[rnln]

Thanks you. You guys answered my question. As for the reason of the question. Currently I have solar tank as primary, feeding warm water to the gas tank. In summer, the gas tank rarely turn on. Now, I hear the gas tank makes a lot of noise when it's boiled, loud and a lot. I drain it twice, still doesn't help. Look like I will need a new tank soon. So, I am thinking of going gas tank again, or electric device as a secondary (and still keep the solar tank as primary). Now, knowing the electric has the sensor (thermostat), I am ensure on the electric system.
Thanks.