What is the correct way to implement infiltration?

I have some struggles with implementing infiltration in my OpenStudio model. If I understand it well, is infiltration the flow of air from the outdoor environment directly into a thermal zone? So this can be caused by opening and closing doors, some cracks around windows, .. ? Does the airflow caused by opening the windows also belong to the infiltration input parameter? During the summer period here in Belgium, a lot of residents open some of their windows at night or during the evening to enhance the effect of free (night)cooling. I was also wondering if there are any default values that represents a good estimate of the airflow caused by infiltration that I can use for the design flow rate ?

In my model I have implemented a mechanical ventilation, but I got the advice to also implement infiltration. I found two ways to implement infiltration into my model. Is there a difference between the two and which one I use best?

First one: space types < General < space infiltration design flow rates

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Second one: Spaces < Airflow < space infiltration design flow rates

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Infiltration is outdoor air UNintentionally entering the building through cracks, leaks, etc. while the building envelope is "sealed" (doors closed, windows closed, etc.). The window opening events would be an example of natural ventilation -- outdoor air intentionally entering the building for some purpose (free cooling, e.g.). You can read this post for more details on modeling natural ventilation in OpenStudio.

The two approaches of assigning infiltration from your screenshots (Space Type input vs. Space input) are essentially the same, but the space type approach will likely be easier and faster. You can read this post for more information on that topic. This post also discusses using blower door results at 50 Pa and converting that to a natural infiltration rate.

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Aaron Boranian

answered 2021-01-07 10:18:08 -0500

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Hi Aaron,

Thank you for this explanation. If I got 4 perimeter zones with one core zone for a shoebox modeling/just one room on the floorplan, how should I set up the infiltration; does that mean I need to set multizone air flow for the core zone; how should I deal with perimeter zone; each of them is actually having 3 outside boundary with one imaginary inner side boundary; is the infiltration calculation based on zone area or the perimeter of the space

Brianna (2021-03-25 16:46:32 -0500) edit

@Brianna I would define a "Perimeter Zone" space type and a "Core Zone" space type that are identical to each other EXCEPT the perimeter has a space infiltration design flow rate object assigned and the core does not (first image in this post). You can read more here about assigning that object, where the second image shows how you can edit on the right the flow calculation method using the drop-down list (flow per floor area, flow per exterior surface area, etc.).