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Why does Controller:OutdoorAir not set OA-Flow to zero as specified ?

asked 2018-06-20 02:54:13 -0500

updated 2018-06-20 07:58:06 -0500

I have a simple airloop as shown below:

image description

If Outdoor DB Temperatures are too low, then the Outdoor Air Flow should be set to zero. Accordingly the minimum OA Flow rate has been set to 0 m³/s. See here:

image description

Nevertheless, in the following chart you see that outdoor air flow at the outdoor air IN node never reaches zero, but apparently reaches a minimum greater than zero, the origin of which is totally unclear: image description

Can anyone explain what the reasons might be and how to enforce zero outdoor air flow when outdoor air is too cold ?

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In the AirTerminal:SingleDuct:VAV:NoReheat object - do you have the box "Control for Outdoor Air" checked?

mdahlhausen gravatar imagemdahlhausen ( 2018-06-20 12:11:27 -0500 )edit

Is there a zone exhaust fan?

MJWitte gravatar imageMJWitte ( 2018-06-20 16:10:11 -0500 )edit

the entries for the VAV object are:

AirTerminal:SingleDuct:VAV:NoReheat

Name =                      Main Zone VAV Air
Availability Schedule Name =System Availability Schedule
Air Outlet Node Name =      Main Zone Inlet Node
Air Inlet Node Name =       Main Zone ATU In Node
Maximum Air Flow Rate =     9
Zone Minimum Air Flow Input Method= Constant
Constant Minimum Air Flow Fraction= 0.05
Fixed Minimum Air Flow Rate = 0
Minimum Air Flow Fraction Schedule Name =
Design Specification Outdoor Air Object Name =

-- There is only a Var Speed Fan in the airloop.

OS-user-AT gravatar imageOS-user-AT ( 2018-06-21 02:21:34 -0500 )edit

It's going to be impossible to answer this without seeing the complete EnergyPlus idf file. Please post a link here or e-mail to energyplus-support@gard.ocm

MJWitte gravatar imageMJWitte ( 2018-06-21 17:38:16 -0500 )edit

OK, here is a temporary download link

OS-user-AT gravatar imageOS-user-AT ( 2018-06-26 05:33:16 -0500 )edit

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answered 2018-06-27 14:53:05 -0500

The Controller:OutdoorAir references a Controller:MechanicalVentilation object which is always on which references a DesignSpecification:OutdoorAir with OA/person (but this zone has no people) and OA/zone = 9 m3/s. (I should have noticed the Mechanical Ventilation Controller name in the screen shot above.)

In general, the outdoor air flow rate will be the greater of the Controller:Outdoorair Minimum Outdoor Airflow Rate (times Minimum Outdoor Air Schedule) OR the flow requested by Controller:MechanicalVentilation (when it's schedule is active). The schedule in DesignSpecification:OutdoorAir is also applied to any values coming from there which feed into the Controller:MechanicalVentilation. Then add in the economizer and other limits for final OA flow rate. See list of limit checks in order in the Input Output Reference in the introduction to Controller:OutdoorAir.

From this input file, the relevant inputs are:

Controller:OutdoorAir,
Outdoor Air Controller,  !- Name
<snip>
0,                       !- Minimum Outdoor Air Flow Rate {m3/s}
Autosize,                !- Maximum Outdoor Air Flow Rate {m3/s}
<snip>
Always Off Discrete,     !- Minimum Outdoor Air Schedule Name
Always Off Discrete,     !- Minimum Fraction of Outdoor Air Schedule Name
Always On Discrete,      !- Maximum Fraction of Outdoor Air Schedule Name
Controller Mechanical Ventilation 1,  !- Mechanical Ventilation Controller Name

Controller:MechanicalVentilation,
Controller Mechanical Ventilation 1,  !- Name
Always On Discrete,      !- Availability Schedule Name
No,                      !- Demand Controlled Ventilation
ZoneSum,                 !- System Outdoor Air Method
1,                       !- Zone Maximum Outdoor Air Fraction {dimensionless}
Main Zone,               !- Zone 1 Name
OA_Design_Zone,          !- Design Specification Outdoor Air Object Name 1
DataCenter Design Air Dist;  !- Design Specification Zone Air Distribution Object Name 1

DesignSpecification:OutdoorAir,
OA_Design_Zone,          !- Name
Flow/Zone,               !- Outdoor Air Method
0.00,                 !- Outdoor Air Flow per Person {m3/s-person}
,                        !- Outdoor Air Flow per Zone Floor Area {m3/s-m2}
9,                       !- Outdoor Air Flow per Zone {m3/s}
,                        !- Outdoor Air Flow Air Changes per Hour {1/hr}
Always On Discrete;      !- Outdoor Air Schedule Name
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answered 2018-06-21 03:35:27 -0500

A solution that works is as follows:

Add a Maximum Outdoor Air Fraction Schedule to Controller:OutdoorAir which is EMS-controlled (in EMS the condition is defined for setting outdoor air flow to zero). Although this works, this solution is not fully satisfactory because it remains unexplained why EnergyPlus produced a minimum outdoor air flow greater than zero in the first place.

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Asked: 2018-06-20 02:54:13 -0500

Seen: 144 times

Last updated: Jun 27 '18