How to provide no outdoor air except for free-cooling?

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See the documentation for the Controller:OutdoorAir object, which outlines the order that OA limits are applied (excerpt below). In your case, I think setting the Minimum Outdoor Air Schedule Name field to always 0 is the solution (first bullet), which should be overridden by the economizer (second bullet).

There are various minimum and maximum limits on the outdoor air flow rate which are applied in the following order. Limits that fall later in the list can override earlier limits.

• OA flow rate = Minimum Outdoor Air Flow Rate * Minimum Outdoor Air Schedule

• Apply economizer controls

• OA flow rate ≥ Controller:MechanicalVentilation OA flow rate

• OA flow rate ≥ System exhaust flow rate

• (OA flow rate)/(Current mixed air flow rate) ≥ Minimum Fraction of Outdoor Air Schedule

• (OA flow rate)/(Current mixed air flow rate) ≤ Maximum Fraction of Outdoor Air Schedule (even if this reduces the OA flow rate, can override all minimums and economizer)

• OA flow rate ≤ Maximum Outdoor Air Flow Rate

• Apply OA flow rate specified by demand limiting

• Apply OA flow rate specified by EMS

• OA flow rate ≤ Current mixed air flow rate (system flow rate)