HVAC&R Research (2013) 19, 1063–1072 Copyright C  2013 ASHRAE. ISSN: 1078-9669 print / 1938-5587 online DOI: 10.1080/10789669.2013.806173 Experimental study including subjective evaluations of mixing and displacement ventilation combined with radiant floor heating/cooling system MICHAL KRAJ ˇ C ´ IK 1,2,∗ , ROBERTA TOMASI 1,3 , ANGELA SIMONE 1 , and BJARNE W. OLESEN 1 1 International Centre for Indoor Environment and Energy (ICIEE), Department of Civil Engineering, Technical University of Denmark (DTU), Nils Koppels All´ e, Building 402, DK-2800 Kgs, Lyngby, Denmark 2 Department of Building Services, Faculty of Civil Engineering, Slovak University of Technology in Bratislava, Radlinsk´ eho 11, 81368 Bratislava, Slovakia 3 Department of Industrial Engineering, University of Padova, Padova, Italy Sixteen subjects evaluated the indoor environment in four experiments with different combinations of ventilation systems and radiant heating/cooling systems. In the first two tests, the simulated residential room was equipped either by a mixing ventilation system supplying warm air for space heating or by a combination of radiant floor heating and mixing ventilation system. The vertical air temperature distribution was more uniform for floor heating. The discomfort due to cold feet/lower legs was higher for warm air heating, but no significant difference in thermal perceptions between the two mixing ventilation systems was found. The next two tests simulated an office room during summer, ventilated and cooled either by a displacement ventilation system alone or by a displacement ventilation system combined with radiant floor cooling. Displacement ventilation combined with floor cooling had lower floor temperature, warmer supply air, and less homogeneous vertical temperature profile, but it did not result in thermal discomfort on feet/lower legs or discomfort due to a vertical air temperature difference higher than for a displacement ventilation system alone, where the floor temperature was higher, supply air cooler, and vertical temperature profile more uniform. Introduction Nearly zero energy consumption will be mandatory for new buildings in the European Union by December 31, 2020 (Di- rective 2010/31/EU), and installation of heating, cooling, and ventilation systems that are able to utilize low-valued energy from renewable sources is necessary. As the heating demand in these buildings is very low and mechanical ventilation with heat recovery will be more common, there is an interest in us- ing the ventilation air also for space heating. Feist et al. (2005) and Schnieders and Hermelink (2006) reported satisfaction with the indoor climate in spaces heated by warm air and rec- ommended using warm air heating in very low-energy houses. Another option is the use of floor heating, which has been increasingly installed in new low-energy buildings. Floor heat- ing is known to provide comfort advantages, such as homo- geneous thermal conditions, reduced vertical air temperature difference, and reduced radiant temperature asymmetry (Ole- sen 2002; Babiak et al. 2007; Olesen et al. 2011). Hashiguchi Received February 6, 2013; accepted April 24, 2013 Michal Krajˇ c´ ık, PhD, is Research Assistant. Roberta Tomasi, PhD, is Research Fellow. Angela Simone, PhD, Associate Mem- ber ASHRAE, is Researcher. Bjarne W. Olesen, PhD, Fellow/Life Member ASHRAE, is Professor and Leader. ∗ Corresponding author e-mail: michal.krajcik@stuba.sk et al. (2004), in experiments with young and elderly males, showed that the number of subjects who felt comfortable and made no requests for changes in air temperature was larger in a uniform environment created by floor heating compared to an environment with high vertical air temperature differ- ence created by an air conditioner. In subjective evaluations, Olesen and Thorshauge (1979) found no serious discomfort for a subject sitting near the window of a well-insulated room during winter conditions, and both floor heating and warm air heating system provided a comfortable thermal environment. Recently, Krajˇ c´ ık et al. (2012) and Tomasi et al. (2013) performed a series of experimental measurements with warm air heating and mixing ventilation (MV) combined with floor heating in a simulated residential or single office room located in a low-energy building. The experimental measurements in- dicated a homogeneous thermal environment, but different airflow patterns were measured. When floor heating was used, the cool air mixed well with the room air and created condi- tions close to complete mixing, whereas in a warm air heating system, internal heat sources and cold surfaces were important for air motion in the room, and the ventilation effectiveness depended on various factors, such as the position of the air terminal devices (ATDs) or the difference between supply air and room air temperature. In spaces where larger airflows are supplied, displacement ventilation (DV) is assumed to provide better air quality for the same amount of ventilation air, due to its high ventilation