How Much Natural Gas Does an Average Water Heater Use?

How Much Natural Gas Does an Average Water Heater Use?

Time of day, your ambient air temperature and lots of of other factors affect the cost

By Brad Painting

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Water heaters come in different forms, but one of the most common and economical models is a storage tank heated by natural gas. Natural gas is usually cheaper than electrical heating, but its cost is not negligible; the California Energy Commission estimates the annual cost of running a tank-style gas water heater to be $136 to $190.

Quantity

The gas burner works in conjunction with the water heater's thermostat to burn the appropriate amount of natural gas. According to a popular engineering website, natural gas has a net heating value of 11,000 kcal per kilogram, or about 44,000 BTUs per kilogram. A typical water heater might have an energy consumption of about 230 therms per year, or 23 million BTUs. Such a tank would consume about 530 kilograms (or 1,200 pounds) of natural gas each year.

Effect of Size

Larger heaters have greater surface area and thus greater opportunity to lose heat. A larger heater uses more natural gas than a smaller one even if the occupants use the same amount of hot water. The California Energy Commission lists suggested "first-hour ratings" for various house types. First-hour rating tells you how much hot water your tank can supply during an hour of consistent use. A typical two-bathroom, three-bedroom house needs a first-hour rating of 70 gallons. With natural gas water heaters, this requirement is usually met by a 40-gallon tank.

Effect of Demand

The amount of water used in a household greatly affects the quantity of natural gas the water heater burns. Every time you pull water from the tank, the heater must warm ground-temperature water at approximately 50 degrees Fahrenheit up to about 120 F, or to whatever the thermostat setting is. Showers and baths, laundry loads and dishwashers are the main drains on the hot water supply.

Efficiency

Water heaters have less than 100 percent efficiency because as the gas burns in the combustion chamber, much of the heat escapes up the flue. Some tanks have a label that shows the "energy factor," which can range from .5 to .7 -- meaning 50 percent to 70 percent of the fuel's inherent chemical energy is absorbed into the water as heat. Water heaters with lower efficiencies use more fuel.

Variables

Although the energy guide label on water heaters gives an estimated yearly cost, a range of costs would be more appropriate. Even if you know the exact hot water demand for a household, several unpredictable factors come into play. Depending on geography and time of day, the temperature of incoming water can vary, which alters the amount of energy required to heat it. If the tank is an unconditioned room, differences in ambient air temperature will also affect its rate of heat loss. The presence or absence of an insulating jacket for the tank can affect gas consumption by up to 10 percent. Natural gas prices differ from state to state, so factor your area's cost into your estimates.

About the Author

Brad Painting is a regular contributor to DexKnows and specializes in green building design.

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