Glossary of Terms

To help you better understand what certain water heater related parts and terms mean, we’ve created the glossary below. Anode Rod A long magnesium or aluminum rod that is located inside a water heater and attached to the top. Its purpose is to literally “sacrifice” itself to protect the inside lining of the tank from […]

Anthony Barnes

To help you better understand what certain water heater related parts and terms mean, we’ve created the glossary below.

Anode Rod

A long magnesium or aluminum rod that is located inside a water heater and attached to the top. Its purpose is to literally “sacrifice” itself to protect the inside lining of the tank from corrosion. An anode rod has a life expectancy of about 5 years after which is should be replaced.

Ball/Gate Valve

Two common types of valves which open or close to allow the flow of water. A ball valve has a handle which you turn 90 degrees to open/close. A gate valve has a wheel which you turn until it fully opens or closes. Similar to an outdoor water spigot you would connect a garden hose to.


Natural gas input is measured in BTUs (British Thermal Units) per hour. Measures the required energy  to raise the temperature of 1 pound of water from 60 to 61 degrees at one atmospheric pressure. Most residential gas water heaters are rated between 35,000 to 75,000 BTU.

Circuit Breaker

An automatically operated electrical switch that protects an electrical circuit from overloading or short-circuiting damage. Multiple circuit breakers are located inside a breaker panel in most garages. You may manually switch off a breaker to cut off the flow of electricity to that circuit.

Cold Water Inlet

The pipe through which cold water enters the water heater. Generally located at the top of the water heater.

Cold Water Shut-off Valve

Typically located on the cold water inlet atop the water heater, this valve opens or closes the supply of cold water to the tank. Prior to water heater repair or replacement, you would turn this valve to the closed position.


When water vapor is turned back into liquid form (reverse of evaporation) most commonly due to the water inside the water heater being colder than outside the tank (as well as other reasons). This is temporary and normal and is often confused for a water heater leak.


The slow decomposition of metal due to a chemical reaction that occurs during prolonged contact with water. Corrosion is responsible for many water heater parts failing, including the inside of the tank. It’s the most common reason why a water heater will require replacement.

Dip Tube

A plastic tube that runs from the cold water inlet pipe down to the bottom of the tank. The tube carries the cold water to the bottom where it can be heated instead of mixing in with the hot water that’s near the top of the tank and ready to be used.

Earthquake Strap

For those that live in an area prone to earthquakes, an earthquake strap (or straps) may often be required. Essentially a metal belt, it wraps around your water heater and attaches to the wall to prevent the water heater from tipping over and ripping out gas lines. Important for safety as well as allowing you to use the contents of the hot water tank as emergency drinking water.

Electric Water Heater

Simply put, it’s a water heater powered by electricity. They can be the most common tank-type, tankless, or small point-of-use variety. Unless otherwise specified, when we mention an “electric water heater” on this site, we are referring to the tank-type version.

Energy Guide Label

This yellow label is attached to every water heater to allow you to compare the estimated energy use of various models to see how much it would cost to run and how efficient a model is.

Expansion Tank

Attached to the cold water inlet, the expansion tank prevents pressure build up due to thermal expansion inside the water heater by absorbing the expanding water. This allows the pressure to not stress piping or force open the T&P valve to vent water.

Gallon Capacity

The amount of water the hot water tank can store. Make sure it’s enough for your needs before purchasing a new water heater.

Gas Burner

The part in a gas water heater that produces the flame to heat the water.

Gas Flue Tube

A cylindrical chimney that runs through the center of a gas water heater tank which carries combustion gases through vent piping and discharges them outside.

Glass Lining

A hard, protective porcelain compound surface that lines the inside of a water heater tank to help prevent corrosion from water.

Gas Water Heater

A natural gas powered water heater. Available in the both tank-type and tankless variations. Unless otherwise specified, when we mention a “gas water heater” on this site, we are referring to the tank-type version. Gas water heaters are also available with three types of venting:

  • Standard-Vent – Draws indoor air from around the water heater and vents vertically through the roof of the house. Also referred to as atmospherically-vented.
  • Direct-Vent – Draws all needed air from outside the house and vents horizontally through an outer wall.
  • Power-Vent – Draws indoor air from around the water heater and uses an electric blower to vent the air outside.

