# Improvement of the Chimney Effect in Stack Ventilation
## 1 Introduction  
Natural ventilation (NV) is the cheapest strategy for distributing fresh air inside a building. It is still the most popular system found in Poland and other Central and Eastern European countries. Natural ventilation is driven by two physical phenomena, wind [1] and buoyancy (stack effect) caused by the temperature difference between indoor and outdoor air temperatures [2]. However, [PERSON] et al. [3] showed that the NV is mainly affected by wind direction and velocity. The most popular type of natural ventilation used in countries with a cold climate is gravity ventilation, also known as stack ventilation. This type of natural ventilation is analyzed in this paper. Stack ventilation should work when one or all the natural forces are available. Unfortunately, the two main drivers causing stack ventilation flows are stochastic; therefore, stack ventilation may be difficult to control and predict, as well as analyze and design. Other disadvantages of stack ventilation are the reduced control of air distribution within the building and the lack of its effectiveness in summer conditions with minimal wind. Despite the difficulty in control, stack ventilation is still relied upon to meet the need for fresh air in many buildings. The study [4] shows that stack ventilation has become a new trend in building design in the architectural community. Furthermore, stack ventilation has been used in many types of buildings, even in highly controlled indoor climate hospitals [5]. According to the standard [6], stack ventilation may cease to function properly if the outside air temperature \(\mathrm{T_{e}}\) exceeds 12 \({}^{\circ}\)C or when we talk about the so-called wind silence or the so-called light wind, which has been checked on the basis of previous own research by [PERSON] [PERSON], [PERSON] [PERSON]. [7]. To find out the number of days with unfavorable climatic conditions, the number of hours in a calendar year was checked basedon a typical Meteorological Year, during which it is highly probable that gravity ventilation will not work. The following assumptions were made for the analysis:  
* Outdoor air temperature of 12 \({}^{\circ}\)C \(\leq\) T\({}_{\rm e}\)\(\leq\) 20 \({}^{\circ}\)C--assumed value of 20 \({}^{\circ}\)C as the upper criterion, because above this temperature it was assumed that there are warm days and it is possible to ventilate the rooms. * Wind velocity V\({}_{\rm wind}\) = 0-5 m/s. Data from a Typical Meteorological Year for the city of Elblag was used for the analysis. Elblag, due to its location, is not exposed to frequent gusts of wind, as are the cities located on the coast. For the city of Elblag, continuous measurements of the external climate for 30 years were made. The Typical Meteorological Year [8] contains averaged hourly data compiled according to the ISO methodology [9]. In Figure 1, the external temperature range, during which gravity ventilation ceases to be efficient, is marked in black dots. On the other hand, the red dots show the wind velocity, which may be too low to maintain the efficiency of gravity ventilation at the desired level. The analysis shows (Figure 1) that in a year there are over 5500 h during which at least one mechanism occursand the movement of the ventilation air does not work. This is as much as 60% of the entire calendar year. On the other hand, if the mechanisms causing the movement of the ventilation air do not work at all (according to assumptions: 1. Outside air temperature 12 \({}^{\circ}\)C \(\leq\) T\({}_{\rm e}\)\(\leq\) 20 \({}^{\circ}\)C and 2. Wind velocity V\({}_{\rm wind}\) = 0-5 m/s), we obtain 2200 h of occurrence problems with gravity ventilation, which accounts for about 26% of the entire calendar year. The quality of stack ventilation is usually determined by the performance indicator of the ventilation system--air change rate (ACH). The ACH measures how quickly the air in the interior space is beingreplaced by air coming from the outside [10].