Among the various types of gas water heaters on the market today, the tankless continuous flow models are very popular. There are two primary reasons for this popularity: energy efficiency and the fact that they do not run out of hot water.

But how does a gas hot water system work?

In basic terms, when a hot water tap is opened, the water flow is detected by a sensor. This causes the gas burner to fire, which in turn causes the flowing water to be warmed by the heat exchanger. Some models pre-heat the water with a secondary heat exchanger, which utilizes heat produced from exhaust gases. The preset temperature is maintained by a thermostat, which controls how much gas is supplied to the burner.

When shopping for tankless systems, the terminology can sometimes be confusing. You may hear these systems referred to as “tankless,” but the terms “continuous flow, instant or instantaneous” may also be used.

gas hot water system

Tankless Hot Water Heater Installed

Most modern systems employ an Electronic Control Unit (ECU) to regulate the water temperature. This unit controls all the heater’s functions, including ignition, gas flow, temperature control, and modulation. The ECU also provides safety aspects, such as automatic gas cut-off if there is an issue, and the flame sensor. Basically, when a hot water tap is turned on anywhere within the home, the ECU-connected flow sensor detects it, and the water heating process begins.

Once the need for hot water is detected, the ECU will initiate the gas flow, and trigger the igniter. The igniter will spark, until a flame is detected by the sensor. If ignition does not occur within a few seconds, the gas flow is shut off by the ECU, to prevent gas from accumulating.

Heat is then transferred to the heat exchanger from the burner. Water is heated as it passes through the heat exchanger. The design of the heat exchanger typically directs the water in a serpentine pattern, so that it can absorb the maximum amount of heat from the exchanger walls.

Some models include a secondary heat exchanger, which utilizes the heat produced from burner exhaust gases. Although these models generally cost a bit more, they provide even greater energy efficiency. These water heaters are commonly referred to as “condensing” models. However, even many of the non-condensing models are efficient enough to have a 6 Star or better energy rating.

Water Heaters for Residential Units

gas hot water system

As a safety feature, most of these water heaters have the maximum water temperature pre-set at the factory. For most residential units, 50 degrees Celsius is the maximum setting. Some units have controllers that allow a lower temperature to be selected. The temperature of the water is maintained by a sensor on the exiting side of the heat exchanger, which monitors the water temperature. The ECU modulates the gas supplied to the burner, based upon the flow rate and incoming water temperature.

Most water heater units are connected to a home’s power supply, for the purpose of powering the sparking igniter. However, some units are designed to utilize the water itself. In these units, energy is generated from the water as it flows through a small water turbine. Some units also use a battery to power the igniter. Battery and water powered models can operate even when there is a power outage. However, turbine units are larger in size, and more expensive.

So, the entire process is simpler than many people realize. In summary, when a hot water tap is opened, cold water flows in. The sensor detects this flow, and ignites the burner. The water is then heated in the heat exchanger, with the temperature being monitored and controlled by the electronic control unit. The end result is an endless supply of hot water, whenever it is needed.