In order to explain better how does the tankless water heater work, I will take an example of one of the on-demand models from State manufacturer.
Every tankless unit has to meet certain requirements before it can start. This includes the minimum water flow and pressure, gas pressure, properly installed venting system, electricity, gas and water connections. Every State tankless water heater is equipped with the electric board, a small computer that monitors and adjusts parameters, and also turning on or off all the electric components.
Once the hot water tap is open, cold water starts circulating through the water heater and heat exchanger. The tankless heater will activate only if the minimum water flow rate of 0.75 gallons per minute is met, which is detected by the internal water flow sensor.
When this condition is met, the signal is sent to the computer board which then activates the fan.
After the fan has been activated the signal gets to the electric board again, which then energizes the ignition electrode so when activated, it generates sparks, followed by buzzing sound. Now, the gas control valve opens the solenoid valve followed by the “clunk” sound, gas is directed to combustion chamber and once it gets in contact with electrical sparks, the gas lighting occurs.
The flame rod detects the flame and sends the signal to the computer board, which keeps it on as long as there is demand for hot water or until the set temperature is not met, and if there is no safety issue or malfunctioning elements.
During the process of heating, the electric board will modulate the gas control valve and electric fan between minimum and maximum power, based on the demand for hot water and temperature rise. The gas burner has three sections that can be activated separately or all together, depending on the previous requirements.
The water heater turns off automatically only when the set temperature of hot water is met, the hot water tap is closed or there is a problem with the unit.
As said before, the small computer monitors all the input and output parameters and by changing some the unit adjusts its work or turns off. For example, if the water flow rate drops below minimum value, the unit will turn off. Deactivation point is 0.4 gpm. The led diode will turn on red when the unit is on, or off if the unit is off. After the water heater has extinguished the flame, the fan will run for 1-2 minutes to purge the venting system from the remaining exhaust gases.
Picture is courtesy of State Industries