To ensure that the simulated space-conditioning loads are adequately met, adequate capacity must be available in each component of the HVAC system (e.g., supply-airflow rates, cooling coils, chillers, and cooling towers). If any component of the system is incapable of adequate performance, the simulation program will report UMLH, which need to be addressed following the steps in Section 2.6. Adequate capacities are required in the simulations of both the proposed design and the baseline building. If the equipment capacity is not sufficient to meet demands, then UMLH are evaluated at the building level by looking at the UMLH for each thermal zone being modeled. The subsections below describe the procedures that shall be followed to ensure that both the baseline and proposed building models are simulated with adequate space-conditioning capacities.
2.7.1 Specifying HVAC Capacities for the Proposed Design
If loads are not met for more than 300 hours, the software shall require the user to change the proposed design building description to bring the UMLH equal to or below 300. This process might not be automated by the software, in which case the user is required to modify the design model to bring the UMLH within acceptable limits. Two tests must be met:
- Space loads must be satisfied and space temperatures in all zones must be maintained within one half of the throttling range (1°F with a 2°F throttling range) of the scheduled heating or cooling thermostat setpoints. This criterion may be exceeded for no more than 300 hours for a typical year.
- System loads must be satisfied: Plant equipment must have adequate capacity to satisfy the HVAC system loads. This criterion may be exceeded for no more than 300 hours for a typical year.
Equipment sizes for the proposed design shall be entered into the model by the energy analyst and shall agree with the equipment sizes specified in the construction documents. When the simulations of these actual systems indicate that specified space conditions are not being adequately maintained in one or more thermal zone(s), the user shall be prompted to verify the model inputs to reduce the number of UMLHs.
2.7.2 Sizing Equipment in the Baseline Building
System coil capacities in the baseline building are automatically oversized by the program (25% for heating and 15% for cooling). Equipment is sized using design day data and weather files for the building location. These are discussed in Section 3.2.3 of this manual. The coil capacities shall be based on sizing runs for each orientation in accordance to Section 2.6 of this manual.
Oversizing would be carried out at the zone level, where the sizing parameters would be applied to the zone design coil loads, but not to the supply airflow. The system sizing calculations would sum the zone design airflow rates to obtain a system level airflow rate. The design conditions and the outdoor airflow rate would be used by the simulation program to calculate a design mixed air temperature. The thermostat setpoint plus the design supply air temperatures (SATs) would allow for the calculation of system design heating and cooling capacities. The sizing option would be specified as “Coincident,” which specifies that the central system airflow rate will be sized on the sum of the coincident zone airflow rates. There would be no oversizing factor specified at the system level or the central plant level. Plant capacities shall be based on coincident loads.
For cooling sizing runs, schedules for internal loads including those used for infiltration, occupants, lighting, gas and electricity using equipment shall be equal to the highest hourly value used in the annual simulation runs and applied to the entire design day. Schedules for infiltration for the cooling sizing runs shall be equal to the highest hourly value used in the annual simulation runs and applied to the entire design day
For heating sizing runs, schedules for internal loads including those used for occupants, lighting, gas and electricity using equipment shall be equal to the lowest value used in the annual simulation runs. Schedules for infiltration for the heating sizing runs shall be equal to the highest hourly value used in the annual simulation runs and applied to the entire design day.
For the sizing run, thermostat schedules for heating and cooling should be set to the most typical 24 hour profile from the annual simulation run.
Exception: For cooling sizing runs in residential dwelling units, the infiltration, occupants, lighting, gas and electricity using equipment hourly schedule shall be the same as the most used hourly weekday schedule from the annual simulation.
If the automatic oversizing percentage is not sufficient to meet demands, then UMLH are evaluated at the building level by looking at the UMLH for each thermal zone being modeled. The first step would be to determine if the UMLH are high (>300) for the proposed building design as well as the baseline building. If that is the case, the issue is usually related to fan operation, HVAC availability, and occupancy schedules where the HVAC system has an incorrectly specified schedule that makes it unavailable during occupied hours. Optimal start controls, if a part of the building design, can also help eliminate UMLH during startup times. Since the same schedules are used for the baseline design, UMLH are seen in the baseline building as well. Other user inputs that could cause UMLH include incorrectly specified zone minimum airflows, which could result in unmet heating load hours. In this case, the software should notify the user and ask the user to verify schedules of operation. If a space is being conditioned via transfer air, it might be that the temperature of the transfer air is not sufficient to meet space conditioning requirements.
- If this is not the case and UMLH are seen only with the baseline design, the user or software tool is required to incrementally increase system airflows and equipment capacities, following the steps outlined below.
- In the case where UMLH for cooling are a bigger problem, the equipment in the baseline building model is resized by first increasing the design airflow of all zones with significant UMLH (greater than 150 for an individual zone) by 10%, increasing the design airflow of all zones with some UMLH (between 50 and 150) by 5%. Then, the equipment capacity for the system(s) serving the affected zones is increased to handle the increased zone loads. For the central plant, the chiller(s) and towers are resized proportionally to handle the increased system loads.
