Kevin.
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Under the tab of Flow Parameters, you?may use Return-Cap-Ratio to specify the return air flowrate. Also,?you can check the definition of this parameter to make sure that's what you want.
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Hope this helps.

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Shaojie
RETURN-CAP-RATIO
For systems that can have a return fan, specifies the ratio of the return/relief fan capacity to the supply capacity. Applies only when a return fan exists. In many buildings, the return fan is deliberately sized 10% to 20% smaller than the supply fan (RETURN-CAP-RATIO = 0.8 to 0.9) so that the building is pressurized whenever the fans run. The difference between the supply and return air flow exfiltrates from the building (or is exhausted through zone exhaust fans). The default value of 1.0 implies that no building pressurization occurs. The amount assumed to exfiltrate is
FX = [SUPPLY-FLOW] * (1.0 - [RETURN-CAP-RATIO])
In other words, the amount exfiltrated due to building pressurization (or zone exhaust) is assumed to be constant all hours of operation. In the hourly simulation, the return/relief fan operates only when the actual return air flow is greater than (FX - zonal exhaust). The return/relief fan moves only the amount of return air greater than FX. If FX is greater than the outside air ventilation requirements, as defined through other keywords, then FX will override any other outside air requirements including those specified through scheduled values. If you have specified the return fan size, a check will be made to see if any building pressurization is occurring. The difference between the supply flow and return flow is compared to the zonal exhaust quantity. If the zonal exhaust quantity is less than this difference, then the balance is assumed to exfiltrate. Note that infiltration is still assumed to occur, as infiltration is calculated in the LOADS
module. For this reason, the infiltration schedules in LOADS should be coordinated with the fan schedules in SYSTEMS.
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