This paper reports the results of an examination on thermal efficiency and heat loss of hot water supply system for dwellings with gas instantaneous hot water heater, condensing type gas instantaneous hot water heater and mid-night electrical power water heater. The authors carried out experiments in laboratory of heater and system efficiency when hot water supply quantity, ambient temperature and piping system (header-conduit and point divergence) change. As the results, 1) The system efficiency with header-conduit piping system is about 5% higher than point divergence piping system. 2) The boil up temperature has a great influence on the heater efficiency of mid-night electrical power water heater. 3) The changes of hot water supply quantity and ambient temperature have a little influence on heater and system efficiency of system with gas instantaneous hot water heater and condensing type gas instantaneous hot water heater. 4) Condensing type gas instantaneous hot water heater is more efficient than gas instantaneous hot water heater on every mode by about 14%.
