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# Turn off outside air for air loop during unoccupied period

In case of an office building, my system should be running 6am to 10pm, and should cycle with the load at night, except I want to make sure I don't have any outside air during the night cycle (unless economizer is activated for cooling).

Let's call my schedule HVAC Operation Schedule, and it has a value of 1 between 6 am and 10pm, and 0 otherwise

I've set an AvailabilityManager:NightCycle for the airloop an assigned HVAC Operation Schedule as fan schedule.

I figured that to turn off the outside air I should assign the HVAC Operation Schedule to the Controller:OutdoorAir.

Instinctively, I have set this schedule to the Maximum Fraction of Outdoor Air Schedule Name field. This made sense to me: if I say the maximum is zero, it should be turned off!

This doesn't seem to be doing what I want, since when cycling at nighttime, it's apparently still pulling Outside Air.

After investigation in the documentation and confirming it by looking at the DOE Commercial reference buildings, it seems that the field I should use is Minimum Outdoor Air Schedule Name. From the docs:

Minimum Outdoor Air Schedule Name

The name of a schedule which uses decimal values (0.0 1.0). These values are multiplied by the minimum outdoor air flow rate. This schedule is useful for reducing the outdoor air flow rate to zero during unoccupied or start up hours. If this field is not entered, the minimum outdoor air flow rate either remains constant during the simulation period (Minimum Outdoor Air Control Type = FixedMinimum) or varies in proportion to the supply air flow rate (Minimum Outdoor Air Control Type = ProportionalMinimum).

I'm having trouble following the logic here... Can someone enlighten me?

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Not sure but I think Maximum Fraction of Outdoor Air Schedule Name can be used for Economizer Free Cooling condition. it applies for VAV system when ambient conditions is appropriated for Economizer operation. -> air flow of supply fan in VAV system is mostly bigger than Ventilation Requirements (Minimum outdoor air flow ). This Maximum Fraction of Outdoor Air Schedule Name can be bigger than 1.0 (fraction of minimum outdoor flow). Please look at in Engineer Documentation for more investigate here outdoor air damper

( 2016-02-17 16:05:04 -0500 )edit

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We've found this to be an issue as well. The documentation for the Controller:OutdoorAir object states that the input for Minimum Fraction of Outdoor Air Schedule Name "overrides Minimum Outdoor Air Schedule and Minimum Outdoor Air Flowrate." However, we found this IS NOT true and that using Minimum Outdoor Air Schedule by itself works fine. This field should be set to the same HVAC Operation Schedule you defined above.

The documentation for the AvailabilityManager:NightCycle object doesn't explicitly say it, but I'm guessing that when it cycles on fans during unoccupied periods, it is programmed to bring in minimum OA. This would explain why setting the MINIMUM OA to zero through the Minimum Outdoor Air Schedule input field instead of the MAXIMUM OA would cause night cycling to stop introducing outside air.

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This was addressed in v8.6, see PR #5794.

( 2017-07-24 09:59:29 -0500 )edit

It doesnt seem that this has been addressed in v 9.1.0. Using the approach outlined above I see minimum outside air being provided to each zone when NightCycle turns the system on. This is also what is happening in the 2004 90.1 large office reference model.

( 2019-05-21 12:30:44 -0500 )edit
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The OA controller is not sensitive to nightcycle operation vs regular operation. If any of the minimum OA criteria are non-zero, it will introduce outdoor air. The economizer may also operate during night cycle. That's the case for RefBldgLargeOfficeNew2004_Chicago - it's the economizer.

By the way, the above answer stating that Min Fraction of Outdoor Air Schedule does not override the Minimum Outdoor Air Schedule isn't exactly correct. Minimum Fraction of Outdoor Air Schedule Name will only have an impact if it greater than min OA flow that results from all of the other OA controls

( 2019-05-29 15:37:19 -0500 )edit

In addition to above, availability managers only override fan availability schedule internally in EnergyPlus. If same fan schedule is used elsewhere in the idf such as in controller or as any HVAC schedule, or elsewhere, availability manager won't have any impact there. In controller or other objects, fan schedule values will still be same as specified in the idf (can be verified by looking Maximum Fraction of Outdoor Air Schedule Name schedule values in eplusout.csv). One approach could be to use EMS (perhaps with AirLoopHVAC Availability Status) to modify HVAC Operation Schedule. This may have timestep delay issues though.

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