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What Does a Thermocouple Do in a Water Heater?

Fact checked by Stephen Conklin

what does a thermocouple do in a water heater

It’s okay for newbie owners to ask, “What does a thermocouple do in a water heater?” After all, this component is so tiny that many folks cannot fathom its role.

However, a thermocouple for gas heater units is a type of sensor that automatically shuts off the fuel supply when the pilot light dies out. It prevents the gas from filling the combustion chamber and reduces the risk of a spark-induced explosion.

How? Please read on to learn more.

Table of Contents

Overview of Thermocouple and Its Purpose


Appreciating a thermocouple’s importance in a gas-powered water heater’s safe operation requires knowledge of the device, including the thermocouple’s purpose.

  • Gas water heaters have burner assemblies that heat the storage tank’s bottom and raise the water temperature.
  • The water heater has a pilot tube emitting a flame that ignites the burner. Next to the pilot tube is the thermocouple, with its two metal bands directly in the pilot flame.
  • The thermocouple’s principal function is to cut off the gas supply, preventing fuel from accumulating in the water heater’s combustion chamber. This action makes a thermocouple on a furnace a safety mechanism.

So, how does the thermocouple do this?

How Does a Thermocouple Work?

1. Seebeck effect


A thermocouple in hot water heater appliances works on the “Seebeck Effect” principle.

The theory states that two chemically diverse and adjacent metals will produce an electrical voltage proportionate to the heat the metals receive.

Hence, the hotter the air surrounding the metals, the higher the electrical voltage generated.

And since the metals are directly in the pilot flame, the thermocouple continues to generate electricity. This action keeps the gas valve open, allowing fuel to flow through the pilot tube and the burner.

Should the flame in the pilot tube’s tip die out, the metals “cool” and can no longer produce electricity. This event closes the fuel valve to prevent gas from filling the combustion chamber. It would be disastrous if the thermocouple fails in its principal duty.

2. A temperature control device


In hindsight, we can look at the thermocouple as a rudimentary temperature control device. It might seem like outdated technology, but it works.

It’s worth noting that not all gas water heaters have a thermocouple. Newer models often feature a different mechanism (i.e., mercury sensor) to cut off the gas supply should the pilot fail.

Sadly, thermocouples are not indestructible. These components require the same care and attention you would give to the water heater as a whole.

Although you can replace a failing thermocouple, prevention still tops the list.

Consider scheduling annual professional maintenance to identify potential thermocouple issues before they go out of hand.

Signs Indicating a Faulty Thermocouple


You can clean thermocouple devices to keep them in tip-top shape. Moreover, you might want to increase your knowledge of the indicators of a possible failing thermocouple. Learning about these signs can help you decide on the solution.

  • Issues in the pilot tube, including tiny holes, cracks, dents, and an unusual coloration
  • Breaks or deformities to the thermocouple’s electrical connectors
  • Damage to the thermocouple’s wirings, including stripped sheaths, corrosion, cracks, and brittleness

Unfortunately, some owners of gas water heaters cannot identify these signs. We recommend a water heater technician check the thermocouple’s integrity and operating performance.


An indispensable safety feature sums up the response to the riddle, “What does a thermocouple do in a water heater?” A failing thermocouple could pose a serious threat to a household’s safety. It could create gas-related explosions and fires.

Appreciating a thermocouple’s mechanism of action and relevance to safe gas water heater operation is crucial for homeowners. It allows them to care for and maintain this device as much as they would the water heater.

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