G3.1.3 System-Specific Baseline HVAC System Requirements
Electric air-source heat pumps shall be modeled with electric auxiliary heat and an outdoor air thermostat. The systems shall be controlled to energize auxiliary heat only when the outdoor air temperature is less than 40°F. The air-source heat pump shall be modeled to continue to operate while auxiliary heat is energized.
The boiler plant shall use the same fuel as the proposed design and shall be natural draft, except as noted in Section G22.214.171.124. The baseline building design boiler plant shall be modeled as having a single boiler if the baseline building design plant serves a conditioned floor area of 15,000 ft2 or less, and as having two equally sized boilers for plants serving more than 15,000 ft2. Boilers shall be staged as required by the load.
Hot-water design supply temperature shall be modeled as 180°F and design return temperature as 130°F.
Hot-water supply temperature shall be reset based on outdoor dry-bulb temperature using the following schedule: 180°F at 20°F and below, 150°F at 50°F and above, and ramped linearly between 180°F and 150°F at temperatures between 20°F and 50°F.
The baseline building design hot-water pump power shall be 19 W/gpm. The pumping system shall be modeled as primary-only with continuous variable flow and a minimum of 25% of the design flow rate. Hot-water systems serving 120,000 ft2 or more shall be modeled with variable-speed drives, and systems serving less than 120,000 ft2 shall be modeled as riding the pump curve.
Electric chillers shall be used in the baseline building design regardless of the cooling energy source, e.g. direct-fired absorption or absorption from purchased steam. The baseline building design's chiller plant shall be modeled with chillers having the number and type as indicated in Table G126.96.36.199 as a function of building peak cooling load.
Table G188.8.131.52 Type and Number of Chillers
|Building Peak Cooling Load||Number and Type of Chillers|
|≤300 tons||1 water-cooled screw chiller|
|>300 tons, <600 tons||2 water-cooled screw chillers sized equally|
|≥600 tons||2 water-cooled centrifugal chillers minimum with chillers added so that no chiller is larger than 800 tons, all sized equally|
Chilled-water design supply temperature shall be modeled at 44°F and return water temperature at 56°F.
Chilled-water supply temperature shall be reset based on outdoor dry-bulb temperature using the following schedule: 44°F at 80°F and above, 54°F at 60°F and below, and ramped linearly between 44°F and 54°F at temperatures between 80°F and 60°F.
Chilled-water systems shall be modeled as primary/secondary systems with constant-flow primary loop and variable-flow secondary loop. For systems with cooling capacity of 300 tons or more, the secondary pump shall be modeled with variable-speed drives and a minimum flow of 25% of the design flow rate. For systems with less than 300 tons cooling capacity, the secondary pump shall be modeled as riding the pump curve. The baseline building constant-volume primary pump power shall be modeled as 9 W/gpm, and the variable-flow secondary pump power shall be modeled as 13 W/gpm at design conditions. For computer room systems using System 11 with an integrated fluid economizer, the baseline building design primary chilled-water pump power shall be increased by 3 W/gpm for flow associated with the fluid economizer.
The heat-rejection device shall be an axial-fan open-circuit cooling tower with variable-speed fan control and shall have an efficiency of 38.2 gpm/hp at the conditions specified in Table 6.8.1-7. Condenser-water design supply temperature shall be calculated using the cooling tower approach to the 0.4% evaporation design wet-bulb temperature as generated by the formula below, with a design temperature rise of 10°F;
Approach10°F Range = 25.72 — (0.24 × WB)
where WB is the 0.4% evaporation design wet-bulb temperature (°F); valid for wet bulbs from 55°F to 90°F.
The tower shall be controlled to maintain a leaving water temperature, where weather permits, per Table G184.108.40.206. floating up to the design leaving water temperature for the cooling tower. The baseline building design condenser-water pump power shall be 19 W/gpm and modeled as constant volume. For computer room systems using System 11 with an integrated fluid economizer, the baseline building design condenser-water-pump power shall be increased by 3 W/gpm for flow associated with the fluid economizer. Each chiller shall be modeled with separate condenser-water and chilled-water pumps interlocked to operate with the associated chiller.
Table G220.127.116.11 Heat-Rejection Leaving Water Temperature
|Climate Zone||Leaving Water Temperature|
|5B, 5C, 6B, 8||65°F|
|0B, 1B, 2B, 3B, 3C, 4B, 4C, 5A, 6A, 7||70°F|
|0A, 1A, 2A||80°F|
Fans in parallel VAV fan-powered boxes shall run as the first stage of heating before the reheat coil is energized. Fans in parallel VAV fan-powered boxes shall be sized for 50% of the peak design primary air (from the VAV air-handling unit) flow rate and shall be modeled with 0.35 W/cfm fan power. Minimum volume set points for fan-powered boxes shall be equal to 30% of peak design primary airflow rate or the rate required to meet the minimum outdoor air ventilation requirement, whichever is larger. The supply air temperature set point shall be constant at the design condition.
|Method 1—Part-Load Fan Power Data|
|Fan Part-Load Ratio||Fraction of Full-Load Power|
|Method 2—Part-Load Fan Power Equation|
|Pfan = 0.0013 + 0.1470 × PLRfan + 0.9506 × (PLRfan)2 — 0.0998 × (PLRfan)3 |
Pfan = fraction of full-load fan power and
PLRfan = fan part-load ratio (current cfm/design cfm).
Minimum volume set point shall be 50% of the maximum design airflow rate, the minimum ventilation outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards, whichever is larger.
Fan volume shall be reset from 100% airflow at 100% cooling load to minimum airflow at 50% cooling load. Supply air temperature set point shall be reset from minimum supply air temperature at 50% cooling load and above to space temperature at 0% cooling load. In heating mode supply air temperature shall be modulated to maintain space temperature, and fan volume shall be fixed at the minimum airflow.
If the proposed design HVAC systems have humidistatic controls, then the baseline building design shall use mechanical cooling for dehumidification and shall have reheat available to avoid overcooling. When the baseline building design HVAC system does not comply with any of the exceptions in Section 18.104.22.168. then only 25% of the system reheat energy shall be included in the baseline building performance. The reheat type shall be the same as the system heating type.