In American homes, conventional storage gas water heaters remain the most common type of water heating system.
Most models have a reservoir from 20 to 80 gallons. Turn on the hot water tap, and hot water is released from the top of the tank. Cold water enters the bottom of the tank, so the tank is always full.
In addition to natural gas, water heaters may be fueled by propane or fuel oil.
Natural gas and propane water heaters. A gas burner under the tank heats the water. When water temperature falls, a thermostat opens the gas valve. When the water reaches a predetermined temperature, the valve closes.
Oil-fired water heaters. Power burners mix oil and air in a vaporizing mist, ignited by an electric spark.
Conventional storage water heaters have become more efficient over the years, but they are still less efficient than a tankless water heater because water is constantly being heated in the tank. Standby heat loss occurs and energy is wasted even when a hot water tap isn't running.
Some storage water heater models with heavily insulated tanks significantly reduce standby heat loss. These particular models have a thermal resistance (R-Value) of R-12 to R-25.