Understanding Ventilation and Infiltration

I’ve been trying to find out how much of the time Honeybee simulates the windows to be open when I define natural ventilation through the SetEPAirflow component. In doing so, I’ve realized several behaviors regarding ventilation and infiltration rates that I can’t explain.

Apologies in advance for the number of questions that are about to follow, but it would be really helpful to get some insights on any of the discussed topics. (Definition: Infiltration and Ventilation.gh (582.0 KB) )

  1. Honeybee defines infiltration through Energyplus’ DesignFlowRate model, which calculates infiltration with this formula:
    The EnergyPlus documentation says that by default it uses the coefficients 1,0,0,0 for A,B,C,D. This should result in a constant flow rate that only varies according to the infiltration schedule, but I am getting slight deviations:

    What is causing this?

  2. Below are the schedules for a “Closed Office”. The assumption appears to be that the infiltration drops to a quarter when the space is occupied, however I would expect the infiltration to be higher when occupied due to the increased heat loads and use of exterior doors.

    What is the reasoning behind this?

  3. In the context of figuring out whether occupants are getting enough fresh air, should I be using “Current” or “Standard” density volume flow rates?

  4. Neither the “Current” nor the “Standard” density volume flow rate match what I would expect given the specified infiltration rate and facade area (the building is a single zone). They are both off by about a factor of 2:

    Is my calculation wrong or are there other factors that I am missing?

  5. Does Zone Ventilation Current Density Volume Flow Rate only refer to natural ventilation or could it also refer to air-conditioning?

  6. Below is the Zone Ventilation Current Density Volume Flow Rate I get when using SetEPAirflow.

    Does this answer my initial question, namely, can I assume that every time this value is above 0, EnergyPlus is modelling the windows to be open?
    If so, does the above graph mean that the windows are almost always open during occupancy, and sometimes even outside of that? This would seem unrealistic.

  7. When using air-conditioning (IdealAirLoads), I am getting these outputs:
    Zone Ideal Loads Outdoor Air Standard Density Volume Flow Rate
    Zone Ideal Loads Supply Air Standard Density Volume Flow Rate
    Zone Mechanical Ventilation Standard Density Volume Flow Rate
    Aren’t they all the same?

  8. Finally, there seems to be a mismatch between the infiltration schedule and the infiltration output (and possibly also ventilation output):

    It looks like the time frames of the weekends are shifted?

I’m going to be cheeky and tag you guys here :sweat_smile:
@mostapha @chris @MingboPeng

Sorry, I know it’s a lot to ask (literally and figurativey) but I searched the forum and couldn’t find the answers. I thought it would be better to ask them in one go instead of separate threads since some of the topics relate.

I’ve gone about trying to answer some of my own questions, but the only additional information I found was that in 7) the “Outdoor Air” flow rate was identical to the “Mechanical Ventilation” flow rate. I still can’t explain the “Supply Air” flow rate being different.

@MaxMarschall ,

You really should break this up into multiple posts. In my view, any post that has more than 3 parts is not only difficult for someone to answer but is also difficult for the other forum users to understand and apply to their own issues.

Also, it’s very tough to know what you have plotted on those charts without showing or telling us where the data came from. For example, in 1, are you looking at the loss/gain energy from infiltration, which is a function of both the indoor/outdoor temperature difference as well as the flow rate? Or are you looking at some other E+ output that is specifically concerned with flow rate.

If you can break this up into at least two posts (maybe one for ventilation and one for infiltration) and give us some sense of where the data came from, I think we can post more helpful responses.


Hi @chris, fair point - I have now created several new threads (should I delete the current one?):

Re 1) I was outputting the ZoneInfiltrationCurrentDensityVolumeFlowRate. I just realised that if I output the ZoneInfiltrationStandardDensityVolumeFlowRate (‘standard’ instead of ‘current’), I get constant values. So that answers the first question.