Indoor air quality in rooms with cooled ceilings.: Mixing ventilation or rather displacement ventilation?

Experimental investigations and practical experiences in Europe have proved that hydronic cooled ceilings are able to remove high cooling loads without impairing thermal comfort. As hydronic cooled ceilings cannot remove latent loads and pollutants, e.g., CO2, VOCs, odors, additional ventilation has to be applied. Often, displacement ventilation is used, which is able to provide lower pollutant levels in the occupation zone than mixing flow systems, if the occupants are causing most of the pollution. Unfortunately, the advantage of the displacement flow, in respect to the air quality, can vanish when cooled ceilings are used to remove the major part of the sensible cooling load. For these applications, a combination of a cooled ceiling with a mixing ventilation system might be more appropriate. This paper presents the results of an investigation examining the distribution of tracer gas in a test chamber which is equipped with a radiant cooled ceiling. There is both a displacement flow system and a mixing flow system available, so that the concentrations of the tracer gas within the occupation zone characterizing the air quality can be compared directly and evaluated under similar conditions. The vertical air temperature rise is taken into consideration as well, as it influences the displacement flow and is an important issue for assessing thermal comfort. The results show the interaction of the portion of the cooling load being removed by the supply air, the air quality in the occupied zone and the vertical air temperature rise. The figures and tables presented show which of the two supply air systems investigated have advantages over the other.