If you received an alert that your air handler had shut down, your first step would probably be to log on to your BAS and try to restart it. Building power looks ok, no other equipment is affected, and you don’t notice anything unusual on the graphic, but your AHU start/stop command does nothing. Now what?

Your maintenance office might have a shelf of dusty 3” binders filled with every job TOP from the last 20 years, crammed with mechanical drawings and sensor cutsheets. Maybe that’s your next stop. If you’re lucky, you might be able to find the right job (from 2015) and then locate the engineer’s original Sequence of Operation. But as you unfold the drawing, you remember that the Energy Team made some modifications to the programming last year, but you aren’t sure what they were. A vendor tech might have made some tweaks, too, the last time there was a TI on one of your floors.

If you need to have a clear and current picture of how your equipment is programmed, the document you need is a System Software Design Specification. The System SDS describes in regular language how the sequence was implemented within the custom logic. You’ll know what causes alarms and how to clear them. It will tell you how operating modes are determined, and what behaviors you can expect to see (including the all-important startup and shutdown conditions). Any system with custom-programmed behavior should have an SDS: Air systems, water systems, alarm monitoring systems, and especially power loss/recovery systems.

Maintaining an SDS throughout the life of the equipment is ideal, but if you don’t already have one, we can as-built it for you based on your current programming. You’ll still need to troubleshoot the AHU, but you’ll have the information you need to identify the problem and get your building back up and running.