# Improvement of the Chimney Effect in Stack Ventilation
## 5 Measurements of Gravity Ventilation Efficiency (ACH)
### ACH Measured for TYPE III--Chinney Cap  
Stack ventilation works best when the temperature outside is much lower than in the building [18], if it is close to the temperatureisable, the draft force in the ventilation ducts becomes too low to effectively remove air from the rooms [19]. The spring-autumn transition period is the worst in terms of stack ventilation efficiency [17]. To improve the technical efficiency, understood as efficiency, of stack ventilation, among others, chimney caps are used. They are designed to improve the chimney effect. The research [20] shows that in the period of negative temperatures, the efficiency of gravitational ventilation is at a satisfactory level. However, based on research [17], it is known that during the occurrence of wind, gravitational ventilation and more precisely the rate of air changes, is three times higher than in the case of windless weather. For the purposes of the analysis, the following assumptions were made:  
* Outside air temperature \(\geq\)12 \({}^{\circ}\)C.
* The wind velocity range from 2 to 5 m/s was adopted for the analysis. * The intensity of solar radiation \(\leq\)50 W/m\({}^{2}\). To improve gravity ventilation in the building, a rotary type of chimney cap was installed. The evaluation of the operation of the stack ventilation system was made based  
Figure 15: Gravity ventilation efficiency—solar chimney TYPE II. on continuous measurements. Measurements for the system with a chimney cap were carried out from October to November. The sensor recording the airflow in the ventilation system was placed at the outlet of the vent. The average value of the ventilation air velocity at the outlet from the V5 vent was 0.75 m/s. The maximum value of the air velocity was V = 2.15 m/s and it corresponded to the maximum value of ACH = 2.44 1/h. However, the minimum value of the air velocity was V = 0.1 m/s and corresponded to the minimum value of ACH = 0.11 1/h (Figure 16). The average value of the gravitational ventilation efficiency for a chimney with a chimney is 0.85 1/h. The research shows that the rotary-type chimney increases the efficiency of gravity ventilation for the outside temperature above 12 \({}^{\circ}\)C and the wind velocity of 2-5 m/s compared to the chimney without the root by about 35% on average. Based on the conducted research, it was noticed that for the maximum values, the chimney cap improves the draft in the ventilation duct by 0.40 1/h (Figure 17). However, in the case of a solar chimney for an outside temperature above 12 \({}^{\circ}\)C, a wind velocity of 0-2 m/s and a solar radiation intensity above 100 W/m\({}^{2}\), we notice an average improvement in ventilation efficiency by 14% than in the case of a traditional chimney (for the same external climate conditions). For the maximum values, the solar chimney improves the draft in the ventilation duct by 0.36 1/h (Figure 17). Figure 16: Gravity ventilation efficiency—chimney with a cap TYPE III. Figure 17: Summary of ACH results average value and maximum value for the three types of chimneys. To summarize, it can be stated that the scope of the research was sufficient to notice the influence of different types of chimneys on ACH. Due to the popularity of the solutions and the effect obtained, it was found that the chimney cap is the optimal solution to improve gravity ventilation in this type of buildings. Additionally, ACH results for windy and no wind periods were compared. Based on the results presented in Figure 18, we can observe a clear influence of the chimney cap on the air exchange rate in the room. In the case of a windless period, all three types of chimneys achieve an approximate ACH value (Figure 18). The ACH results for sunny and cloudy periods were also compared. Based on the results presented in Figure 19, we can see the dominant influence of the solar chimney on the air exchange in the room. In the case of the cloudy period, the ACH value for the solar chimney reduces by approx.