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

asked 2019-04-04 10:50:43 -0600

BORM gravatar image

updated 2019-04-04 14:31:03 -0600

I'm trying to simulate an air loop in Openstudio 2.7 that recirculate indoor air, but use outdoor air only when economizer is ON.

The HVAC system provides air for offices (picture 1).

The problem is the air loop provides continuously an outdoor air flow rate (picture 3) whenever economizer is not always ON (picture 2).

Is there a way to set outdoor air flow rate to zero when economizer is not ON?

Picture 4 provides information about ControllerOutdoorAir.

  • Maximum flow rate allowed has been hard sized
  • No schedule for Fraction for outdoor air was applied.

Picture 1 shows the actual air loop.

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Picture 2 shows the air economizer status ON.

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Picture 3 shows the outdoor air mass flow rate for the air loop.

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Picture 4 ControllerOutdoorAir parameters

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What are the ControllerOutdoorAir object's settings?

MatthewSteen gravatar image MatthewSteen  ( 2019-04-04 13:55:59 -0600 )edit

It has been updated in the question.

BORM gravatar image BORM  ( 2019-04-04 14:29:25 -0600 )edit

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answered 2019-04-04 14:56:59 -0600

updated 2019-04-04 14:57:57 -0600

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)

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Asked: 2019-04-04 10:50:43 -0600

Seen: 118 times

Last updated: Apr 04 '19