# Improvement of the Chimney Effect in Stack Ventilation
## 5 Measurements of Gravity Ventilation Efficiency (ACH)  
The experimental research was aimed at determining the efficiency of gravity ventilation (ACH) in transient conditions, i.e., in conditions where the temperature of the outside air is too high to cause air movement in the system by natural convection, and when the wind velocity is too weak to convection forced air movement in the ventilation system.  
Based on the conducted experimental studies, the efficiency of gravity ventilation (ACH) for TYPE I was estimated for two different conditions of the external climate. The ACH efficiency was measured for the time interval in which the dominant air exchange mechanism in the ventilation system was wind--in the first approach, and the intensity of sunlight--in the second approach. The measured ACH performance allowed us to compare the results with type II and type III.  
In the measurement of ACH when the dominant mechanism was wind, for the purposes of the analysis, the following assumptions were made:According to [6], stack ventilation may not work properly if the outside air temperature exceeds 12 \({}^{\circ}\)C.  
Based on previous research [17], it was noticed that when the wind velocity remained within the range of 0-3 m/s, that is, when we talk about wind silence or so-called light wind, it does not contribute to the movement of ventilation air in the rooms. The analysis of the efficiency of gravity ventilation was extended by the wind velocity range from 2 to 5 m/s. The intensity of solar radiation did not exceed the value of 50 W/m\({}^{2}\).  
The evaluation of the operation of the stack ventilation system was made based on continuous measurements. Measurements for the traditional system were carried out from August to September. The sensor recording the airflow in the duct was placed at the duct outlet. To determine ACH, the ventilation air velocity at the outlet from the duct was assumed. The average value of the ventilation air velocity at the outlet from the V5 was 0.59 m/s. The maximum value of the air velocity was 1.79 m/s and it corresponded to the maximum value of ACH = 2.03 1/h. On the other hand, the minimum value of the air velocity was V = 0.05 m/s and corresponded to the minimum value of ACH = 0.09 1/h (Figure 13). The average value of the gravity ventilation efficiency for a traditional chimney is 0.67 1/h.  
Figure 12: Air temperature measured in the vent.  
For the ACH measurement when solar radiation intensity was the dominant air exchange mechanism in the ventilation system, the following assumptions were made:  
* Outside air temperature above 12 \({}^{\circ}\)C.
* The wind velocity range from 0 to 2 m/s was adopted for the analysis.
* The intensity of solar radiation is above 100 W/m\({}^{2}\).  
The average value of the ventilation air velocity at the outlet from the V5 was 0.48 m/s. The maximum value of the air velocity was V = 1.47 m/s and it corresponded to the maximum value of ACH = 1.67 1/h. On the other hand, the minimum value of air velocity was V = 0.02 m/s and corresponded to the minimum value of ACH = 0.02 1/h (Figure 14). The average value of the gravity ventilation efficiency for a traditional chimney was 0.61 1/h.  
Figure 14: Gravity ventilationefficiency—chimney TYPE I. ACH measurement when the dominant mechanism was the solar radiation.  
Figure 13: Gravity ventilation efficiency—chimney TYPE I. ACH measurement when the dominant mechanism forcing the ventilation air movement was wind.