Modeling Optional Ventilation Fan

edit

While doing simulation for a system that has an optional ventilation fan, should we model the fan in the baseline as well. ASHRAE appendix G does not say anything about modeling this fan and nor does it have any guidance on allowance for fan power. In that case, should the ventilation fan be modeled same in baseline as in proposed? And if we are not supposed to model the OA fan at all in the baseline design, should we add the static pressure of the outside fan to the static pressure for the primary fan for baseline to get a realistic calculation of fan power?