How Does a Gas Water Heater Work?

Gas water heaterGas water heater

Find out how a gas water heater works and see what is the purpose of each component in gas water heating. Typical gas water heaters, found in most homes in North America, are built to store water inside a water tank and heat water utilizing a gas burner. This article will talk about the working principle of both traditional tank-type and tankless gas water heaters.

Before you buy a gas-powered tank-type heater, let's see first how they work, what components and features they offer to ensure they are the right choice, easy to service and troubleshoot.

Due to advanced electronics, the most innovative models usually require professional assistance if something goes wrong. Still, less expensive and older models contain mostly basic features that are easier to service and maintain.

There are various gas water heaters that come in many models and sizes, where the tank capacity ranges from 20 gallons to over 100 gallons.

Smaller models are usually installed close to the fixture or application, while larger models (whole-house) are centrally located and can deliver hot water to multiple fixtures or applications.

Main components of gas water heaters

  • Storage tank with glass lining
  • Insulation
  • Gas burner
  • Gas control valve with the thermostat
  • Igniter (standing pilot flame, electronic ignition, or hot surface igniter)
  • Thermocouple or thermopile
  • Gas combustion chamber
  • Gas manifold
  • Arrestor plate
  • Dip tube
  • Anode rod
  • Drain valve
  • Coldwater inlet
  • Hot water outlet
  • Draft diverter
  • Flue tube/heat exchanger
  • Venting system
  • Fan (power vented models)

How does a gas tank-type water heater work?

In short: Gas water heaters work by bringing the cold water inside the tank, heating with the gas burner, and moving hot water from the tank to the fixture using home plumbing.

What fuel types can gas water heaters use?

Gas water heaters run on natural or liquid propane gas, with or without electricity. Natural gas is connected to a supplier's gas line, while propane is stored inside a propane tank situated on your property.

It is also possible to make the gas conversion and convert a unit from natural gas to liquid propane.

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Cold water delivery

Cold water from the main water supply enters the tank through the water inlet and dip tube.

Dip tubes are long plastic pipes connected to the water inlet and have a purpose to deliver cold water to the lower part of the tank. Better ones are designed to create turbulent water flow and reduce the sediment buildup inside the tank. These are known as self-cleaning dip tubes.

Gas ignition and gas combustion

The gas used to heat water is supplied by the gas pipe and through the gas control valve, which also includes a thermostat. The gas valve supplies gas to the main burner and pilot light (if equipped), which usually burns non-stop.

Once the thermostat senses that the water temperature inside the storage tank is lower than what was set, the thermostat signals the gas valve to open and start the ignition.

With the gas burner ON, the flame starts heating water stored above inside the tank. For safety purposes, a thermocouple enclosed in the flame senses its presence keeping a gas valve open. If there is no flame, a thermocouple reacts and sends the signal to interrupt gas delivery.

Temperature control

Thermostats are part of gas control valves and are designed to control and monitor the temperature of the stored water.

Once water is heated, hot water due to natural law rises to the upper part of the tank, where it is drawn off by the water outlet pipe and then sent to the fixture or a device.

When the thermostat is satisfied, the gas control valve closes and shuts off the water heater until the next call for heating.


Venting systems consist of the exhaust flue that runs through the center of the tank, flue baffle, and venting pipe connected to each other. The venting system starts from the burner and terminates through the outside wall or roof, so all exhaust gases are safely removed to the outer atmosphere.

Standard atmospheric vent models use a vertical vent pipe through which hot gases can escape out naturally. There are also power-vent models that use electric blowers to push the fumes through the vent system.


The metal tank is surrounded by a thick 2-3" non-CFC polyurethane foam to reduce heat losses from the hot water stored inside the tank.

Safety elements for protection

There are also several safety elements that make a gas water heater running properly and users safe.

Anode rods or sacrificial anodes are designed to protect the steel tank from rusting. Anodes are fastened to the top of the tank and must be replaced if consumed. One of the indications that the element doesn't perform well anymore is the presence of the rotten egg odor.

Temperature and pressure relief valves (T&P valve) have the purpose of protecting a water heater from excessive temperatures and pressure. Once the temperature and pressure approach the limit set by the manufacturer, the T&P valve opens, relieving excessive hot water, reducing the pressure, and preventing damages, even explosion.

A drain valve is located at the bottom of the water heater and is utilized for draining and flushing sediments collected inside a water heater tank. This helps the water heater operate efficiently.

How does a gas tankless water heater work?

Gas tankless water heaterGas tankless water heater

Gas tankless water heaters work by running cold water through the heat exchanger, where it picks up heat from the gas burner, and supplying hot water to one or more fixtures or devices.

Tankless heaters do not store hot water but heat water on demand (on-call) and deliver in endless supply. According to tankless manufacturers, gas tankless water heaters can save up to 50% of heating costs.

A gas tankless water heater starts as soon as you open a hot water tap. The water flow activates the flow sensor, sending the signal to the PC board to open gas and start the ignition.

Electric sparks from the ignition electrode fire the gas burner, heating water running through the heat exchanger. The igniter sparks until the flame rod detects a flame.

When we compare how the gas tank-type water heater works vs. tankless and see how they are designed, you will see that tankless devices are more advanced than the tank-type.

Tankless models are equipped with an electronic control unit, a small computer that controls and monitors all system functions.

Some of them are also Wi-Fi and Alexa voice control compatible.

Water is heated as long as the hot water tap is open and the minimum water flow rate is satisfied. As with tank-type heaters, exhaust gases must be released to the outside atmosphere.

Related: How does an electric water heater work?

Can a gas water heater work without electricity?

Most homeowners know that gas water heaters do not use electricity as fuel, but they are still concerned about asking questions such as "can I shower during a power outage?"

The answer is yes, but it might be hard to find a tank-type water heater where all of its components do not require electricity to function. One of the main components that doesn't depend on electricity is the standing pilot light. The standing pilot can function normally during the power outage, while electric igniters, such as hot surface igniter, cannot.

However, most gas tankless water heaters are equipped with the elements, such as the PC board or remote controllers that require electric current to function. Some models, such as portable or older models from Bosch, are independent of the electrical current and can be used during the power outage.

So, no matter if the model you have, the best way to use a gas water heater and enjoy hot water with no interruption is to install a battery backup system or buy a generator.

Is it safe to leave a gas water heater ON all the time?

Yes, it is, even if you are going on vacation or have to be absent for a long time. It is actually recommended to turn the thermostat to "vacation" mode or lower the temperature.

This is especially important if you live in a cold climate because your water inside the water heating system is protected from freezing. However, if the water heater was off for some time, later when you want to use hot water again, it might cost you more to reheat cold water. Also, water that was stagnant for a long time might develop bacteria and an unhealthy situation.

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