Glossary of Terms


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Anode Rod

A long, replaceable metallic rod located inside the tank, typically attached at the top of the water heater near the cold water inlet. It helps to prevent corrosion inside the tank, and will last four to six years before needing replacement.


A gas powered burner used to heat water in a gas water heater.

Circuit Breaker

An electrical switch designed to protect a circuit from damage caused by an overload or short circuit. The switches can be flipped automatically or manually. Every house is equipped with a circuit breaker box. If you own an electric water heater this is where you would shut it off.

Cold Water Inlet

This is where the cold water is supplied to the water heater. It is usually found at the top of the water heater.

Cold Water Shut-Off Valve

A gate or knob-type valve that is located on the cold water inlet pipe, usually close to where it enters the tank. It is used to shut off the cold water supply to the tank when needed, during repair or replacement of the water heater.


Combustion is a chemical reaction that occurs in a gas-powered water heater. There are 3 elements necessary for combustion to take place: A fuel source (gas), fresh air (oxygen), and ignition (pilot light). When combustion happening properly, the by-products are vented outside (heat, carbon dioxide, water vapor, and light). If an incomplete combustion occurs (normally due to lack of oxygen) the lethal gas, carbon monoxide is produced.


Corrosion is the gradual decomposition of metals, and other materials, due to a chemical reaction with their environment. In the case of water heaters, it refers to the breaking down of various components of a water heater due to a reaction with the water stored inside the tank. Corrosion is one of the most common reasons that a water heater needs to be replaced.


Also known as Descaling. Is necessary with gas-fueled water heaters. When water is heated the minerals in the water attach to the water heaters components, particularly the heating elements. The mineral build-up is known as lime scale (or scale) and negatively impacts the heat exchange efficiency. Flushing the unit as part of regular maintenance will remove the lime scale.

Dip Tube

An attachment to the cold water inlet that extends inside the tank. It allows the cold water coming into the tank to be deposited near the bottom of it, so that already-heated water within the tank remains near the top, where it can be drawn off.

Electric Water Heater

A water heater that is powered by electricity rather than gas. Although both tank-type and tankless water heaters can be powered by electricity, when “electric water heaters” are mentioned on this site we are generally talking about tank-type water heaters.


An electric element, similar to the elements found on an electric stove, used to heat water in an electric water heater. Most electric water heaters employ two elements, one near the top of the tank and one near the bottom, to heat the water.

Energy Factor (EF)

A rating system that determines a water heater’s energy efficiency. The higher the EF rating the more efficient the water heater. It is based on the amount of hot water produced from one unit of fuel.

Flow Rate

The amount of water a tankless water heater is capable of heating at any given time measured by GPM (Gallons per Minute).

Gas Control Valve

Also known as the gas inlet valve, is located on the outside of a gas water heater near the bottom. It contains the pilot light controls and the thermostat.

Gas Water Heater

A water heater that is powered by gas rather than electricity. Although both tank-type and tankless water heaters can be powered by gas, when “gas water heaters” are mentioned on this site we are generally talking about tank-type water heaters.


The abbreviation for Gallons per Minute. GPM is one of two critical measurements used to properly size a tankless water heater. See Flow Rate.

Hard Water

Hard water is defined by the quantity of dissolved minerals (calcium and magnesium) within the water. Different areas have different levels of water hardness. The more minerals within the water, the harder the water. If you live in an area with hard water, you should flush your gas-fueled water heater more frequently to prevent lime build-up.

Heat Exchanger

A heat exchanger is a sophisticated piece of technology that’s designed to efficiently transfer heater from one medium to another. They are used in tankless water heaters to heat water instantaneously as it flows through the device.

For more information on heat exchangers, click here.

Heater Drain Valve

A valve near the bottom of the water heater that can be used to drain water out of the tank in order to flush out sediment or to drain the tank.

High-Temperature Cut Off Switch

A safety device in place on electric water heaters to shut down the power if the water becomes too hot or there is a malfunction. There is usually a red button located in the center of the switch that allows you to reset the device.

Hot Water Outlet

Where hot water leaves the tank, also usually found at the top of the tank, near the cold water inlet.


Used in tank-type water heaters to help prevent heat from escaping, thereby keeping the water hotter for longer with less fuel use.

Main Burner Orifice

This orifice controls the amount of fuel that is released into the combustion chamber of a gas water heater. For water heaters that need to be adjusted for high altitude environments, it may be necessary to decrease the size of this orifice for proper combustion to take place.

On/Off Control

On a gas-powered water heater, the thermostat can also act as the on/off control, or the on/off control can be a separate knob located near the thermostat. On an electric-powered water heater, power is turned on and off either at a main circuit breaker box or in a separate on/off switch box located near the water heater.

Pilot Light

A small gas flame kept alight in order to provide an ignition source for a more powerful gas burner. Pilot lights are used in gas water heaters.


Sediment is a collection of minerals found in water that has settled at the bottom of the tank. A large amount of sediment can greatly reduce the efficiency of your water heater, clog the drain valve and cause strange noises to emanate from your storage tank. To prevent sediment from building up inside the tank, flush your water heater at least once a year.

Standby Heat Loss

Heat loss that occurs when hot water is in a storage tank waiting to be used. Energy is used to keep the water warm reducing the efficiency of the water heater.

Storage Tank

Where the water is stored and heated. It is a solid, water-tight tank that is wrapped with a layer of insulation, then the outer skin of the water heater.

Tankless Water Heater

Tankless water heaters use a heat exchanger to heat water instantaneously without the need for a storage tank.  When a hot water demand is made (a tap is opened or a washing machine is turned on), it quickly heats the water that flows through the unit, thus providing a continuous supply of hot water.

For more information about tankless water heaters, click here.

Tank-Type Water Heater

Tank-type water heaters are the most common type of water heater. They use a storage tank that is fed cold water. That water is then stored and heated. The water inside the tank is kept continuously hot and therefore continuously ready for use. The water inside the tank is then drawn off to the separate hot water lines when a hot water demand is made.

For more information about tank-type water heaters, click here.

Temperature and Pressure Relief Valve

A safety mechanism designed to open automatically to release water or steam that might result from overheating or excess pressure within the tank. The valve is usually found on the side of the water heater near the top of it, with a plastic extension tube that runs from the valve down the side of the water heater.

Temperature Rise

The difference between the incoming ground water temperature and the heated output temperature.


A sensor that measures the water temperature on a gas water heater. It attaches from the temperature control to the pilot light. When the thermostat senses the tank water dropping below a set point, it triggers the burner to fire and heat the water. The Thermocouple is also a safety device that senses when the pilot light is on. If the pilot light is not lit, it will prevent the gas valve from turning on.


Used to adjust the temperature at which the water inside the tank is heated. On a gas-powered water heater (natural gas or propane) it is a knob located near the bottom of the water heater, where the gas inlet attaches. On an electric water heater, there are usually two thermostats (one each for the upper and lower heating elements), located on the side of the water heater, and each of which should be set to the same temperature.

Venting System

On a gas-powered water heater, the ducting through which combusted gasses are vented to the outdoors. On high-efficiency gas-powered models, venting may be assisted by an electric motor.

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