I installed a 125k btu natural gas heater approx. 3 years ago. It performs quite remarkable. It doesen't require electricity and uses an inline flow switch to turn on the main burner valve. It has a stainless steel heat exchanger which holds a 10 year warranty. It also incorporates an automatic varable power output feature. Burner output is proportional to water flow demand for max. fuel consumption. We have poor water here in New Orleans and folks change out conventional heaters quite regular. I install it in such a manner as to be able to acidize unit due to potential scalling. Took an initial reading from a particular faucet in gallons per minute to use as a benchmark to determine when flowrate is restricted due to calcium scale. If additional info. is needed, please contact.