Older BAS are typically replaced when BAS controllers and/or operator interface (the “brains”) are at the end of their useful life. This typically means that the brains are no longer supported by the manufacturer. BAS are custom-built, using many components besides the “brains.” These include actuators/valves, sensors/switches, various types of wiring, conduit/EMT/J-boxes, enclosures including power supplies/transformers, etc. For the sake of this subject, I will refer to these components as field devices. How much of the system beyond the brains should also be replaced?
The simplest approach to designing a BAS retrofit is, of course, to specify that everything be replaced. However, this may not provide the best value to the owner. The cost of field devices represents a substantial portion of a BAS installation, yet they are neither part of the system’s brains nor do they all share the same expected length of useful life. Instead, field devices need to be evaluated to determine the optimum combination of replace vs. reuse … the devil is in the details. The following are suggestions about how to negotiate these details without the need for excessive design efforts.