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What are the solutions for modeling natural ventilation in summer?

asked 2024-01-24 07:38:43 -0500

updated 2024-01-25 10:35:17 -0500

Hello everyone,

I am trying to model a window opening in the summer. From what I have observed, the existing solutions seem to be primarily designed for natural ventilation in winter (for example, AirflowNetwork_Multizone_HorizontalOpening.idf). The only solution I have identified is to adapt the model (EMSAirflowNetworkOpeningControlByHumidity.idf), which requires me to define a ventilation network. Do you see any other solutions?

Regards, Simon

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answered 2024-01-31 18:34:27 -0500

Jamie Sullivan's avatar

There are two basic paths here in EnergyPlus:

  1. Use the simple ZoneVentilation objects to apply ventilation to your zone when it's hot (by far the simplest option)
  2. Use the AirflowNetwork to try to better account for interzonal airflow etc.

As Denis notes, there are a lot of uncertainties in ventilation modelling regardless of what you do, and to be honest one of the first things you'll need to accept is that whatever you do your modelled ventilation will be "wrong", whether due to the inaccuracies in the calculations or the uncertainty in occupant behaviour. The only thing you can do is try to manage this with sensitivity analysis, and factor this into your analysis.

Both of these work on the setpoint system, and will apply ventilation/open the windows when the temperature rises above your setpoint. I'm not sure why you're finding that the windows are only opening in winter - possibly if you're running the humidity control EMS example then with your weather file it's only hitting the humidity setpoints in winter?

Anyway, if you are wanting to use the AirflowNetwork then looking at the use of the EMS is advisable, because the AFN's basic window controls are terrible.

In brief, the window opening control logic of the AFN is deeply flawed and is prone to both overcooling and runaway overheating. This is because EnergyPlus has no good way of determining how much ventilation is needed, and how much the windows should be opened, other than toggling them open and closed each timestep. Rooms with significant ventilation potential – such as spaces with sliding doors and cross-ventilation – can readily achieve very high air changes (e.g. >70ACH) in windy conditions. If this happens when the air outside is relatively cold, this can cause the zone temperature to plummet rapidly and fall below the heating setpoint (more likely to occur when the setpoint is relatively high such as 20°C). At which point, the heating is turned on, the temperature rises again, and as the conditions that caused the overheating in the first place still exist the ventilation setpoint is reached and the windows open again, causing the model to bounce between heating and ventilating. EnergyPlus attempts to control window opening by allowing opening area to scale based off the difference between indoor and outdoor temperature, reducing window opening area when the outdoor temperature is significantly lower than the indoor.

The main tool EnergyPlus uses to try to scale window openings is the window opening factor. The stated intent of this parameter is to try to prevent excessive temperature swings by reducing the window opening area if the difference between the indoor and outdoor temperatures is large. The problem is that ΔTin-out doesn’t actually make any sense as a window control parameter, and this becomes obvious when you think about it and realise that you can increase ΔT by increasing the indoor temperature. In other words, the worse it’s overheating, the less E ... (more)

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answered 2024-01-24 08:27:44 -0500

updated 2024-01-31 08:27:35 -0500

For small-scale residential (or similar), I have found BEopt's EMS solution to be the most practical, given the uncertainties one needs to deal with (eg occupant perception + forecasting, subsequent actions). You can look up the EMS entries in a BEopt-generated IDF (and adapt to suit your needs/hypotheses). In a nutshell (this may have been revised in recent years):

  • at each timestep, BEopt evaluates house/space thermal conditions
  • BEopt evaluates free cooling potential from balanced ventilation + air infiltration alone (eg off-season)
  • if insufficient, infiltration ACH is overridden (eg 1, 2 or 3 ACH), a proxy for opening windows

Revised ACH are notoriously tough to pin down:

  • climate dependent (e.g. hot/humid? windy?)
  • microclimate dependent (e.g. wooded area? urban canyon?)
  • design dependent (single-sided ventilation vs cross-ventilation, number/location of windows)
  • static pressure distribution, discharge coefficients of openings (e.g. awning vs casement)
  • occupant dependent (eg presence/absence, night cooling)

A lot of uncertainties (regardless of the calculation method), but at least one can simulate/evaluate a range of likely outcomes. Hope this helps.

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Dear Denis,

Thank you for your response. For the context, I am trying to model an individual house. For one of the living spaces, I am looking to model window openings for passive cooling in the summer. At the moment, my best approach to achieving this is:

  • Adapting the model EMSAirflowNetworkOpeningControlByHumidity.idf to a new EMSAirflowNetworkOpeningControlByTemperature
  • Retrieving/Adapting elements from the HVAC model into my individual house IDF.

NB : it seems that "AirFlowNetwork" window opening regulation is only operational for the winter period

smartiz's avatar smartiz  ( 2024-01-30 09:13:18 -0500 )edit

@smartiz, I have no direct experience with EnergyPlus' EMS Airflow Network control of openings (so I'm really hoping you'll get some useful insights from others). Yet you finished your initial question with "Do you see any other solutions?", so I thought I'd chime in. I brought minor edits to my initial answer, based on your comments.

Denis Bourgeois's avatar Denis Bourgeois  ( 2024-01-31 08:24:20 -0500 )edit

answered 2024-02-01 00:52:11 -0500

Dave's avatar

I generally don't comment in conversations like this because it might be perceived as 'commercial' input. But relative to the EnergyPlus airflow network and the specifics of this thread I think I'd be doing you a disservice by not mentioning that: (1) AFN modelling in EnergyPlus is widely regarded as very powerful, but also very difficult and perhaps too time-consuming for larger buildings; and (2) you are likely to find it an order of magnitude faster and easier in DesignBuilder, which also has simplified EMS tools that make that easier too. There are 3 free online tutorials on this page that will help you understand the differences between the different natvent methods available in EnergyPlus: and you can find more encyclopaedic information on natvent in DesignBuilder's program help: I hope that helps!

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answered 2024-02-01 01:47:40 -0500

Keigo's avatar

Why does no one mention ZoneVentilation:WindandStackOpenArea? It's the simplest model to try first, with no EMS or AFN, but it seems that this bug has not been fixed yet.

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I'll note several points to be aware of if using it:

  • Window opening areas for wind driven and stack ventilation aren't the same
  • Default values for "opening effectiveness" appear to align with cross-ventilation scenarios - single sided is much lower (0.025 in CIBSE)
  • Autocalculated discharge coefficients for stack effect are for a single large opening rather than two separate openings
  • Height difference to neutral pressure level has no guidance for appropriate values (I suggest using half the difference between upper and lower openings, which will align the equation with CIBSE's)
Jamie Sullivan's avatar Jamie Sullivan  ( 2024-02-01 15:20:32 -0500 )edit

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Asked: 2024-01-24 07:38:43 -0500

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Last updated: Feb 01