Abstract: Instant hot water gas water heater become more and more popular in market at present, not only stand for the high-tech in the area now and future, but also lead to the direction of the gas water heater area, so many famous manufacture takes a lot of effort to develop this kind of gas instant hot water heater. Some famous brands has put the instant hot water gas water heater as a symbol of their R&D strength, which has been sent in market and got the marketed and recognized by consumers. Now there are two factors need to be solved of this kind of water heat：less product type and high cost. The article describes the design principle and test plan of instant hot water gas water heater, effectively solved the waiting time and water waste problems by add circulating pump, thermostatic water tank, and water pipes system, greatly improved the use of comfort, and provides reference for the intelligent, comfortable and healthy development of gas water heater.

Key words: Gas water heater, Circulating water system, Instant hot water