Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
When we get the "no apparatus dew-point" warning, it suggests that the user-specified Design Outlet Air Humidity Ratio of the cooling coil object is too low. If it is autosize, Zone Cooling Design Supply Air Humidity Ratio in Sizing:Zone or Central Cooling Design Supply Air Humidity Ratio in Sizing:System will be used.
I think the Design Outlet Air Humidity Ratio should be revised manually, but EnergyPlus kindly revises the coil outlet air condition automatically to 90% relative humidity at the same outlet enthalpy (Engineering Reference, Github).
This automatic revision does not affect the coil total cooling capacity because the enthalpy remains the same as the user-specified design coil outlet condition. In other words, the user-specified design coil outlet air temperature & humidity ratio determine the coil cooling capacity. This is also true even if Zone Load Sizing Method in Sizing:Zone is Sensible Load Only No Latent Load and Type of Load to Size On in Sizing:System is Sensible.
I've seen many EnergyPlus users not pay much attention to Zone Cooling Design Supply Air Humidity Ratio and Central Cooling Design Supply Air Humidity Ratio, but they are very important parameters, especially when equipment is autosized. They affect not only the cooling coil capacity, but also the coil CHW flow rate, pump CHW flow rate and the chiller capacity.
