| Summary: | According to the WHO the radioactive gas radon must be controlled indoors. E.g. by naturally driven suction systems based on thermal buoyancy, also denoted the chimney effect, which exploits the difference of indoor- and outdoor temperature to lower radon levels indoor. This paper presents four case studies showing that the efficiency of such systems to control radon level indoors varies, as the outdoor temperature varies throughout the year. The chimney effect was the driving force in the four single-family houses used as case studies. In two cases it was used to increase the indoor air change rate to dilute the radon concentration indoor, in one case it was used to drive a suction system under the ground slab to prevent radon from infiltrating through the ground slab, and in one case both techniques were used in combination. Measurements showed a correlation between a low radon level indoors and an increased difference between the indoor- and outdoor temperature, with the highest temperature indoor. Likely, the temperature difference can provide the needed suction in such systems. Without this driving force – out of the heating season – an increase of the indoor radon level was seen to occur. The needed suction to lower the radon level indoor did not occur in all cases. However, the efficiency of the system was seen to vary throughout the year, and was limited in periods with little difference between the indoor- and the outdoor temperature. Such needs to be taken into account, when the effect of naturally driven suction systems to reduce the radon level indoor are evaluated.
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