1.gas and water interlock valve 2.apply to 6L,7L,8L,10L,12L's capacity 3.copper of raw materials 4.cold and hot water pipe 1.gas and water interlock valve2.apply to 6L,7L,8L,10L,12L's capacity3.copper of raw materials4.including summer and winter 3 knobs5.there isn't summer and winter 2 knobs6.solenoid valve's quality you can option(A grade,B grade and C grade) copper valve gas water heater