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How Do Hot Water Heaters Work? (Gas and Electric)


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Water heaters are an essential part of any household, providing hot water to showers, dish and clothes washers, and sinks. But how does a water heater work? 

Knowing the essential parts and how they work to provide your home with hot water can help you understand when there's a problem. This article will breakdown the anatomy of the traditional tank-style hot water heater, and explain how it provides your household with hot water.

Water heater tank is the most crucial part

How Do Hot Water Heaters Work?

Although tankless water heaters are gaining in popularity, most homes still use the traditional tank-style water heater. These large metal cylinders heat and hold water until there's a demand for hot water in your house.

Open a hot water faucet and water starts flowing through your home's hot water pipes. As hot water leaves your water heater, cold water enters the tank.

The cold water drops the temperature in the tank and your water heater begins heating the water. 

How the water is heated depends on the type of water heater you own. An electric water heater uses submerged heating elements to bring the water to temperature, where a gas heater uses a burner.

Electric Current

How an Electric Water Heater Works

Electric water heaters are hardwired to your electrical panel and require a 240-volt/30-amp dedicated circuit. Which means no other appliances can use the circuit.

You'll find two water lines located on top of the tank. The first is the cold water inlet line, which supplies the tank with fresh, cold water. 

The other water line is the hot water outlet line which distributes hot water into your home's plumbing.

When there's a demand for hot water (an open faucet), hot water travels out the hot water outlet line, and cold water enters your tank through the cold water inlet line to fill the void.

As the cold water enters your tank, it flows through a long dip tube that runs to the bottom of the tank. Since hot water is less dense than cold water, the water rises to the top as its heated.  

The upper thermostat is responsible for keeping the water in the top part of the tank at the set temperature. Since the tank has been in standby (holding the hot water until there's a demand), it's typically not necessary for the upper thermostat to fire the upper heating element. Instead it instructs the lower thermostat to turn on.

The lower thermostat senses the drop of temperature as the cold water enters the tank and fires the lower heating element on to heat the water.

During periods of high demand, where the water in the top of the tank drops below the set temperature, the upper thermostat will instruct the lower thermostat to shut down the lower heating element. The upper heating element will fire and heat the water at the top of the tank.

When the tank is in stand-by (no hot water demand, but the water heater keeps the water to temperature) the lower heating element will cycle on and off to keep the water warm.

It's important to note that the upper thermostat controls the lower thermostat, and that both heating elements are never on at the same time.

Watch the Video

Gas Burner

How a Gas Water Heater Works

Since not all areas have access to natural gas, manufacturers have gas water heater models which can be powered by either natural gas or propane.

Gas water heaters also have a cold water inlet line that supplies the tank with cold water as the hot water leaves the tank through the hot water outlet line.

Just like an electric water heater, as the water enters the tank, it travels through a long dip tube to the bottom of the water heater.

Near the bottom of the tank is a gas control valve with a thermostat that protrudes into the tank. When the thermostat senses the temperature has fallen below the set point, the main gas valve opens.

Gas mixes with air and the burner ignites at the bottom of the tank and begins heating the water. An exhaust vent runs vertically down the center of the tank, and as it vents the fumes, it also transfers heat to the water.

The gas burner continues to burn until the water reaches the set temperature. 

Watch the Video

Hot Water Heater Parts

Electric and gas water heaters share many of the same parts, but because they heat the water entirely differently, they also have parts that are unique to each.

Common Water Heater Parts

You'll find these parts in both electric and gas water heaters:

Tank - The average water heater has a 50-gallon tank, but there are smaller and larger sizes available. The tank is made of steel and has an interior glass lining to protect it from rust and corrosion. In order to reduce heat loss, the tank is wrapped in insulation and encased in a thin outer shell.

Cold Water Inlet Line - This water line is located at the top of your water heater and connects to a dielectric nipple. It feeds the water heater fresh, cold water as the hot water exits the tank.

Shut-Off Valve - A shut-off valve is located on the water inlet line which allows you to easily turn off the water coming into your tank. Although, most water heaters have a shut-off valve, if your's doesn't, you'll need to shut-off your water at your water main.

Dielectric Nipple - There are two dielectric nipples. One that connects the cold water inlet line to the dip tube, and the other that connects to the hot water outlet line. Since two different metals will be connected together (copper lines to the steel tank) a chemical reaction will take place and corrode/rust the tank. By connecting these metals to dielectric nipples, your tank will be protected.

Dip Tube - A long plastic tube called the dip tube connects to the dielectric nipple on the cold water inlet. As the cold water enters, it travels to the bottom of the tank through the dip tube where it will be heated.

Temperature Pressure Relief Valve - This is a safety device to release pressure from within your tank should the temperature ever reach unsafe levels where the tank may explode. You can read more about the T&P Valve HERE.

Anode Rod - Sometimes called a sacrificial anode rod, its designed to sacrifice itself to protect your tank from corrosion. The end of the anode rod can be found on the top of the water heater, and the rod hangs vertically inside your tank. The anode rod is typically made of magnesium or aluminum and has a steel core.

Drain Valve - Located on the side exterior, near the bottom of the tank is a drain valve. This valve allows you to drain the tank of water for maintenance or replacement. If it's been awhile since you've drained your tank, the sediment may cause the drain valve to clog. Read this article to learn how to unclog a drain valve.

Hot Water Outlet Line - The hot water leaves the tank through this line and begins traveling through your household hot water plumbing. The hot water outlet line is located on the top of the water heater and connects to a dielectric nipple.

Electric water heater thermostat

Electric Water Heater Parts

These parts are unique to electric water heaters.

Heating Elements - Electric water heaters typically use 2 heating elements to heat the water. One is located near the top of the tank, and the other, near the bottom. They are controlled by the thermostat and are fully submerged within the tank. Learn more about heating elements HERE.

Upper Thermostat - The upper thermostat is larger than the lower thermostat and is essentially a temperature activated switch which controls both the upper and lower elements. Think of the upper thermostat as the boss of the lower thermostat. If the water in the top part of the tank is at the set point temperature, the upper thermostat will instruct the lower element to turn on.

Lower Thermostat - After receiving instruction from the upper thermostat to turn on, the lower thermostat will turn on the lower heating element if it's below the set point temperature. Learn more about water heater thermostats HERE

Gas water heater control valve

Gas Water Heater Parts

These parts are unique to gas water heaters:

Gas Control Valve - Located on the back of the gas control valve is a thermostat which protrudes into the tank. The thermostat turns the gas control on and off. When the gas control is on, gas is released to the burner. Learn more about gas control valves HERE.

Gas Burner - At the bottom of the tank is a gas burner that heats the water. The heat is transferred to the water at the bottom head of the tank and through an exhaust flue which runs from the bottom of the tank to the top where it vents outside.

Exhaust Flue - As the gas burns, other gases are generated. The exhaust flue vents these gases from the burner to the outside of your house. The exhaust flue acts like a chimney and runs from the bottom of your tank through the top where it eventually makes its way outside. The hot gases that travel through the tank in the exhaust flue transfer heat to the water.

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