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BEopt currently only models the most common type of CFIS system, where the blower fan will run for a set number of mins/hr to ensure that the mechanical ventilation requirement is met regardless of how much heating or cooling occurs for a given hour. For example, if it's the swing season and there is no call for heating/cooling, the blower fan will still run for a portion of the hour to provide ventilation-only air. Conversely, if it's the peak of the summer/winter and the system is running continuously, it's assumed there is an automated outdoor air damper that will close after sufficient outdoor air has been provided.

That said, the underlying modeling framework that BEopt uses (OpenStudio-HPXML) is in the process of allowing for more CFIS control scenarios. There are a lot of ways that CFIS systems can be controlled and many of them are not very good because they either provide too much or too little ventilation (i.e., do not provide equivalent levels of outdoor air). But we want to provide opportunities to analyze them.

Specifically, we are looking to allow inputs that describe:

1. Whether the CFIS system has an automated flow control of outdoor air (if not, you can get too much ventilation when the HVAC system is running 100%)
2. Whether blower fan control strategy runs at a fixed interval regardless of heating and cooling runtimes or whether it includes heating and cooling runtimes in the fan operation.
3. The strategy for meeting the remainder of the ventilation target when the HVAC system has not run enough for the given hour -- using the blower fan (which is what BEopt currently assumes), using a supplemental exhaust/supply fan, or no strategy (which can result in too little ventilation)

Long story short, there are plans underway to support the CFIS control strategy you are asking about, but it will be some time before it's available in BEopt.