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How to define a good AirflowNetwork Crack for a AirBoundary wall?

asked 2024-02-04 20:15:25 -0600

paula's avatar

updated 2024-02-19 09:16:42 -0600

Hello,

I'm using Energy+ (self-taught) to help us design a small home in the countryside. I have an L-shaped open plan kitchen+living space that I split into two rectangular zones using an AirBoundary wall (making them convex to have FullInteriorAndExteriorWithReflections running). I am also using AirflowNetwork to simulate natural ventilation.

Somewhere on the internet (I can't find the link anymore) I read that I could use an AirflowNetwork:MultiZone:Surface:Crack with the AirBoundary wall to have the air flowing through it.

Doubts:

1 - is it indeed okay to emulate a big opening with an AirflowNetwork:MultiZone:Surface:Crack?

2 - If so, what would be reasonable values of Air Mass Flow Coefficient and Air Mass Flow Exponent? I could not find an example for this case in the ExampleFiles.

Thank you.

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answered 2024-02-18 17:22:06 -0600

Jamie Sullivan's avatar

Personally, I just set up a permanent opening for open zone connections like that.

So draw a "door" covering most of the wall, and assign the airboundary construction to it. Then in the AFN you can assign a normal AFN surface and opening to it, just one set up to be always open rather than a normal door which might be linked to the ventilation schedule to open and close.

The opening properties are much simpler to think about than normal, as the opening will always be fully open and so most of the inputs don't matter. One thing to remember though, iirc, is that you only want to apply the opening to one surface, not both of them. If you assign the AFN opening to both of the door surfaces, then it will double your airflow.

(I'm not really sure if a discharge coefficient of 0.62 is correct for this - I'm just using it as a generic default window opening coefficient - but actually working out what discharge coefficients should be for different situations is not straightforward)

e.g.

Schedule:Compact,
    Always On,               !- Name
    Fraction,                !- Schedule Type Limits Name
    Through: 12/31,          !- Field 1
    For: AllDays,            !- Field 2
    Until: 24:00,            !- Field 3
    1.0;                     !- Field 4

AirflowNetwork:MultiZone:Surface,
    In_Ghost_Door_Hallway1_KitchenLiving_1,  !- Surface Name
    GhostOpening,            !- Leakage Component Name
    ,                        !- External Node Name
    1,                       !- Window/Door Opening Factor, or Crack Factor {dimensionless}
    Constant,                !- Ventilation Control Mode
    ,                        !- Ventilation Control Zone Temperature Setpoint Schedule Name
    0,                       !- Minimum Venting Open Factor {dimensionless}
    0,                       !- Indoor and Outdoor Temperature Difference Lower Limit For Maximum Venting Open Factor {deltaC}
    100,                     !- Indoor and Outdoor Temperature Difference Upper Limit for Minimum Venting Open Factor {deltaC}
    0,                       !- Indoor and Outdoor Enthalpy Difference Lower Limit For Maximum Venting Open Factor {deltaJ/kg}
    3e+05,                   !- Indoor and Outdoor Enthalpy Difference Upper Limit for Minimum Venting Open Factor {deltaJ/kg}
    Always On;               !- Venting Availability Schedule Name

AirflowNetwork:MultiZone:Component:DetailedOpening,
    GhostOpening,            !- Name
    1e-05,                   !- Air Mass Flow Coefficient When Opening is Closed {kg/s-m}
    0.65,                    !- Air Mass Flow Exponent When Opening is Closed {dimensionless}
    NonPivoted,              !- Type of Rectangular Large Vertical Opening (LVO)
    0,                       !- Extra Crack Length or Height of Pivoting Axis {m}
    2,                       !- Number of Sets of Opening Factor Data
    0,                       !- Opening Factor 1 {dimensionless}
    0.62,                    !- Discharge Coefficient for Opening Factor 1 {dimensionless}
    0,                       !- Width Factor for Opening Factor 1 {dimensionless}
    0,                       !- Height Factor for Opening Factor 1 {dimensionless}
    0,                       !- Start Height Factor for Opening Factor 1 {dimensionless}
    1,                       !- Opening Factor 2 {dimensionless}
    0.62,                    !- Discharge Coefficient for Opening Factor 2 {dimensionless}
    1,                       !- Width Factor for Opening Factor 2 {dimensionless}
    1,                       !- Height Factor for Opening Factor 2 {dimensionless}
    0;                       !- Start Height Factor for Opening Factor 2 {dimensionless}

Using the AFN does have some dangers however - I'd suggest checking for instances of overcooling and clashes between ventilation and heating, and if you do have them see my answer here for how to fix EnergyPlus's opening controls. Or just use the simple ventilation objects instead, as they are much easier to use.

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Thanks, @Jamie Sullivan; indeed, this solution is simpler than what I was thinking.

paula's avatar paula  ( 2024-02-19 11:52:47 -0600 )edit

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Last updated: Feb 19