HOW Does a GAS TANKLESS Water Heater WORKS?!
Here is how a Whole House Gas Tankless Water Heater works:

Whole house gas tankless water heaters apply the same principle to heat water as standard gas water heaters, but without a storage tank.
The tankless gas heats water as it flows through the tubing in the heat exchanger inside the heater.


When you open a faucet or turn on the shower, instant command for hot water tap is turned on in the home, cold water is drawn into the water heater, a sensor detects water flow and activates the gas burner which warms and heats up the heat exchanger, hot water is instantly brought in within seconds. . .and keep on flowing continuously.
That incoming cold water encircles the heat exchanger and leaves the heater at its set-point temperature, combustion gases safely exit through a dedicated, sealed vent system.


With traditional tank type water heaters, you have hot water until the hot water tank's supply is depleted, then you must wait for the storage tank of cold water to re-heat before you again have hot water.
But with a tankless hot water heater, the time it takes for hot water to arrive at the tap is done immediately, a real on-demand switch command : as soon as the faucet is turned on, hot water is delivered : instantly, efficiently, and continuously . . . until you turn off the faucet.
The best thing about the hot water that comes from a tankless model is that you will receive an endless supply of hot water for as long as the faucet is being turned on.