASHRAE User’s Manual indicates that an exceptional calculation method is not required for natural ventilation and gives some further examples.

Perform sufficient analysis to document that loads can be met when credit is taken for passive cooling and natural ventilation using a simulation tool capable of ensuring thermal conditions are met with natural ventilation. A simple load calculation is not sufficient.

Service water heating

Hot water demand

Hot water demand savings from low-flow fixtures must be derived from WE Prerequisite or Credit Indoor Water Use Reduction calculations.


Lighting power density, method

Lighting power must be determined using the same categorization procedure (building area or space-by-space method) in both the proposed and baseline designs.

Lighting power density, multifamily

ASHRAE 90.1–2010 does not allow credit for lighting within dwelling units. Therefore, the lighting within these units must be modeled identically in both cases unless an exceptional calculation method is pursued. If credit is attempted, the lighting must meet prescribed illuminance levels. Refer to the ENERGY STAR’s Multifamily High Rise Program Simulation Guidelines for examples.

Lighting power density, luminaire wattage

Table G3.1.6 requires that the proposed lighting power include all components shown on the plans and be determined in accordance with Sections 9.1.3 and 9.1.4. Ensure that the lighting calculations include all task lighting except where specifically exempted by ASHRAE 90.1 and that all power used by the luminaires, including lamps, ballasts, transformers, and controls, is taken into account. For track and other flexible lighting systems, use the specified wattage of the transformer supplying the system. The sum of lamp wattages will not necessarily meet the requirements of G3.1.6.

Lighting power density, additional lighting power

ASHRAE 90.1, Section 9.6.2, addresses the use of additional lighting power for decorative lighting, in retail areas, or when additional controls have been installed.

Additional lighting is allowed only when using the space-by-space method and if it is “installed and automatically controlled, separately from the general lighting, to be turned off during nonbusiness hours.”

Therefore, the general lighting system must be separate and capable of providing general illumination to the space, and the additional lighting must have automated controls that shut it off during nonbusiness hours even when the general lighting remains on.

In retail applications, a common mistake is that the lighting may not be used for any purpose other than to highlight the merchandise.

Project teams can model the additional lighting power up to what has actually been designed, and no more; the baseline building must be modeled equal to what has been designed or up to the lighting allowance from ASHRAE 90.1, Section 9.6.2, whichever is less (i.e., credit may not be taken for unclaimed additional lighting power).

Note that only the sales area can be used in the lighting power allowance. For example, do not use the entire project floor area (which may include space with other purposes, such as checkout areas, corridors, or dressing rooms) to determine the allowance.

ASHRAE 90.1–2010 now allows an additional lighting power allowance based on the application of additional controls and using the control factors found in Table 9.6.2. This additional allowance may be used anywhere in the building and is based on the total wattage in the given space to which the control method is being applied.

Unlike the retail allowance, this allowance is earned with the application of the control methods and may be added to the baseline whether or not the project designs up to the full allowance.

Automatic lighting controls

ASHRAE 90.1, Table G3.1(g), indicates that only automatic lighting controls, such as occupancy sensors, that are in addition to the required minimum control (Section 9.4.1) may be taken for credit.

One of the most common errors is taking credit for an occupancy sensor located in a conference room; this is already a requirement of the baseline building. ASHRAE 90.1–2010 lists additional spaces that must have occupant sensors or timer switches that automatically turn off lighting.

ASHRAE 90.1–2010 has added requirements for the lighting system and controls for buildings. Project teams are encouraged to read the standard, the User’s Manual, and the lighting compliance forms to ensure that all mandatory measures have been met; these are prerequisites to LEED certification.

Exterior lighting

Exterior lighting is divided into allowances for tradable and nontradable surfaces. No credit may be taken for lighting reductions on nontradable surfaces. A lighting power allowance cannot be claimed in the baseline building for surfaces that are not provided with lighting in the actual design, and lighting fixtures cannot be double-counted for different exterior surfaces.

Energy rates

Energy rates

Project teams must consistently use either actual utility rates or their state’s average energy prices, published by the U.S. Department of Energy’s Energy Information Administration for commercial building customers. The sources may not be mixed.