AirLoopHVAC Autosizing does not recognise load from ZoneHVAC:OutdoorAirUnit

asked 2023-05-20 08:19:15 -0500

Keigo
3699 ●1 ●43 ●21 https://www.linkedin.c...

updated 2023-05-20 18:13:59 -0500

A bug report. As per the title. A sample idf file is here (V22-2-0). It's based on the ExampleFile: OutdoorAirUnitwithAirloopHVAC.idf, but I revised it a little bit. There are 5 conditioned zones in the ExampleFile, and only SPACE4-1 has both ZoneHVAC:OutdoorAirUnit and AirLoopHVAC. Originally, ZoneHVAC:OutdoorAirUnit for SPACE4-1 has HeatExchanger:AirToAir:Plate and Coil:Cooling:Water, but I removed them, which is irrerevant to the bug I want to report. Below is the air-side schematic. Please imagine that a kitchen has VAV system, Exhaust fan and Fresh air makeup fan with preheat coil. I want to model that kind of system. So, the ZoneHVAC:OutdoorAirUnit here is meant to be Exhaust fan and Fresh air makeup fan with preheat coil.

image description

AirLoopHVAC VAV Sys 4-1 and AirTerminal:Singleduct:VAV:Reheat are autosized, but it seems that they do not recognise the fresh air load from ZoneHVAC:OutdoorAirUnit, which results in so many unmet hours in the annual run.

Some notes:

The sequence described in ZoneHVAC:EquipmentList is ZoneHVAC:OutdoorAirUnit first and ZoneHVAC:AirDistributionUnit second.
I tried chaning Account for Dedicated Outdoor Air System in Sizing:Zone from "No" to "Yes", but in vein.
I tried modelling "Exhaust fan and Fresh air makeup fan with preheat coil" with AirLoopHVAC instead of ZoneHVAC:OutdoorAirUnit i.e. there are two AirLoops in one zone. EnergyPlus allows two AirLoops in one zone, but autosizing does not work well. The VAV system (one AirLoopHVAC) also did not recognise the fresh air load from the Fresh air makeup fan with preheat (another AirLoopHVAC) during sizing run. I will report this as a separate post when I have time.
ZoneVentilation:DesignFlowRate is not appropriate because I need to model a preheat coil on the fresh air makeup.

Is there any way to make AirLoopHVAC recognise the load fromZoneHVAC:OutdoorAirUnit?

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Posted issue as https://github.com/NREL/EnergyPlus/issues/10045

Keigo ( 2023-05-26 08:27:09 -0500 )edit

It sounds like you might want to try a DesignSpecification:AirTerminal:Sizing object to modify the size of the air terminal unit to account for the additional outdoor air load.

ericringold ( 2023-05-26 09:58:47 -0500 )edit

@Eric Ringold If I increase Fractions in DesignSpecification:AirTerminal:Sizing, does the size of the central coil and fan also increase accordingly?

Keigo ( 2023-05-26 10:11:29 -0500 )edit

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answered 2023-11-23 10:14:02 -0500

Keigo
3699 ●1 ●43 ●21 https://www.linkedin.c...

Just confirmed that DesignSpecification:AirTerminal:Sizing is not useful in my case. I added it in the updated idf file (V23-2-0). The files are here.

DesignSpecification:AirTerminal:Sizing,
    DSATS,                   !- Name
    1,                       !- Fraction of Design Cooling Load {dimensionless}
    1,                       !- Cooling Design Supply Air Temperature Difference Ratio {dimensionless}
    1,                       !- Fraction of Design Heating Load {dimensionless}
    1,                       !- Heating Design Supply Air Temperature Difference Ratio {dimensionless}
    8.3142857143;            !- Fraction of Minimum Outdoor Air Flow {dimensionless}

ZoneHVAC:AirDistributionUnit,
    SPACE4-1 ATU,            !- Name
    SPACE4-1 Supply Inlet,   !- Air Distribution Unit Outlet Node Name
    AirTerminal:SingleDuct:VAV:Reheat,  !- Air Terminal Object Type
    SPACE4-1 VAV Reheat,     !- Air Terminal Name
    ,                        !- Nominal Upstream Leakage Fraction
    ,                        !- Constant Downstream Leakage Fraction
    DSATS;                   !- Design Specification Air Terminal Sizing Object Name

image description

The design air flow rate and the cooling capacity of the AirLoopHVAC VAV Sys 4-1 increased as expected. However, the outdoor air flow rate in the AirLoop is slightly higher than the minimum outdoor air flow rate of 0.42m3/s in winter and much larger in summer. It should be constant at 0.42m3/s. The chart below shows the outdoor air flow rate of the two idf files during SummerDesignDay and WinterDesignDay as an example.

image description

EnergyPlus is not good at autosizing multiple HVAC systems in one zone. Hopefully the bug(s) will be fixed as soon as possible.

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AirLoopHVAC Autosizing does not recognise load from ZoneHVAC:OutdoorAirUnit

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AirLoopHVAC Autosizing does not recognise load from ZoneHVAC:OutdoorAirUnit