3.1.1.1 Exceptions to HVAC System Requirements

There are several important exceptions to the HVAC mapping rules that apply to zones with unusual internal heat gains, different schedules, or unique outside air needs. Where attributes make a building eligible for more than one baseline system type, use the predominant condition to determine the system type for the entire building except as noted in exceptions below. These exceptions should be applied in the order defined below, to avoid inconsistencies in baseline system definition-

1. Exception (a) for mixed residential and non-residential buildings
2. Exception (d) for heated only zones
3. Exception (e) for baseline system 9 and 10
4. Exception (f) for computer rooms
5. Exception (c) for laboratory spaces
6. Exception (b) for internal loads

a. Mixed Residential and Non-Residential Buildings:

The baseline HVAC system must be determined separately for buildings with both residential and non-residential zones. Additional system type(s) are required to be used if the non-predominant conditions (use type) apply to more than 20,000 ft² of conditioned floor area. Residential building types include dormitory, hotel, motel, and multifamily. Residential space types include guest rooms, living quarters, private living space, and sleeping quarters. Other building and space types including common areas associated with residential buildings are considered nonresidential.

b. Internal Loads:

This exception is triggered for HVAC zones with total non-coincident peak internal gains (excluding ventilation or envelope loads) that differ by more than 10 Btu/h-ft² (31.5 W/m²) from the average of the other HVAC zones by the system or when the weekly operating hours of the HVAC system are different by more than 40 equivalent full load hours (EFLH) per week from the other HVAC zones served by the system. A full load hour is an hour during which the zone is occupied and supply and return fans operate continuously. The baseline system for such spaces would be system type 3 or 4, depending on the heating source for the main building. This exception does not apply to computer rooms, see (f) below for the exception related to computer rooms.

When multiple proposed systems with schedules varying for less than 40 hours are combined into a single baseline system as a whole floor variable air volume (VAV), the baseline system fan schedule is defined to include the earliest start hour and latest end hour, so that all HVAC zones are designated to have HVAC system availability. Section 3.7.2.1 has more details regarding HVAC availability.

This exception doesn’t apply to computer rooms (see f. below).

The process for calculating internal loads for spaces is defined below-

1. Step 1: Eliminate zones that meet both exceptions (internal loads and EFLH)

   a. Find all zones whose  peak coincident internal loads are more than 10 Btu/ft² from the area-weighted average of all other zones and whose EFLH are more than 40 hrs from the average of all other zones. Remove all of those zones and assign them to baseline system 3 or 4 depending on the climate zone.

   b. Repeat until no zones meet this criteria.

2. Step 2: Eliminate zones that meet the EFLH exception

   a. Find any zones whose EFLH is more than 40 hrs from the average of all other zones.  Remove the zone with the highest EFLH greater than 40, and assign to baseline system 3 or 4 depending on climate zone.

   b. Repeat until no zones meet this criteria.

3. Step 3: Eliminate zones that meet the internal loads exception

   a. Find any zones whose peak coincident internal loads are more than 10 Btu/ft² from the area-weighted average of all other zones.  Remove the zone with the highest peak internal load greater than 10 Btu/ft², and assign system 3 or 4 depending on the climate zone.

   b. Repeat until no zones meet this criteria.

4. Step 4: Assign remaining zones to the appropriate multizone system.

Example 1: For a floor with zones with peak internal gains specified as 9 Btu/h-ft², 14 Btu/h-ft², 16 Btu/h-ft², and 34 Btu/h-ft². The difference between the zone’s peak internal loads and the average for all other zones is shown in Table 5 . Zone D would be subject to the exception since its coincident peak internal gains differ by more than 10 Btu/h-ft² from the average for all other zones and the baseline system for this space would be system type 3 or 4 (depending on building heating source). Zone D would then be removed from the list and the the difference between remaining zone’s peak internal loads and average for all other zones would be calculated again. The difference is now under ^. 10 Btu/h-ft² and would hence stay on the same baseline system.

