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How set infiltration rate (ACH) in AFN model?

asked 2023-12-11 01:24:25 -0500

Rafael's avatar

updated 2023-12-11 01:25:45 -0500

Hi all!

I would like to run AirflowNetwork (AFN) model for ventilation in summer conditions, however, during the winter HVAC system is activated. Heating loads are quite sensible to infiltration rate, thus, I would like to simulate controlling the air change per hour (ACH), but is not possible in a model which has AFN configurated.

Somedody knows how to set both AFN and ZoneInfiltration objects without problems? at least any mode to configurate ACH while AFN is included in the model.

Thank you so much!

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answered 2023-12-11 19:53:15 -0500

updated 2023-12-11 19:53:55 -0500

It is not possible to use AFN and ZoneInfiltration objects in the same model. (There is one exception where you can model ZoneInfiltration if you use AFN's MultiZoneWithDistributionOnlyDuringFanOperation option, but the infiltration will only be modeled when the HVAC system fan is off.)

There is an AirflowNetwork:MultiZone:SpecifiedFlowRate object where you can explicitly specify an infiltration airflow rate, however it cannot currently be scheduled so it cannot vary throughout the year.

Related issue:

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Hi! thanks a lot for your answer. I tried to simulate with AirflowNetwork:MultiZone:SpecifiedFlowRate object and volumentric air flow but EP crashed everytime. I dont know why.

Rafael's avatar Rafael  ( 2023-12-12 21:49:36 -0500 )edit

Hi @Rafael

I recently simulated a model with both AirflowNetwork:MultiZone:Surface:Crack and AirflowNetwork:MultiZone:SpecifiedFlowRate objects. While it crashed with EP 22.1, it worked correctly with EP 23.1. Which version are you currently using?

Additionally, I believe each surface can only have assigned one object between crack and specified flow rate. To introduce the second type, you have to remove the first one that is assigned to the surface.

MatteoMerli's avatar MatteoMerli  ( 2023-12-13 07:06:46 -0500 )edit

@Rafael Is it possible you're hitting this bug? That was fixed in E+ 23.1. If it's not that, please provide a link to your IDF file because it may be an unknown bug.

shorowit's avatar shorowit  ( 2023-12-13 09:41:15 -0500 )edit

Hi! Oh! I see! I'm using V22.2. I will check with the newer and I will tell you. Thank you so much!

Rafael's avatar Rafael  ( 2023-12-13 22:18:50 -0500 )edit

Hi again! I tried with v23.2 but I received this error: * Severe * AirflowNetworkSolver: L-U factorization in Subroutine FACSKY. The denominator used in L-U factorizationis equal to 0.0 at node = COC. One possible cause is that this node may not be connected directly, or indirectly via airflow network connections (e.g., AirflowNetwork:Multizone:SurfaceCrack, AirflowNetwork:Multizone:Component:SimpleOpening, etc.), to an external node (AirflowNetwork:MultiZone:Surface). Please send your input file and weather file to EnergyPlus support/development team for further investigation.

Thank you!

Rafael's avatar Rafael  ( 2023-12-14 00:34:06 -0500 )edit

answered 2024-01-08 15:42:51 -0500

Jamie Sullivan's avatar

There is a trick that allows you to do this actually. While the basic zone infiltration/ventilation objects do not work in an AFN model, HVAC does.

What you can do then, if you want to model the baseline fresh air/infiltration rate simply like this, is to instead deliver it through your HVAC system. Use DesignSpecification:OutdoorAir to specify your desired infiltration rate (which can itself be scheduled), and then apply that to e.g. a basic Ideal Loads system. Make sure the System Availability Schedule is always on (so the system can always provide air), and if you want to schedule the heating/cooling use the Heating Availability Schedule and Cooling Availability Schedule inputs.

Note that this obviously does have limitations - the air provided here will not interact with the AFN and affect the pressures throughout the airflow network at all. It is just adding a certain amount of fresh air completely separate to whatever air is being provided from window opening. That being said, in most situations where there is significant natural ventilation through the windows for cooling it would likely overpower the infiltration anyway. And given how uncertain any natural ventilation estimates are (both in terms of the ventilation modelling and the assumed occupant behaviour) worrying about details like that probably isn't worth it.

Example use:

    infrate,                 !- Name
    AirChanges/Hour,         !- Outdoor Air Method
    0,                 !- Outdoor Air Flow per Person {m3/s-person}
    0,                       !- Outdoor Air Flow per Zone Floor Area {m3/s-m2}
    0,                       !- Outdoor Air Flow per Zone {m3/s}
    0.35,                    !- Outdoor Air Flow Air Changes per Hour {1/hr}
    Always On;               !- Outdoor Air Schedule Name

    Masterbedroom1,          !- Zone Name
    ,                        !- Template Thermostat Name
    Always On,               !- System Availability Schedule Name
    50,                      !- Maximum Heating Supply Air Temperature {C}
    13,                      !- Minimum Cooling Supply Air Temperature {C}
    0.0156,                  !- Maximum Heating Supply Air Humidity Ratio {kgWater/kgDryAir}
    0.0077,                  !- Minimum Cooling Supply Air Humidity Ratio {kgWater/kgDryAir}
    NoLimit,                 !- Heating Limit
    ,                        !- Maximum Heating Air Flow Rate {m3/s}
    ,                        !- Maximum Sensible Heating Capacity {W}
    NoLimit,                 !- Cooling Limit
    ,                        !- Maximum Cooling Air Flow Rate {m3/s}
    ,                        !- Maximum Total Cooling Capacity {W}
    HVAC_day,               !- Heating Availability Schedule Name
    HVAC_day,               !- Cooling Availability Schedule Name
    None,                    !- Dehumidification Control Type
    0.7,                     !- Cooling Sensible Heat Ratio {dimensionless}
    60,                      !- Dehumidification Setpoint {percent}
    None,                    !- Humidification Control Type
    30,                      !- Humidification Setpoint {percent}
    DetailedSpecification,   !- Outdoor Air Method
    0.00944,                 !- Outdoor Air Flow Rate per Person {m3/s}
    0,                       !- Outdoor Air Flow Rate per Zone Floor Area {m3/s-m2}
    0,                       !- Outdoor Air Flow Rate per Zone {m3/s}
    infrate,                 !- Design Specification Outdoor Air Object Name
    None;                    !- Demand Controlled Ventilation Type
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Nice idea. Agreed that the interaction between infiltration and natural ventilation is not important, but there are other components (supply/exhaust only ventilation, duct leakage imbalance, etc.) where the interaction is important.

shorowit's avatar shorowit  ( 2024-01-08 17:35:00 -0500 )edit

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Asked: 2023-12-11 01:24:25 -0500

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Last updated: Jan 08