Good building mechanical design requires a firm grasp of the mechanical engineering fundamentals (thermodynamics, psychrometrics, the fan laws, etc.). The same can be said about the BAS design needed to control these mechanical systems. However, engineers are not normally taught control theory and its application to commercial buildings. Therefore, BAS designs often lack the same engineering foundation as that for the associated mechanical systems. This month, I will revisit an important aspect of control fundamentals.
Control loops are the foundation of how a BAS performs temperature control. There are often thousands of control loops in even a moderately-sized BAS, and there’s usually three or more control loops in a single VAV box controller. There are two types of control loops: open and closed. Open loop control bases its action on an input, but that action does not have an effect on the input. Examples include time clock control, reset schedules based on outside air temperature, etc. Open loops are important to temperature control but they should never be used when closed loop is the right approach.