ASHRAE-90.1-2022- - Page 215
Extracted Content
load hours for the budget building design . Alternatively, unmet load hours exceeding these limits may be approved by the building official, provided that sufficient justification is given indicating that the accuracy of the simulation is not significantly compromised by these unmet loads. j. Determining the HVAC System. Each HVAC system in a proposed design is mapped on a one-to-one correspondence with one of eleven HVAC systems in the budget building design . To determine the budget building system, do the following:
- Enter Figure 12.5.2 at “Water/Ground” if the proposed design system condenser is water or evaporatively cooled; enter Figure 12.5.2 at “Air/None” if the condenser is air cooled. Closed-circuit dry coolers shall be considered air cooled. Systems utilizing district cooling shall be treated as if the condenser water type were “Water.” If no mechanical cooling is specified or the mechanical cooling sys- tem in the proposed design does not require heat rejection, the system shall be treated as if the condenser water type were “Air.” For proposed designs with ground-source or groundwater-source heat pumps, the budget system shall be water-source heat pump ( System 6).
- Select the path that corresponds to the proposed design heat source: electric resistance, heat pump (including air source and water source), or fuel -fired. Systems utilizing district heating (steam or hot water) shall be treated as if the heating system type were “Fossil Fuel.” Systems with no heating capability shall be treated as if the heating system type were “Fossil Fuel.” For systems with mixed fuel heating sources, the system or systems that use the secondary heating source type (the one with the smallest total installed output capacity for the spaces served by the system ) shall be modeled identically in the budget building design, and the primary heating source type shall be used in Figure 12.5.2 to determine budget system type.
- Select the budget building design system category. The system under “Single-Zone Residential System” shall be selected if the HVAC system in the proposed design is a single-zone system and serves a residential space . The system under “Single-Zone Nonresidential System” shall be selected if the HVAC system in the proposed design is a single-zone system and serves other than residential spaces . The system under “All Other” shall be selected for all other cases. k. Kitchen Exhaust. For kitchens with a total exhaust hood airflow rate greater than 5000 cfm, use a
demand ventilation system on 75% of the exhaust air. The system shall reduce exhaust and replacement air system airflow rates by 50% for one half of the kitchen occupied hours in the baseline building design . If the proposed design uses demand ventilation, the same airflow rate schedule shall be used. The maximum exhaust flow rate allowed for the hood or hood section shall meet the requirements of Section 6.5.7.2.2 for the numbers and types of hoods and appliances provided in the proposed design .
12.5.3 Modeling Building Envelope Air Leakage. The air leakage rate of the building envelope ( I75Pa ) at a pressure differential of 75 Pa (0.30 in. of water) shall be converted to appropriate units for the simulation program using one of the following formulas:
a. For methods describing air leakage as a function of floor area,
IFLR = 0.112 × I75Pa × S/AFLR b. For methods describing air leakage as a function of the area of above-grade walls that separate condi-
tioned spaces and semiheated spaces from the exterior,
IAGW = 0.112 × I75Pa × S/AAGW c. When using the measured air leakage rate of the building envelope at a pressure differential of 75 Pa
(0.30 in. of water) for the proposed design, the air leakage rate shall be calculated as follows:
I75Pa = Q/S
where I75Pa = air leakage rate of the building envelope in cfm/ft [2] at a fixed building pressure differential of 75 Pa (0.30 in. of water) Q = volume of air in cfm flowing through the building envelope when subjected to a pressure differential of 75 Pa (0.30 in. of water), in accordance with ASTM E779, ASTM E1827, or ASTM E3158 S = total area of the building envelope in ft [2], including the lowest floor, any below-grade walls or above-grade walls, and roof (including vertical fenestration and skylights ) IFLR = adjusted air leakage rate of the building envelope cfm/ft [2] at a reference wind speed of 10 mph and relative to the gross floor area AFLR = gross floor area, ft [2]
ANSI/ASHRAE/IES Standard 90.1-2022 (I-P) 213