Nowadays, many heating system is adapted in Japanese houses, such as air-conditioner, floor heating system, panel heating system, and so on. In these systems, the need of floor heating is increasing by residents selecting open-plan houses. But, there are a few studies on floor heating, using heat pump, which uses less energy than other heating systems. We have been developing the floor heating system, using low temperature hot water and stocking heat energy in PCM, and keeping inner thermal environment stable in houses which are well-insulated and airtight. We have been measuring and evaluating two houses using this system. In conclusion, both houses kept inner temperature stable between 17°C and 24°C during heating period using floor heating in a short time every day. It is shown that making hot water in low temperature, using with air-conditioner, and operating low power at starting time of heating contribute to low energy consumption.