Target superheat needs indoor wet bulb. Students, buy the probe.

Asked by ms_almonte · Aug 2

Every summer a few of my former students come back from their first jobs saying they "checked the superheat" on a piston system and it was fine. Then I ask what their target was and they look at me. First, superheat by itself is just a number. On a fixed orifice system you compare it to a target, and the target comes from the manufacturer's chart or a target superheat chart, using indoor wet bulb and outdoor dry bulb. Second, wet bulb, not dry bulb. Wet bulb tells you how much moisture the coil is dealing with, and that moves the target a lot. A sticky July afternoon in Paterson and a dry day in September want different numbers. Third, measure the return air at the unit, not in the hallway. Buy a decent wet bulb probe before you buy the fancy gauges. Ask me how I know which one matters more.