Any approved computer program may be used to show compliance with the Energy Code using the performance method. The following steps are an outline of the typical computer program procedure:
1. Collect all necessary data on each component.
a. For the building envelope, the area of each fenestration, wall, door, roof, ceiling, and floor is needed. For each component, the applicable energy characteristics needs to be defined including U-factor, solar heat gain coefficient (SHGC), solar reflectance, and thermal mass values.
b. For HVAC systems, the equipment type and efficiency are required. For hydronic space heating, the specific water heater type and efficiency are required. For fan-forced air conditioning systems, the location and amount of insulation on the duct system are needed.
c. For DHW systems, the water heater type, number, efficiency, and area served are required, along with the information about the hot water distribution system. Additional information will be required for features such as solar thermal systems and drain water heat recovery devices. More information is in Chapter 5.
d. For PV systems, size and location information--such as roof slope and orientation-- are required. Battery storage capacity and control information must be described if battery storage is proposed. Refer to Chapter 7 for more information.
2. Enter the basic building envelope data such as square footage, number of stories, occupancy type, and climate zone. Define each opaque surface with the orientation, area, and thermal performance properties. Add the doors and fenestration associated with each opaque surface, including any fixed shading such as overhangs and side fins. Enter the data of the equipment and distribution systems for the space conditioning and water-heating systems. The input values and assumptions must correspond to the information on the final approved plan set, and the inputs must be at least as energy efficient as the relevant mandatory measures. (Software compliance programs may not automatically check for compliance with mandatory measures.)
3. Launch a computer simulation to calculate the source EDR1, efficiency EDR2, and total EDR2 of the standard design and the proposed design.
For additions and alterations, compliance is based on TDV energy, and not the EDR criteria that is used for newly constructed buildings. In existing buildings, where the values of installed features are unknown, default values may be used based on the year of the construction. Refer to Table 8-1, Default Assumptions for Year Built (Vintage), at the end of this chapter. The proposed design complies if all mandatory measures are met and the total TDV energy use is the same as or less than the standard design TDV energy budget.
When creating a computer input file, use the space provided for the project title information to concisely describe the building being modeled. User-designated names should be clear and internally consistent with other orientations and/or surfaces being analyzed. Title names and explanatory comments should assist in the compliance and enforcement processes.
Approved compliance software programs automatically calculate the standard design efficiency based on data entered for the proposed building.
The program defines the standard building by modifying the geometry of the proposed building and inserting the features of Table 150.1-A of the Energy Standards. Details on how the proposed and standard design energy budget are established can be found in the Residential ACM Reference Manual.
Note the details of how the standard design efficiency is determined. Deviations from the prescriptive requirements will be reflected in the compliance margin. For example, if the prescriptive requirements from Table 150.1-A include a heat pump space heating system, and the proposed building is modeled with a central gas furnace, it will significantly increase the heating source energy and result in a compliance penalty.
The standard design assumes the same total conditioned floor area and volume as the proposed design and the same gross exterior wall area as the proposed design, except that the wall area in each of the four cardinal orientations is divided equally. The standard design uses the same roof/ceiling area, raised floor area, slab-on-grade area, and perimeter as the proposed design, but uses the standard insulation R-values from Table 150.1-A of the Energy Code.
The standard design includes all features of the prescriptive compliance tables, including quality installation of insulation, walls with the prescriptive U-factor, below-deck roof insulation or radiant barrier, and a solar PV system.
Total fenestration area in the standard design is equal to that in the proposed design if the fenestration area in the proposed design is less than or equal to 20 percent of the conditioned floor area (CFA), Otherwise, the fenestration area is equal to 20 percent of the CFA. Fenestration area in the standard design is evenly distributed among the four cardinal orientations. SHGC and U-factors in the standard design are the same as those listed in the prescriptive tables, with no overhangs.
The standard design includes minimum efficiency heating and cooling equipment, as well as the minimum duct insulation R-value required for Option B from Table 150.1-A of the Energy Code. Ducts are assumed to be sealed as required by §150.0(m). The standard design also assumes correct refrigerant charge as required by §150.1(c)7A.
For water-heating systems that serve dwelling units, the standard design is a NEEA Tier 3 heat pump water heater with a uniform energy factor equal to 2.0, and the distribution system meets all mandatory requirements specified in §150.0.
For consistency and ease of enforcement, the way building features are reported by compliance software programs is standardized. Energy Commission-approved compliance software programs produce compliance reports in a standard format. The principal report is the certificate of compliance (CF1R-PRF-01-E).
The CF1R-PRF-01-E includes two feature summary sections, one for required special features and modeling assumptions, and a second for features requiring HERS field verification and/or diagnostic testing. These sections provide a general overview during compliance verification by the local enforcement agency and the HERS Rater. Items in the special features and modeling assumptions section indicate that if such features or assumptions used for compliance are not installed, the building would not comply, and they call for special consideration by the local enforcement agency. Items in the HERS verification section rely on diagnostic testing and verification by an approved HERS Rater to ensure proper field installation. Diagnostic testing and verification by HERS Raters is separate from local enforcement agency inspections.
Some modeling techniques and compliance assumptions applied to the proposed design are fixed or restricted. At other times, professional judgment may be acceptable or necessary.
Enforcement agencies can reject a particular input if the permit applicant cannot substantiate the value with supporting documentation or demonstrate that appropriate professional judgment was applied.
A simplified approach or assumption should be consistent with what is used by the compliance software to generate the standard design. If simplification increases the predicted energy use of the proposed building or reduces the compliance margin when compared to a more detailed modeling assumption, the simplification is acceptable. Simplification must reflect a worse case than a more detailed model and result in the same or lower compliance margin.
Any unusual modeling approach, assumption, or input value should be documented with published data and conform to standard engineering practice.
For assistance evaluating the appropriateness of input assumptions, contact the provider of your compliance software or the Energy Commission’s Energy Standards Hotline (Title24@energy.ca.gov or (800) 772-3300).
Example 8-1
Question
Three different-sized windows in the same wall of a new home are designed without exterior shading. They have the same National Fenestration Rating Council-rated U-factors and SHGC values. Is it acceptable to simplify the computer model by adding the areas of the windows and inputting them as a single fenestration area?
Answer
Yes. Because modeling the windows as a single window will not increase or decrease the modeled energy consumption, this is acceptable. However, plan checking and finding errors when windows are combined is much more difficult. If the software has a multiplier, identical window sizes with identical shading features can be combined. Otherwise, it is best practice to model each window individually.