Air distribution in a space and diffuser selection is simple, right? It is so simple that it is typically one of the first tasks that a young aspiring HVAC engineer or designer is assigned to do in their first job. It really can’t get much simpler than looking in an air device manufacturer’s catalog and looking at the terminal velocity of 50 or 100 or 150 ft per minute, and then placing a diffuser in an architect’s reflected ceiling plan to get the “throw” needed at the wall or an adjacent diffuser’s termination velocity point, right? It is engineering fluid flow and heat transfer reduced to a simple, tried and true design rule of thumb that is quick and easy.
However, is this all defensible in court? Defensible? Really? Who thinks about that in a time crunch to get a job designed and out the door? Isn’t the terminal velocity method the standard of care acceptable by every engineer and the way to ensure good air distribution and comfort? Doesn’t terminal velocity air device selection ensure meeting ASHARE Standard 62.1 and ASHRAE Standard 55? Well, maybe yes, but maybe no. It depends.