I am working on a laboratory project and we are using ASHRAE 90.1 2013 for the energy credit.  In 2013 it states the baseline lab exhaust shall be modeled as constant horsepower reflecting constant volume stack discharge with outdoor air bypass. Can you advise how we calculate the baseline lab exhaust HP?  Is it matched to proposed as a process load or based on the baseline overall fan power for the system as a regulated load and split out somehow.