Table 5. Example 1 Calculations

Example 2: An office building with baseline system 5 has the following thermal blocks on one floor:

• Thermal Block A: Zones with predominantly office occupancy, to be occupied 50 hours per week;
• Thermal Block B: Zones to be occupied 55 hrs per week;
• Thermal Block C: Zones for help desk, to be occupied 65 hours per week.
• Thermal Block D: Zones to be occupied 100 hours per week

Following the methodology outlined above, the difference between the zone’s EFLHs and the average for all other zones is shown in Table 6. Since the difference from the average is greater than 40, Thermal Block D would be modeled with baseline system 3 and the  difference between remaining zone’s EFLH and average for all other zones would be calculated again. The difference is now under ^. 40 hours and hence the remaining zones would hence stay on the same baseline system.

Table 6. Example 2 Calculations

Example 3: An office building with baseline system 5 has the following thermal zones on one floor:

• Zone A: Occupied 25 hours per week;
• Zone B: Occupied 30 hours per week;
• Zone C: Occupied 35 hours per week;
• Zone D: Occupied 45 hours per week;
• Zone E: Occupied 80 hours per week;
• Zone F: Occupied 90 hours per week;
• The coincident peak internal loads for these zones are 6 Btu/h-ft², 8 Btu/h-ft², 10 Btu/h-ft², 20 Btu/h-ft², 16 Btu/h-ft², and 28 Btu/h-ft²

Following the methodology outlined above, following would be the steps for elimination

1. Step 1: Eliminate zones that meet both exceptions (internal loads and EFLH)

   a. Based on this, zone F is eliminated and assigned baseline system 3 or 4 depending on the climate zone.

2. Step 2: Eliminate zones that meet the EFLH expection
3. Step 3: Eliminate zones that meet the internal loads exception

Table 7. Example 3 Calculations

c. Laboratory Spaces:

All zones with laboratory spaces in a building having a total laboratory exhaust rate greater than 15,000 cfm, use a single system of type 5 or 7 serving only those spaces. .The baseline system serving laboratory spaces shall be system 5 (PVAV with hot water reheat) or 7 (VAV with hot water reheat) depending on the size of the building. If the building is more than 5 floors or >150,000 ft² use system 7. Otherwise, use system 5. The lab exhaust fan shall be modeled as constant horsepower reflecting constant volume stack discharge with outdoor air bypass.

d. Heated Only Zones:

Thermal zones designed with heating only systems in the proposed design, serving storage rooms, stairwells, vestibules, electrical/mechanical rooms, and restrooms not exhausting or transferring air from mechanically cooled thermal zones in the proposed design shall use system type 9 or 10 in the baseline building design. If a space type doesn’t fall in the list of “storage, stairwells, vestibules, electrical/mechanical rooms or restrooms,” then, despite being heated only, it would be modeled as heated and cooled. This rule applies even if the total area of such zones is below 20,000 ft².

e. Baseline System 9, 10:

If the baseline HVAC system type is 9 or 10, all zones that are mechanically cooled in the proposed building design shall be assigned to a separate baseline system determined according to Table 3 by using the climate zone and floor area of the mechanically cooled zones. This rule applies even if the total area of such zones is below 20,000 ft².

f. Computer Rooms:

Standard 90.1-2019 defines computer rooms as a room whose primary function is to house equipment for the processing and storage of electronic data and that has a design electronic data equipment power density exceeding 20 W/ft2 of conditioned floor area. This exception would also apply to server closets or telecom equipment closets if these requirements are met.

All zones with computer rooms would be modeled with systems 3 or 4 with the exception of-

1. Computer rooms in buildings with a total computer room peak cooling load for the proposed building>3,000,000 Btu/h
2. Computer rooms in buildings with a total computer room peak cooling load for the proposed building >600,000 Btu/h where the baseline HVAC system type is 7 or 8.

Computer rooms meeting either of these two exceptions will use system 11 with the heating source determined by climate zone as described in footnote 2 to Table 4.

These special systems serve just the HVAC zones containing spaces that trigger the exceptions. The rest of the building/floor is served by the baseline building HVAC system as indicated in Table 3.