On a recent project status call an engineering firm’s mechanical engineer stated, “We always design for the worst case.” The focus of the call was to troubleshoot operational problems associated with an air handler that was experiencing problems due to low load conditions. Weeks of effort and multiple attempts to get the unit to operate had resulted in repeated failures. On the call were the design engineer, the installer, the air handler’s manufacturer’s representative, the commissioning agent, and various other entities. Basically, everyone agreed the unit would work properly if it was seeing higher load conditions. The problem was it was still early in a phased construction project and the existing conditions were far from maximum load. Quite the opposite, the loads were fairly small, so the unit was tripping safeties associated with low load demands.
What struck me as interesting was that in reality the “worst case” scenario was not what would be encountered at rated design loads, but was obviously occurring at the opposite end — at loads associated with “Day 1” conditions. Some of the recommended solutions would have significant negative impacts on the unit and system including customizing what was supposed to be a standardized unit, writing new complex controls programming to implement an equally complex new sequence-of-operations, and abandoning some of the unit’s features and components due to oversizing to try and get the unit to perform “adequately” at low loads. There was no longer any discussion regarding optimization, reliability, or energy efficiency.