Optimal Start/Stop of HVAC Systems

Under night setback or setup conditions in both heating and cooling seasons, the building setpoint can be varied during unoccupied periods. However, when the systems are turned back to occupied mode, the building air must be returned to operating temperature as quickly as possible. Because all buildings, and zones within a building, have different thermal inertia, an optimum start algorithm must be used to anticipate the latest time to turn the system back on. These algorithms look at the lowest zone temperature, the outdoor air temperature, and the time lag to heat (or cool) to the setpoint in order to improve the estimate of the start time. They then retrieve and analyze historical temperature and thermal time lag information. These same algorithms can also be used to turn off HVAC systems at the end of the day while the building is still occupied, a technique that is especially effective for buildings with high thermal inertia.

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