Anatomy of a Tank Type Gas Water Heater

  • 01 of 07

    Anatomy of a Gas Water Heater

    Water heater tank
    Julia Nichols/Getty Images

    The standard "tank type" water heater is found in most homes and with just a little maintenance, provides years of trouble-free operation. Unlike tankless water heaters that have no storage tank, the tank type water heater heats cold water and stores the hot water until it is needed. This tutorial will give you a clear understanding of the components which make up the tank type gas water heater.

    The gas water heater works by a law of physics called convection which loosely paraphrased, states that heat rises. Cold water is supplied to the tank and injected at the bottom of the tank through a dip tube. The denser cold water stays there and is heated by the gas burner. As the water heats up, it naturally rises and is drawn off by the hot water discharge pipe.

    You'll notice in the diagram above that the hot water pipe is much shorter than the cold water pipe. This ensures that only the hottest water is being used from the tank. The hot water heater has simple parts and some safety features.

    Let's take a look at each major component of the hot water tank.


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  • 02 of 07

    Water Supply and Discharge

    Digital illustration of inside of a gas water heater
    Dorling Kindersley/Getty Images

    Cold Water Supply: Cold water is provided to the tank by a cold water supply line and controlled by a shutoff valve. It is important to know where the water supply shut off valve is located so maintenance can be performed on the tank.

    Hot Water Discharge: This is the business end of the hot water heater, and the hot water line is what supplies all your sinks, tubs, showers, and appliances needing hot water.


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  • 03 of 07

    Construction of the Hot Water Tank

    Interior of a gas water heater dip tube
    Dorling Kindersley/Getty Images

    The tank jacket itself is made of steel and encloses a pressure tested water storage tank. Between the storage tank and the tank jacket is insulation to reduce heat loss of the heated water. It is a good idea to supplement the insulation by adding a ​fiberglass insulation tank jacket to the outside of the hot water heater. These are inexpensive and easy to install.​​​​

    Inside the tank, you will see a dip tube. The dip tube is where the cold water supply enters the tank to be heated by the gas burner. Since cold air and cold water is denser than hot air or hot water, the cold water sets at the bottom of the tank until it is warmed by the burner. Then it is heated enough to rise (through convection) to the top of the tank where the hot water hangs out. 

    In glass-lined tanks there will also be a metal rod in the tank, usually magnesium or aluminium) called a sacrificial anode. The anode rod is bolted and fastened to the top of the tank and extends deep into the tank. Its purpose is to draw corrosion to itself instead of the metal tank. Some models do not have a separate anode but combine the function of the anode with the hot outlet. Plastic line tanks do not have an anode.

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  • 04 of 07

    Gas Burner Control and Burner Assembly

    Gas water heater pilot light
    Scott Akerman/Flickr/CC BY 2.0

    The natural gas or propane is supplied by a pipe having its own gas shutoff valve. Just like you need to know where the water supply shutoff valve is located, you need to know where the gas line shutoff is located too. The gas line fees into a gas burner control module that serves as a kind of thermostat for the water heater. It also controls the ignition of the pilot light.

    From the control module, we now proceed to the gas burner assembly. This includes the pilot light and gas burner itself. The pilot light and burner adjustment are key to proper and energy efficient operation of the water heater. The gas flame should about 1/2 inch in height and should have blue tips.

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  • 05 of 07

    Gas Combustion Exhaust Flue

    Inside of a gas hot water heater, flue
    Dorling Kindersley/Getty Images

    The exhaust flue serves two purposes. It exhausts combustion gasses from the burner, and it serves as a type of heat exchanger helping to heat the water in the storage tank. The flue must be properly exhausted to the outside, and there are specific code requirements for the type of flue construction and acceptable details.

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  • 06 of 07

    Temperature and Pressure Relief Valve

    Inside of a gas hot water heater, pressure relief valve
    Dorling Kindersley/Getty Images

    A safety feature of the hot water heater includes the pressure relief valve and discharge pipe. It operates like the radiator cap on your car. The purpose of this valve is to relieve excessive temperature or pressure build-up inside the tank if it approaches the limits of the tank's safe design range. This valve is located on top of the tank and often is threaded directly into the tank top itself. To test the valve lift up on the handle slightly and hot water should discharge out of the overflow pipe.

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  • 07 of 07

    Tank Drain Valve

    Inside of a gas hot water heater, drain valve
    Dorling Kindersley/Getty Images

    The hot water tank can build up sediments in the bottom of the tank if left unmaintained, and by draining the tank using the tank drain valve, these sediments cannot build up. And if you don't have sedimentation, then that helps to prolong the life of your tank and improve your water quality. Flushing a tank isn't particularly hard.

    How to Prepare the Tank for Flushing:

    • Turn off power to the electric water heater. Do this by turning off the circuit breaker or fuse powering the heater.
    • Turning gas pilot control valve to "pilot" setting.
    • Shut off the water supply to the water heater.
    • Opening nearest hot water faucet.
    • Attach the hose to drain valve.