How Does a Water Heater Work?

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Conventional Storage Water Heaters

Conventional storage water heaters remain the most popular type of water heating system for the home. In this section, you'll find basic information about how storage water heaters work, what criteria to use when selecting the right model, and some tips on installation, maintenance and safety.

How They Work

A single-family storage water heater offers a ready reservoir — from 20 to 80 gallons — of hot water. It operates by releasing hot water from the top of the tank when you turn on the hot water tap. To replace that hot water, cold water enters the bottom of the tank, ensuring that the tank is always full.

On top of the tank are two thin pipes: one pipe is the hot-water outlet, and the other is the cold-water inlet. A large pipe in the middle is called a vent pipe. A pressure/temperature-relief valve is also on top of the tank and is connected to an open pipe that runs down the side of the tank. Another valve near the bottom of the outside of the tank is the thermostat and gas valve. The graphic below shows the parts inside the tank, which include a large tube called a flue tube/heat exchanger. Inside this tube is a jagged insert called a flue baffle. Beside the flue tube/heat exchanger is a thin tube called the anode rod. At the bottom of the tank is a gas burner, and beneath the burner are combustion air openings.

Conventional storage water-heater fuel sources include natural gas, propane, fuel oil, and electricity. Natural gas and propane water heaters basically operate the same way. A gas burner under the tank heats the water. A thermostat opens the gas valve as the water temperature falls. The valve closes when the temperature rises to the thermostat's set-point. Oil-fired water heaters operate similarly, but they have power burners that mix oil and air in a vaporizing mist, ignited by an electric spark. Electric water heaters have one or two electric elements, each with its own thermostat. With two electric elements, a standby element at the bottom of the tank maintains the minimum thermostat setting while the upper demand element provides hot-water recovery when demand heightens.

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