Gas Water Heaters
December 2, 2009

How a Gas Water Heater Works

Gas water heaters work on one principle. A large gas burner sits below a large tank of water and warms it up. Making it work right and work safely is another matter.

Gas water heaters can either be stand alone units that work only on internal controls, or they can be hooked to electric to run electric ignitions and pollution control devices. Most modern units need electricity for recycling the unburned fuel to increase efficiency also.

Let’s start with the basics. Cold water runs into a tank that ranges from 10 to over 50 gallons in capacity. The line bringing in the cold water hooks to the top of the tank to a fitting at the end of an internal line that runs to near the bottom of the tank.

This way, the cold water gets warmed as it descends into the tank, but the cooler water still goes to the bottom leaving the hot water near the top. Usually, the cold water has a cutoff valve to make maintenance, draining, and replacement easier.

The exit pipe for the hot water comes out of the heater near the cold water inlet. The difference being that the hot water is taken from the top of the tank to maximize the amount of hot water available.

Near the top of the tank is a lever attached to a pop-off valve. The purpose of this valve is to make sure that the tank doesn’t explode if the pressure builds too high while the water is heating up.

This valve will release pressure and water in the same way that a breaker stops an electrical fire by stopping the flow of electric. If this valve looks rusty or won’t move, you may want to have it replaced for safety reasons.

At the bottom of the tank is a faucet. The purpose of this faucet is to allow you to drain the tank for maintenance and replacement. It also can come in handy if you need to attach a hose to get hot water somewhere else.

This faucet can be used to get fresh water if an emergency arises. It will be hot, but it can be used for everything including drinking after it cools.

Coming into the bottom of the tank is a gas line. It connects to a regulator that forces the gas through an orifice designed to feed the proper amount of gas (either natural or propane) to the burner under the water tank.

If your heater has an electric ignition, it will be near the burner and will glow bright orange or red when preparing to ignite the burner.

If your unit is not equipped with electric ignition, there will be a small tube running from the regulator to near the burner. At the end of it will be a place for a small flame to be lit. This is your pilot light. On most water heaters, this pilot is about 3 to 6 inches from the access opening in the bottom of the unit.

This pilot must be lit for the water heater to function and heat water. A small wire with an elongated bulb will be near the pilot. This is a thermocouple.

It’s function is to tell the regulator that the pilot is burning hot enough to ignite the burner. This signal causes gas to be released to the burner. A thermostat inside the heater tells the heater when to use the ignition system to add more heat to the stored water.

This ignition process is the same for electric or mechanical ignition systems. If the heater requires electric, it will have a small electric motor near the top of the tank. A series of ducts will redirect the fumes produced by the burner to be re-burned before exiting the unit. This raises efficiency from about 80 to about 93%.

High efficiency water heaters like high efficiency furnaces are vented nearly horizontally with PVC pipe through a nearby wall. Older style heaters need a flu to provide updraft to carry the hot fumes safely away from the house’s interior.