Fossil Fuel Storage Water Heaters

Energy Source Gas or Propane Fuel Oil
Minimum Efficiency Recommended .63 EF .61 EF
Maximum Efficiency Available .86 EF .66 EF
Expected Life 9-13 years 8 years
Approximate Cost to Install $400-$800 $1,000-$4,000

Fossil fuel storage water heaters are the most common hot-water systems. They burn propane, natural gas, or oil; in many locations, gas is the most cost-effective fuel for water heating. Gas-fired water heaters last, on average, only nine to 13 years, but with proper maintenance, you can make them last three times as long. Oil is rarely used in home water heaters, except in combined systems.

1 Vent

2 Insulated hot water outlet

3 Draft hood

4 Temperature and pressure relief valve

5 Insulated cold water inlet line

6 Flue

7 Gas supply line

8 Burner in combustion chamber

9 Control panel and thermostat

A standard fossil fuel water heater uses house air for combustion and exhausts its combustion byproducts by natural draft. To increase the number of installation options, some new models use a fan to help vent combustion gases. A fan permits direct venting out a house wall, instead of straight up through the roof. These induced-draft or fan assist models can be installed almost anywhere. They use a little more energy to run their fans.

Sealed-combustion water heaters draw all their combustion air from the outdoors, which eliminates any chance of backdrafting. This feature is especially helpful in tight homes, where appliances compete for less combustion air. In addition, sealed combustion heaters can save energy because they don't steal heated or cooled indoor air from the house.

Special Features

Look for a model with a high Energy Factor. These models will have multiple flues for greater heat transfer surface, or a more submerged combustion chamber (so heat doesn't escape to the air, but is transferred to the water instead). A few larger gas units now come with a condensing flue to extract more energy from the hot exhaust. Electronic ignition instead of a pilot light will save you a few dollars in energy costs per year. However, they won't work during blackouts.

If you have an electric storage water heater in a tight space and want to convert it to gas, you can use an add-on burner for the electric tank. A system is available in which a tankless gas water heater is installed outdoors and piped inside to the old electric storage tank, which is disconnected from its power supply.


For units that use inside air for combustion, be sure to provide ample clearance so that nearby air can flow easily, and make sure that everyone in the family knows not to put combustibles nearby.

Courtesy of DOE