3.1 Overview

The HVAC system in the baseline building depends on the primary building activity, the size of the building and the energy source used for heating in the proposed design. Figure 6.1.2-1 shows the HVAC system types that apply in each case. Details about these systems are provided in subsequent sections.

Many of the building descriptors have a one-to-one relationship between the proposed design and the baseline building, for example, every wall in the proposed design has a corresponding wall in the baseline building. For HVAC systems, this one-to-one relationship generally does not hold. The HVAC system serving the proposed design and the baseline building may be completely different, each with different components, etc.

For systems 1 through 4 and 9 and 10, each thermal block shall be served by a separate system. For systems 5 through 8, a separate air handler shall serve for each floor. Systems 5 and 6 are packaged equipment with DX cooling and fans combined in the same box. For these systems, each floor will have its own system.

There are several important exceptions to the HVAC mapping rules that apply to spaces with unusual internal heat gains, different schedules, special pressurization requirements, or unique outside air needs. 

There are several important exceptions to the HVAC mapping rules that apply to spaces with unusual internal heat gains, different schedules, special pressurization requirements, or unique outside air needs. These special cases are addressed through the exceptions to Section G3.1.1 of the PRM. These address mixed use buildings, spaces in buildings with unusual loads or schedules, laboratories with fume hoods and commercial kitchens.  The exceptions are described below:

• Mixed residential and nonresidential buildings. If a building has both residential and nonresidential spaces with their total conditioned floor area greater than 20,000 ft², such as a residential tower with retail and restaurants at the base, then the HVAC system type is determined separately for the residential and nonresidential portions.
• Special spaces in buildings. Examples are computer rooms or guard stations. In these cases, the baseline building system for the special space is system type 3 or 4, depending on the heating source for the main building. This exception applies when a space has significantly different operating conditions or thermal loads. The threshold for internal loads is a difference of more than 10 Btu/h-ft². The threshold weekly operating hours is when the difference is more than 40 hours. 
• Laboratories. For laboratory spaces with a minimum exhaust of 5,000 cfm, the HVAC system type should be 5 or 7 (depending on heating source) serving only the laboratory spaces. For all electric buildings, the heating is electric resistance.
• Kitchens. Baseline building systems that serve kitchens with an exhaust hood airflow rate greater than 5,000 cfm shall use either baseline building system 5 or 7. System 5 is a packaged VAV and 7 is a VAV with a chilled water plant. System 7 would be used if the baseline building has a chilled water plant, otherwise system 5 would be used. The baseline building shall have a demand ventilation system that operates on at least 75% of the exhaust air. The system shall reduce exhaust and replacement airflow rates by 50% for one half of the kitchen's occupied hours. If the proposed design uses demand ventilation, the same airflow schedule shall be used. 
• Heated only spaces. A heating and ventilating system (with no cooling) shall be used for heated only storage, stairwells, vestibules, electrical/mechanical rooms and restrooms not exhausting or transferring air from mechanically cooled thermal zones (systems 9 or 10). When unheated storage or other qualifying spaces exist in a building, the remaining spaces are modeled as heated and cooled and follow the system map in Figure 6.1.2-1. The floor area to be used in determining the baseline building system types should exclude the heated only spaces.

When the rated building used purchased heating (hot water or steam) and/or purchased chilled water, the HVAC system mapping is modified as follows:

• Purchased heat, but not purchased chilled water. The purchased heat is assumed to be produced by fossil fuels, and the mappings in Figure 6.2.1-1 apply with purchased heat substituted as the heating source.
• Purchased chilled water, but not purchased heat. The mappings in Figure 6.2.1-1 apply with the following modifications:
  • Purchased chilled water is substituted for the cooling source.
  • Systems 1 and 2 are constant volume fan coil units with fossil fuel boilers. 
  • Systems 3 and 4 are constant volume single zone air handlers with fossil fuel furnaces. 
  • System 7 is used in place of System 5. 
  • System 8 is used in place of System 6. 
• Both purchased chilled water and heat. The purchased heat is assumed to be produced by fossil fuels and the mappings in Figure 6.2.1-1 apply with the following modifications:
  • Purchased heat and purchased chilled water is substituted for the heating and cooling sources. 
  • System 1 is constant volume fan coil units. 
  • System 3 is constant volume single zone air handlers.
  • System 7 is used in place of System 5.

This chapter specifies the rules that apply to the proposed design and to the baseline building for each building descriptor.