3.1.1.2 Process for Determining the Baseline System

This section provides guidance for determining the baseline HVAC system for a proposed design to address all requirements and exceptions specified in Section 3.1.1.

Step 1: Determine Predominant Building Type

The building type for use in Table 3 is determined by the “predominant occupancy” type. The predominant occupancy is defined as the occupancy with the greatest conditioned floor area (CFA) and other occupancy types are defined as the “non-predominant occupancy”. The building types include:

a. Residential

b. Public Assembly

c. Retail

d. Hospital

e. Heated-Only Storage

f. Other Non-Residential

Once the building type is determined based on the predominant occupancy as described above, the appropriate baseline system is chosen from Table 3 based on:

a. Number of floors (including floors above grade and below grade but not including floors solely devoted to parking).

b. Gross conditioned floor area (CFA).

c. Climate zone

Step 2: Determine Non-Predominant Building Type(s)

If the CFA of a non-predominant occupancy exceeds 20,000 ft², a separate baseline system is to be chosen from Table 3 for the non-predominant occupancy. The appropriate system for the non-predominant condition is based on the same three criteria as used for the predominant condition as they apply to the non-predominant occupancy. There could be more than one non-predominant occupancy if each exceeds 20,000 ft². For example:

A building with 100,000 ft² residential, 35,000 ft² retail and 35,000 ft² public assembly, would have the predominant occupancy as residential and non-predominant occupancies as retail and public assemble. The baseline systems for all 3 occupancies would be determined based on the number of floors, gross conditioned floor area and climate zone.

Step 3: Determine Other Exception Area

As described in the section above, the ‘other exception area’ will be determined in the following order:

Exception (d) for heated only zones
Exception (e) for baseline system 9 and 10
Exception (f) for computer rooms
Exception (c) for laboratory spaces
Exception (b) for internal loads

Spaces qualify for additional systems if they have unusual internal loads (Section 3.1.1.1 Exception (b)), are certain laboratory spaces (Section 3.1.1.1 Exception (c)), are certain heated only spaces (Section 3.1.1.1 Exception (d)) or are cooled spaces within a heated only building using systems 9 or 10 (Section 3.1.1.1 Exception (e)). The HVAC systems used for these exception areas is determined as described in Section 3.1.1.1.

Step 4: Calculation of number of floors for a building:

Calculation of number of floors based on occupancy type:
- If a mixed occupancy building does not have enough area of each (greater than 20,000 ft²) to qualify for additional system type, all floors should be counted as the predominant type.
- If a mixed occupancy building has enough area of each (where area of each occupancy type is greater than 20,000 ft²) to qualify for more than one system type, then a mixed use floor should be counted in both. So, a five-story building with two floors of residential occupancy, two floors of non-residential occupancy, and one floor of both residential and non-residential occupancy would be defined as three floors of residential occupancy and three floors non-residential occupancy.
Calculation of number of floors for above-grade and below-grade floors:
- Both above and below grade floors will be counted for the calculation of number of floors.
Calculation of number of floors for partially conditioned floors:
- A floor (above grade or below grade) with any conditioned area should be counted as a floor. However, floors devoted solely to parking (above grade or below grade) would not be included. For example, an unconditioned below- or above-grade parking garage where only the elevator, lobbies, or stairwells are conditioned at each floor

Other example scenarios:

If a building qualifies for two baseline HVAC systems in accordance with Section G₃.1.1 (b) and both systems use hot-water boilers for heating; the same boiler(s) will serve both systems.
If a building in climate zone 2A has two floors of retail at 30,000 ft² and 15 floors of high rise residential at 225,000 ft² it will have two system types. System 4 (PSZ-HP) will serve the retail floors and system 2 (PTHP) will serve the residential floors.
A 6 story dormitory building in climate zone 4a, includes 40,000 ft² dorm rooms, 27,000 ft² common spaces (corridors, lounge, library, common kitchen and dining, study rooms, etc.). 6,000 ft² of the common spaces are storage and mechanical spaces that are heated-only. The predominant occupancy would be residential and the dorm rooms would be modeled with System 1 (PTAC). The common spaces would qualify as non-predominant non-residential (Standard 90.1-2019, Section G₃.1.1b) and would be modeled with system 7 (VAV with reheat). The heated only zones will be modeled with baseline system 9.
A 6 story dormitory building in Climate Zone 4a, includes 19,000 ft² dorm rooms, 23,000 ft² common spaces (corridors, lounge, library, common kitchen and dining, study rooms, etc.). 6,000 ft² of the common spaces are storage and mechanical spaces that are heated-only, hence the effective area for common spaces is 17,000 ft² The predominant occupancy would be residential since the floor area of dorm rooms is more than that of the common spaces or the heated-only spaces and those would be modeled with System 1.The common spaces (excluding the heated-only areas), are less than 20,000 ft^2,hence do not qualify for the occupancy exception and are modeled with system 1. The heated only zones will be modeled with baseline system 9.
Project is a mixed use 9 story hotel and retail in climate zone 5a. Hotel portion includes 250,000 ft² of guest rooms, 170,000 ft² of common spaces (corridors, reception, restaurant, etc.), and 70,000 ft² event spaces (convention center). The two lower floors (110,000 ft²) are occupied by various retail tenants.

The predominant occupancy is residential and the guestrooms would be modeled with baseline system 1. The other areas (convention center, retail, and common areas) are all non-predominant conditions, each over 20,000 ft² and thus need to be considered independently according to Standard 90.1-2019 Section G₃.1.1b. The 70,000 ft² convention center qualifies as public assembly < 120,000 ft² and would modeled with system 3. The retail area is greater than 20,000 ft² and would be modeled with system 4. The remaining common spaces would qualify as non-residential more than five floors and will be modeled with baseline system 7. Based on the internal loads for the restaurant space type, it might qualify for baseline system 3 (Standard 90.1-2019, Section G₃.1.1c).