This chart shows that thermal mass really works. This is a really busy house. The doors are opening and shutting all day long as people come and go. There is a gas-powered instant hot water heater that provides heat for the radiant flooring. We placed data loggers on the outlet pipe of the hot water outlet from the heater. We also placed two within the home and one outside in ambient air temperatures. We started logging data at 9pm, but it was not until 6am that we saw the first rise in outlet temp on the hot water outlet pipe. After that first rise, the heater cycles on a regular basis throughout the day. Our theory is that the thermal mass works when the doors are shut. At night, there is no foot traffic and the doors remain shut. The concrete slowly gives off heat. During the days, with the doors opening and shutting, the concrete loses its gained heat.