Re-cooling zone air after mixing with return air

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It depends if your are using constant flow or variable flow with fixed proportions.

For constant flow:

I believe your application actually warrants using an AirTerminal:SingleDuct:ConstantVolume:FourPipeInduction, for which you'll set the Induction Ratio to $\frac{80\%}{20\%} = 4$. If you don't have a heating coil in your terminal unit, you might have to include one anyway and zero it out, as I believe it's required.

For help and examples:

• This object is available in the OpenStudio Application as of 1.9.0 (release candidate right now) if you need help setting it up.
• The example file 5ZoneFPIU.idf uses one of these.

For variable flow

Probably try to debug your error, using a VAV terminal and a ZoneHVAC unit is likely the way to go.