How a Gas Tank-Type Water Heater Works

Drain valve being checked at bottom of gas tank water heater

The Spruce / Kevin Norris

The standard "tank-type" water heater is found in most homes, and with just a little maintenance, it can provide years of trouble-free operation. While tankless water heaters that heat water as it is needed are becoming more popular, the tank-type is far less expensive and still preferred by most homeowners. Tank-type water heaters are available in both gas and electric versions.

Basics of Gas Water Heater Operation

As the name suggests, a tank-type water heater heats cold water and stores the hot water until it is needed by various plumbing fixtures and appliances in the home. A gas water heater works by a law of physics knowns as convection—which defines how heat rises. In the case of a water heater, the cold water enters the tank through a cold water supply tube to force a constant supply of cold water into the tank. The dense cold water at the bottom of the tank is heated by a gas burner located below the sealed tank. As the water grows warmer, it rises in the tank, where it is drawn off by the hot water discharge pipe to provide hot water wherever it is called for. The hot water discharge pipe is much shorter than the dip tube, since its goal is to funnel off the hottest water, which is found at the very top of the tank. 

The gas burner that heats the water is controlled by a gas regulator assembly mounted on the side of the water heater, which includes a thermostat that measures the temperature of the water inside the tank and turns the burner on and off as needed to maintain the set temperature of the water. 

An exhaust flue runs through the center of the tank to allow exhaust gases to flow up through the tank and out of the house via a chimney or vent pipe. The hollow flue is fitted with a spiral metal baffle that captures heat and transmits it to the surrounding water to maximize the efficiency of the appliance. 

A close examination of each component demonstrates the ingenious simplicity of the traditional tank-type gas water heater. 

The Tank

The tank of a water heater consists of a steel outer jacket that encloses a pressure-tested water storage tank. This inner tank is made of high-quality steel with a vitreous glass or plastic layer bonded to the inside surface to prevent rusting. In the center of the tank is a hollow exhaust flue through which exhaust gases from the burner flow up to an exhaust vent. In most designs, a spiral metal baffle inside the flue captures heat from the exhaust gases and transmits it to the surrounding tank. 

Between the inner storage tank and the outer tank jacket is a layer of insulation designed to reduce heat loss. You can also 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, but it is important to avoid blocking the burner access panel and the flue hat at the top of the tank.

Inside the Tank

In addition to the long dip tube that supplies cold water to the tank, and the shorter hot water discharge pipe through which hot water flows into the plumbing system, there are other key components inside the tank.

In glass-lined tanks, there will be a metal rod in the tank, usually magnesium or aluminum, 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 rust-causing ions in the water to itself, thus preventing the metal tank from corroding. Some models do not have a separate anode rod but instead have a hot water outlet pipe that is coated with magnesium or aluminum to serve the function of an anode. If hot water coming from faucets becomes smelly or discolored, it may be an indication that the anode rod has been consumed. Replacing an anode rod is a fairly easy DIY project. 

Cold Water Supply Pipe and Hot Water Discharge Pipe

Fitted to the top of the tank, there are two water pipes—a cold water supply pipe and a hot water discharge pipe. 

Cold water supply pipe: Cold water is provided to the tank by a cold water supply line controlled by a shutoff valve. It is important to know where the water supply shutoff valve is located so you can close it when maintenance is required.  Shutting off the cold water supply effectively shuts off the water flow entirely, since it is the pressure from the cold water entering the tank that keeps the hot water flowing outward. In many installations, the cold water supply shutoff valve will be identified by a blue handle. 

Hot water discharge pipe: This is the business end of the hot water heater—the pipe that supplies hot water to all your sinks, tubs, showers, and appliances needing hot water. The hot water discharge pipe may also have a shutoff valve, often identified by a red handle. 

Gas Regulator and Burner Assembly

The natural gas or propane that heats the water is supplied by a pipe having its own gas shutoff valve attached to a gas pipe made from steel black pipe or copper tubing. It is important to know where this gas shutoff valve is located, so you can turn off the gas in emergencies or to make repairs. The gas line feeds into a gas regulator that includes a thermostat for the water heater. This valve also supplies gas through a small secondary tube to the pilot light, which serves to light the burner when the regulator valve and thermostat call for it. 

From the gas regulator, gas flows to the gas burner assembly, which you can access through a metal panel on the bottom of the water heater's outer casing. This assembly 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 flames should about 1/2 inch in height and should have blue tips (yellow flames indicate dirty burner jets or an improper air mixture). The pilot light includes a key component known as a thermocouple—a small valve that converts heat to an electrical impulse. On newer water heaters, this component is known as a flame sensor. The thermocouple or flame sensor is a key safety feature, since it senses the presence of the pilot flame and prevents gas from flowing to the burner if there is no pilot flame to ignite it. Replacing a thermocouple or flame sensor is a fairly easy job. 

Exhaust Flue

The exhaust flue, a hollow cylinder running through the center of the tank, serves two purposes. It exhausts combustion gases from the gas burner, and it serves as a type of heat exchanger helping to heat the water in the tank. The flue must be properly exhausted to the outdoors, and there are specific code requirements for the construction of the flue.

Temperature and Pressure-Relief Valve

Another key safety feature of a hot water heater is the temperature and pressure-relief (T & P) 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 limit of the tank's design. 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; tank water should discharge out of the overflow pipe. If it doesn't operate properly, the T & P valve should be replaced.

Tank Drain Valve

The hot water tank can build up sediments in the bottom of the tank over time, leading to a number of problems. A water heater full of sediments will not heat efficiently, and you may hear bubbling, gurgling sounds caused by the moisture-saturated sediments boiling. By periodically draining the tank using the tank drain valve, these sediments are removed and problems are avoided.  Flushing a tank isn't particularly hard.

  1. Turning gas pilot control valve to the "pilot" setting.
  2. Shut off the cold water supply to the water heater.
  3. Open the nearest hot water faucet.
  4. Attach a garden hose to the drain valve and place the open end of the hose in a floor drain or utility sink.
  5. Open the tank drain valve and allow all water to drain out of the water heater tank. You will likely see discoloration in the draining water as the sediments flush out. In extreme cases, you may need to refill the tank with fresh water and drain a second time to remove all the sediments. 
  6. When the tank is empty, shut the tank drain valve, and open the cold water supply valve to refill the tank. Then turn the gas control valve to the ON position and check to make sure the gas burner ignites.