- In the case where UMLH for heating are a bigger problem, the same procedure is followed, with zone airflows resized first, then heating secondary equipment capacity and then boiler capacity as necessary. The capacity of the boiler or furnace shall be increased in proportion to capacities of coils required to meet the increased airflows at the baseline supply air. For heat pumps, the capacity of the coil is increased so that the additional load is not met by auxiliary heat.
2.7.3 Proposed Design with No HVAC, Lighting, SHW System or Receptacle Loads
2.7.3.1 No HVAC System Intended
Standard 90.1-2019 PRM doesn’t address buildings that are intended to have no HVAC system. Portions of a building with no heating and cooling system shall be simulated to be unconditioned for the baseline as well.
2.7.3.2 No Lighting System Designed
For proposed buildings, where lighting neither exists nor is submitted with design documents (including core and shell buildings), lighting shall comply with but not exceed the requirements of Standard 90.1-2019 Section 9.
- If space types are known, lighting power shall be determined in accordance with the Space-by-Space Method for the proposed building, as explained in Section 3.4.3 of this document.
If space types are not known, lighting power shall be determined in accordance with the Building Area Method for the proposed building. The lighting power for the baseline building in such cases shall be determined in accordance with Standard 90.1-2019 Table G3.8.
2.7.3.3 No HVAC System Designed
When the PRM is applied to buildings for which systems have not been designed, such as core and shell office buildings, the systems in the proposed design must be modeled to comply with, but not exceed the requirements of Standard 90.1-2019 Table G3.1 part 10.
In accordance with Standard 90.1 G3.1 part 10, if no heating system exists or has been submitted with design documents, the system type shall be the same system as modeled in the baseline building design and is required to comply with but not exceed the prescriptive requirements as specified in 90.1-2019 Section 6. Similarly, if no cooling system exists or has been submitted with design documents, the system type shall be the same system as modeled in the baseline building design and is required to comply with but not exceed the prescriptive requirements as specified in 90.1-2019 Section 6
Spaces that are cooled but not heated, are required to be simulated with heating. Spaces that are heated but not cooled are required to be simulated with a cooling system, unless they qualify as heated only spaces in accordance with Section 3.1.1.1(d) of this document. Where the space classification for a space is not known, the space shall be categorized as an open office space.
2.7.3.4 No SHW System Designed
In accordance to Standard 90.1-2019 Section G3.1 part 11, proposed designs where no service water-heating system exists or has been designed and submitted with design documents but the building will have service water-heating loads, a service water-heating system is required to be modeled that matches the system type in the baseline building design, as specified in Standard 90.1-2019 Table G3.1.1-2. The SHW system specified in the proposed building should comply with but not exceed the requirements of Section 7.
2.7.3.5 No Power Systems Designed
Where power and other systems covered by Standard 90.1-2019 Sections 8 and 10 are not submitted with design documents, these systems are required to comply with but not exceed the requirements specified in those sections.
2.7.4 Existing Systems
In some cases, a complete HVAC system already exists. An example might be an existing speculative building that is being built out for a tenant. In a case like this, the proposed building must match the existing systems and the baseline building must follow the baseline building modeling rules.
2.7.4.1 Incomplete HVAC System Design, Heating-Only or Cooling-Only
Some buildings, such as retail malls and speculative office buildings, are typically built in phases. For example, the core mechanical system may be installed with the base building while the ductwork and terminal units are installed later as part of tenant improvements. A similar situation can occur with the lighting system or other energy-related features of the building.
There are frequently spaces within a building where the temperature control is not required for oc-cupant comfort but is required to maintain a minimum space temperature for equipment protection. This may require cooling-only or heating-only systems depending on the space needs. Some spaces are heating only to protect against freezing. Other spaces are cooling only to offset high internal heat gain associated with process loads. While these heating-only or cooling-only systems are permitted, they require special consideration when using the Performance Rating Method.
In either of these situations, for the purpose of calculating the proposed building performance rating, the rule is simple: heating or cooling systems that do not exist or that are not yet designed or recorded in the construction documents are assumed to be of the same type that is used in the baseline with characteristics that minimally comply with the applicable mandatory provisions and prescriptive requirements from Standard 90.1 Chapter 6.
- In cases where the space use classification is not known, the default as-sumption is to classify it as office space.
For heated-only storage buildings, and for thermal zones designed with heating systems in the proposed building serving storage rooms, stairwells, vestibules, electrical/mechanical rooms, and restrooms not exhausting or transferring air from mechanically cooled thermal zones in the proposed design (Standard 90.1-2019 G3.1.1 exception e) the baseline HVAC system is a heating and ventilating system with no cooling. For these applications only, cooling is not modeled for the proposed design.