Notes to Table C407.5.1(3) state that chilled water supply temperature shall be modeled at 44°F design supply temperature and 56°F return temperature. Piping losses shall not be modeled in either building model. Chilled water supply water temperature shall be reset in accordance with Section C403.4.3.4. i.e.
Hydronic systems greater than or equal to 300,000 Btu/h (87 930 W) in design output capacity, supplying heated or chilled water to comfort conditioning systems shall include controls that have the capability to:
· Automatically reset the supply-water temperatures using zone-return water temperature, building-return water temperature, or outside air temperature as an indicator of building heating or cooling demand. The temperature shall be capable of being reset by at least 25 percent of the design supply-to-return water temperature difference; or
· Reduce system pump flow by at least 50 percent of design flow rate utilizing adjustable speed drive(s) on pump(s), or multiple-staged pumps where at least one-half of the total pump horsepower is capable of being automatically turned off or control valves designed to modulate or step down, and close, as a function of load, or other approved means.
For applicable IECC standard reference systems, the chilled water temperature shall be reset on an hourly basis, based on the outdoor air temperature. When the outdoor temperature is 80°F or greater, the supply temperature shall be a constant 44°F. When the outdoor temperature is 60°F or below, the supply temperature shall be 56°F. When the outdoor temperature is between 60°F and 80°F, the supply temperature shall ramp between 44°F and 56°F in a proportional manner. If there is dehumidification requirement of the cooling coils, the maximum reset of chilled water temperature should be calculated according to the dehumidification requirement and cooling coils performance.
Figure 10: Default chilled water loop supply water temperature control settings
