The cooling load is determined based on the measured chilled water supply and return temperature differential, which is calculated in the Supply Return Water Differential module (a Setpoint/Miscellaneous module).
This module is designed to ensure chiller startup by providing a fixed temperature differential output of 14°F (7°C) under startup conditions when the chilled water supply and return temperature differential is typically zero. After startup, once the chilled water supply temperature falls to 46°F (or below 10°C), the module begins to output the true chilled water supply-return temperature differential.
This building load is compared to the maximum rated temperature drop across the devices, as defined in the details of the Chiller Selector Module as Max Delta CHWT. This method is inexpensive since it uses only temperature sensors.
The Temperature Differential method may be used only on primary constant speed pumps.