How Does a Water Heater Work?

By Michael Franco

August 24, 2026

It's one of those appliances that quietly does its job in a closet or basement corner, providing hot water on demand without much thought from anyone — until it stops working. Understanding what's actually happening inside your water heater makes it a lot easier to troubleshoot problems, know what's normal and figure out when something needs attention.

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Here's how a traditional tank water heater works from start to finish, and how a tankless model does the same job differently.

Water Heater Basics: The Storage Tank

A traditional water heater is essentially an insulated tank that keeps a large volume of water — typically 30 to 50 gallons for a household — heated and ready to use. Cold water enters the tank through the cold water inlet, which connects to a dip tube that carries the incoming cold water down to the bottom of the tank. This matters because hot water rises and cold water sinks — by delivering cold water to the bottom, the tank avoids mixing it with the hot water sitting near the top, which is where the hot water outlet draws from.

How the Water Gets Heated: Gas and Electric Water Heaters

This is where gas and electric water heaters diverge.

In a gas water heater, a burner assembly sits underneath the tank. When the thermostat detects that the water temperature has dropped below the set point, the gas control valve opens and the burner ignites — either via a pilot light that stays lit continuously, or an electronic ignition system on newer models. The flame heats the bottom of the tank, and that heat transfers to the water through the tank's metal walls. As the water near the bottom heats up, it becomes less dense and rises, creating a natural convection current that circulates heat throughout the tank. Combustion gases are vented out through a flue that runs up through the center of the tank and out of the home.

In an electric water heater, there's no flame or flue at all. Instead, one or two heating elements — metal rods that work like the coils in a toaster — sit inside the tank and submerge directly in the water. When the thermostat calls for heat, electricity flows through the elements and they heat the surrounding water directly through electrical resistance. Most electric water heaters have an upper and lower element, with the upper element typically handling most of the day-to-day heating since hot water is drawn from the top of the tank.

Inside the Tank: The Sacrificial Anode Rod

Steel tanks would corrode quickly if left to their own devices, which is where the sacrificial anode rod comes in. This rod — typically made of magnesium or aluminum — is more reactive than the steel tank itself, so corrosion attacks the rod instead of the tank walls. Over years of use, the anode rod gradually dissolves, which is exactly what it's supposed to do. Once it's fully consumed, the tank itself becomes vulnerable to corrosion, which is one of the leading causes of water heater failure. Replacing the anode rod every few years is one of the most overlooked maintenance tasks that can meaningfully extend a water heater's lifespan.

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Water Heater Safety: Pressure's On

As water heats, it expands — a phenomenon called thermal expansion. In a closed system, that expansion has to go somewhere, and the temperature and pressure relief valve, or T&P valve, is the safety device that handles it. If pressure inside the tank climbs too high, the T&P valve opens automatically and releases water through a discharge pipe, preventing a dangerous buildup. It's one of the most important safety components in the entire system.

Sediment: A Water Heater's Slow Decline

Minerals naturally present in your water supply — primarily calcium and magnesium — settle to the bottom of the tank over time as sediment. This buildup acts as an insulating layer between the burner or lower heating element and the water, making the unit work harder and less efficiently to reach the same temperature. It's also a common cause of the popping or rumbling sounds older water heaters make, as water gets trapped under the sediment layer and boils. Flushing the tank annually removes this buildup and keeps the system running efficiently.

How a Tankless Water Heater Works Differently

A tankless water heater eliminates the storage tank entirely and heats water only as it's needed — hence the term "on-demand" or "continuous flow." When you turn on a hot water tap, a flow sensor detects the movement of water and activates the unit. Cold water passes through a heat exchanger, where either a gas burner or an electric element rapidly heats it to the target temperature as it flows through, delivering hot water directly to the tap with no storage involved.

The major advantage is that tankless units eliminate standby heat loss — the energy wasted by a traditional tank constantly reheating water to maintain its temperature even when nobody's using it. Tankless units also take up significantly less space and generally last longer than tank models, often 20 years or more compared to 10 to 15 for a traditional tank. The trade-off is a higher upfront cost and, in some cases, limitations on how many fixtures can draw hot water simultaneously without a drop in output.

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