# Air Change per Hour due to wind driven cross ventilation

I have set up a simulation with one zone for wind-driven cross ventilation (the zone is not conditioned), and I want to calculate the Air change per hour that is happening in the zone due to cross ventilation. The windows are placed on the opposite walls and have an equal area.
After the first set of simulations, I was calculating the Air change per hour from the “AirFlowVolume” output in the Read EP Result component. If I do that, I am getting crazy results of up to 180 Air changes per hour. The volume of my zone is about 100m3 but the windows are around 2 m2 on both facades. I wouldn’t except such a result.

So, is the “AirFlowVolume” output in the Read EP Result component the right output to calculate the Air Change Per Hour that is happening in my zone?

Hi, have you been able to find the solution to this?
I’m facing the same problem

180 ACH isn’t impossible. It could happen, for example, if you have your windows open all the time, or are calculating the ACH over a small time period.

For example, with a low, constant wind speed of 2.5 m/s hitting your window head on, you’d get 180 ACH:

\begin{align} ACH &= \frac{2.5_{m/s} \cdot 4_{m^2} \cdot 3600_s}{100_{m^3}} \cdot 0.5\\ &= 180 \end{align}

With a simplifying assumption that windows are facing the same direction. Note that the 0.5 factor there is to account for an EP factor that scales the volumetric flow by wind angle to the window normal (around 0.5 for perpendicular, and 0.3 for diagonal angles).

Once you start averaging the rates over longer time periods when your windows are shut, the ACH will go down to more “reasonable” magnitudes.

[Edited to simplify angle assumptions].

Also, take a look at the EP outputs I listed in this post:

I was able to reproduce the ACH from those outputs: