Air distribution, ventilation effectiveness and thermal environment were experimentally studied in a simulated room in a low-energy building heated and ventilated by warm air supplied by a mixing ventilation system. Measurements were performed for various positions of the air terminal devices and at different simulated outside conditions, internal heat gains and air change rates. Floor heating was also simulated and compared with the warm air heating system. Vertical air temperature profiles, air velocity profiles and equivalent temperatures were derived in order to describe the thermal environment. Contaminant removal effectiveness and air change efficiency were used to evaluate ventilation effectiveness. No significant risk of thermal discomfort due to vertical air temperature differences or draught was found. When the room was heated by warm air, buoyancy forces were important for ventilation effectiveness at low air change rates. The effect of increasing air change on the ventilation effectiveness depended on the position of air terminal devices. Depending on the position of air terminal devices, the ventilation effectiveness varied between 0.4 and 1.2, where 1 is complete mixing. When a radiant floor heating system was simulated, the cooler ventilation air introduced to the room mixed well and created uniform conditions with a ventilation effectiveness of about 1. (C) 2012 Elsevier B.V. All rights reserved.