The present research is concerned with effectiveness of stack ventilation system in an average Polish kindergarten. The research was carried out in June (2010), before the cleaning of the ventilation ducts, and in December (2009), after they had been cleaned. Two aspects of possible improvement of effectiveness of stack ventilation were considered, namely the influence of duct cleaning, and the supportive role of window unsealing. It seems that duct cleaning considerably improves effectiveness of ventilation: minimum growth in effectiveness of stack ventilation equalled 10%, whereas maximum was up to 100%. With unsealed windows, the air change rate grew up to 30%. (C) 2011 Elsevier B.V. All rights reserved.