In this study, heat transfer coefficients for radiant floor heating systems were investigated in a special test chamber that is one of the very first implementations of ANSI/ASHRAE Standard 138 in the world. Radiant systems offer high energy- and exergy-efficient sensible heating and cooling potential. These systems can be directly coupled with low enthalpy, renewable, or waste heat resources. Heat transfer coefficients are important design parameters for radiant systems that effect the heat transfer capacity of the heated/cooled surface to the indoors. In this study, radiant, convective, and total heat transfer coefficients for radiant floor heating were investigated experimentally. Experiments were conducted in a special test chamber that was established according to ANSI/ASHRAE Standard 138 with minor differences. The test chamber dimensions are 2.74 x 2.25 x 2.45 m (8.99 x 7.38 x 8.04 ft) and there are no floor coverings during the test. Coefficients were determined through two difference methods. Heat fluxes from the heated floor that calculated via both methods and obtained from the "Design Graph for Sensible Heating and Cooling with Floor and Ceiling" in ASHRAE Handbook-HVAC Systems and Equipment (ASHRAE 2008). According to the experimental results, total heat transfer coefficients for the radiant heating system were obtained between 8.8 and 12.1 W/m(2)K (1.43 and 2.13 Btu/h.ft(2).degrees F).