There are natural gas tankless water heaters that perform exactly as you said. The Department of Energy has said, gas tankless are 30% more efficient than a standard gas tank water heater. A gas tankless water heater heats the water upon demand. You open a faucet or shower and the heater fires up and heats the water and will continue to heat it until you are finished using it. That is correct, a family of eight can get a shower one after the other and everyone will have equally hot water!
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