HVAC fresh air delivery in zone higher/lower than amount requested
Posted by Andy Tindale on 06-Apr-20 03:06 PM

This is known and expected behaviour for EnergyPlus AHU-based Detailed HVAC systems with fixed outside fresh air control including VAV, CAV and Unitary multizone. The reason for the slight differences in zone fresh air from the values requested for individual zones is as follows. When sizing the system EnergyPlus calculates the total fresh for the AHU system by summing the requirements of each of the zones served by the AHU. The flow of fresh air is controlled by the outside air controller during the simulation to give the required total amount. The delivery of air from the AHU to each of the zones is carried out according to heating and cooling loads of each zone and the fresh air is distributed around the building accordingly but this does not guarentee that each zone will receive the requested amount. So for example, zones having high cooling loads might receive more fresh air because they will use a higher proportion of the total fresh air to meet the load. So when you check fresh air delivery at the building level you should see agreement with input data but individual zones will usually have slightly more or less fresh air than requested.