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With the increased interest in radiant floor heating systems, the issue of optimum control has become controversial. Friedlander (1986) and Adelman (1988) recommended outdoor reset control based on the principle that there is a direct, linear relationship between outside temperature and the required supply water temperature. ASHRAE (1987) also states that the supply water temperature should be varied in accordance with outside temperature due to the time-lag between thermostat demand and heat delivery to the space. In contrast, MacCluer (1989) proposed the new concept of proportional flux-modulation: control of heat flow rate, not temperature. This study investigated the control performance of proportional flux-modulation in comparison with four variations of temperature-modulation: outdoor reset control, outdoor reset control with indoor temperature offset, flow-inhibition with outdoor temperature, and flow-inhibition with indoor-outdoor temperature. A series of experiments were conducted comparing flux-modulation and temperature-modulation for three different categories: four seasonal heating studies, three stability studies, and a study of zoning feasibility. The performance of each control strategy was evaluated on the basis of indoor temperature stability in response to various internal and/or external load conditions. The response of each control system was observed in terms of supply and return water temperatures, floor surface temperatures, indoor air temperatures, and start-up patterns from initial conditions. The experimental results demonstrate that proportional flux-modulation favorably compares with temperature-modulation. Proportional flux-modulation demonstrates a much faster response to a sudden internal thermal load change than does any type of temperature-modulation. Proportional flux-modulation shows consistent performance under the effects of dynamic outside conditions, maintaining stable indoor temperature around the setpoint. In contrast, temperature-modulation demonstrates indoor temperature offset from setpoint with a sudden internal thermal load change. In temperature-modulation via mixing valve, maintaining stable indoor temperature around the setpoint is difficult because of the problem of determining the proper reset ratio. In temperature-modulation via flow-inhibition, maintaining stable indoor temperature around the setpoint is difficult because of the problem of determining the proper boiler water temperature.