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.


Electric Water Heaters
December 2, 2009

How Electric Water Heaters Work

Electric water heaters work on a different principle then gas. There is no core in the center, since there is no burner, and no heat exchanger or venting system is required. Electric water heaters typically employ two immersion elements that are similar to the elements in a kettle.

One is near the top of the tank and the other is near the bottom. Both elements do not work at the same time. (Note: There are some water heaters that have only one element. There are also some water heaters designed to have both elements on at the same time. Life can never be simple, can it?)

Thermostat Control Both elements are thermostatically controlled, and the temperature setting is usually the same on the top and bottom. The thermostats are not normally accessible without removing an access panel, and often some insulation.

If the electric power to the water heater is on, there is a risk in removing these access panels and accessing the thermostats. There are live electrical connections adjacent to the thermostats. Unless you are comfortable and skilled working with electricity, you should leave these access panels closed.

These elements provide the heat and, for the most part, the water configuration is the same as on a gas or oil system. Cold water is introduced near the bottom of the tank and hot water is drawn off near the top. The cold pipe may enter the top of the tank or the side of the tank near the bottom.

When the tank is at rest, it will be filled with hot water. When a hot water faucet is turned on, hot water will be drawn off the top of the tank and cold water introduced at the bottom. This cold water near the bottom of the tank will activate the thermostat for the lower element.

The lower element will come on and heat the incoming cool water. The upper element will still be surrounded with hot water and will be satisfied.

If we keep drawing off hot water, the tank will be filled with cool water, because the lower heating element will not be able to warm the water up quickly enough as it comes in and flows through. Eventually, all of the hot water will be drawn out of the tank, and all the water will be cold.

At this point, the thermostats for the upper and lower heating elements will both be calling for heat. The upper element has priority and it will shut off the lower element so that it can operate. As discussed, only one element can work at a time.

The upper element has priority because after the tank has run out of hot water, we want to prepare quickly for the next use of hot water. The upper element heats the water near the top of the tank first, since it is the water that will be drawn off first.

Once it heats the water in the top part of the tank, the thermostat will be satisfied and the upper element will shut off. The lower element now gets electricity and heats up the water in the lower two-thirds of the tank.

Under normal use, the lower element works more often than the upper element, although the upper element has priority.

You can’t tell if one element is burned out without doing tests. It is common for water heaters to work, but not very well, because one element is burned out. Replacement of elements is not a big job and water heaters themselves are not terribly expensive.