Heat Exchanger

A part that allows an efficient means to transfer heat from one source to another. They are most commonly used in tankless water heaters to instantly heat water flowing through it.

Heating Element

Extending directly into the tank of electric water heaters, it is the heat source for the water inside the tank. Most water heaters have an upper and lower heating element. The bottom one does most of the work by heating the cold water than comes in. The top element keeps the upper water hot and gives it a quick boost when some of the hottest water is depleted.

Heater Drain Valve

Located at the bottom of the water heater tank, the heater drain valve is used to flush sediment out of the tank or completely drain the tank in order to make repairs or replace the water heater.

Hot Water Outlet

Found at the top of the tank alongside the cold water inlet, this is what transfers the hot water from the tank into the hot water piping in your home.

Insulation R Value

A water heater’s insulation R Value determines how well it prevents radiant heat loss through the tank. A higher R Value is better.

On/Off Control

With electric water heaters, power is controlled by either switching off the circuit breaker at the main breaker box or via a separate on/off switch that’s part of the water heater unit (if there is one). With gas water heaters, the control knob on the thermostat acts as the on/off control.

Pilot Light

Only used in gas water heaters, the pilot light is a small constantly burning flame next to the burner. When the thermostat senses the need for water heating, the pilot light ignites the gas flowing from the burner.

Pipe Nipple

The threaded fittings on a pipe (typically 3/4″) which are used for water pipe connections. Typically made of brass or galvanized steel.

Point-of-Use Electric Water Heater

Small electric water heaters that are used in low-demand hot water applications such as utility sinks or kitchens where only a limited amount of hot water is necessary at one time. Their small storage tanks (as low as 2 gallons) allow them to be installed in many limited-space locations.


Amount of water that can be heated in an hour. Measured in gallons (GPH).


Also referred to as scale, sediment is comprised of minerals found in water that have dissolved and settled at the bottom of the water tank. If allowed to build up, they can clog or cause the drain valve to leak, reduce heating efficiency, and even cause the storage tank to make strange noises. As part of yearly maintenance, it’s a good idea to flush your water heater to remove most of the sediment.

Storage Tank

The actual tank that stores the water. It is watertight, the inside is lined with a porcelain compound, and the outside is wrapped in insulation as well as the final outer skin.

Tankless Water Heater

Unlike a tank-type model, a tankless water heater uses a heat exchanger to instantaneously heat water based whenever the need for hot water arises (ie: hot water faucet is opened or dishwasher starts). Water is heated as it flows through the heat exchanger to provide a non-stop supply of hot water. The biggest disadvantage is that many units can’t keep up with the demand of an entire family.

Tank-Type Water Heater

This type of water heater is by far the most common and uses a large storage tank to hold water. Cold water comes in, is heated, and then hot water is dispensed out when needed. The thermostat tries to keep the water inside at a specific temperature and ready for use. The biggest disadvantage is that once all the hot water in the tank is depleted, it takes time to reheat/recover to the correct temperature.

T&P Valve

The Temperature & Pressure Relief Valve (T&P valve or TPR valve) is a mandatory safety feature which automatically opens to discharge hot water in case the pressure or temperature inside the tank gets too high. If this excess pressure were allowed to build up, the water heater tank could literally explode. The valve is most often found on the top side of the water heater with a long tube connected to it that runs down the side of the tank to the floor.


Found on residential gas water heaters, a thermocouple is a tube that’s attached to the temperature control on one end and extends into the pilot light on the other. It causes the burner to fire when the thermostat senses water temperature is not hot enough.


This device constantly monitors the water temperature at the bottom of the tank. When the water temperature goes below the set point, it triggers the electric heating element to begin heating or gas flowing to the burner to be ignited. On a gas water heater, you may adjust the desired temperature via a dial or knob. On electric models, two thermostats (top and bottom) are typically accessed behind panels and the desired temperature can be set there.

UEF (Uniform Energy Factor)

A metric developed by the Department of Energy to communicate the efficiency of a water heater. The higher the UEF number, the more efficient that water heater is in it’s category (there are four different categories) and the less it will cost to operate. For example, a 50-gallon water heater with a UEF of 0.96 is more efficient and cheaper to operate than a different 50-gallon model with a UEF of 0.92.

Venting System

The way in which combusted gases are vented to the outside on gas water heater models. See Gas Water Heater above for the different types of systems.

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