During normal operation, the valve is closed. Air goes into the scrubber, through the media and out of the scrubber to the skimmer. When the pH goes above 8.32, the valve is commanded open. Since the air will travel through the least resistance, the air will come from the air of the room to the skimmer through the valve (some air will get through the media but the resistance of the media is much greater than that of the air that goes through the valve when it is open). This method is better for me as I prefer to not turn the skimmer off. This way I can keep the skimmer running at all times.