Traditional water heaters all work in a similar fashion. The primary difference between them is the fuel source, such as gas, electric or even solar energy. To understand how your unit works, the first step is to become familiar with the components of your system, and how they interact.
Components of a Water Heater
Electric and gas water heaters have a number of components in common, including the drain valve, the TPR valve, an internal magnesium anode rod, dip tube and pipes for hot water and overflow/pressure relief. In both cases, the internal tank is wrapped with insulating material to keep the water hotter longer.
Electric water heaters have a separate thermostat, while the thermostat on gas models is built into the gas control valve. Gas water heaters also have a heat limiting device to prevent overheating, a central, internal flue to vent gas and help circulate heat, and a thermocouple to shut the gas off in an emergency.
How a Water Heater Works
The differences between electric and gas water heater types are more apparent when you get down to the basics of operation. In both types, cold water enters the unit through the dip tube at the top and is funneled to the bottom of the tank, where it is heated.
Electric Water Heaters
In electric models, the thermostat is mounted flush against the side of the internal tank. When the thermostat senses that the internal temperature has fallen below the preset threshold, it triggers a switch (or two, in the event of a dual element system) that allows electricity to flow to the heating element. The heating element is in turn submerged in the water of the tank and heats up in the same way that an electric stove burner works, by passing electricity through a resistant material and converting energy into heat. When the thermostat(s) senses that the water has reached the correct temperature, it shuts off the power to the element(s).
Gas Water Heaters
Gas models also have a thermostat, typically a small copper tube with a mercury sensor in the tip. They also have special sensor called a thermocouple that senses whether the pilot light is currently burning. If the pilot is out, the thermocouple will not allow gas to flow to the burner. When the water temperature in the tank falls, the thermostat sends a signal to the gas control valve, which checks the signal from the thermocouple to ascertain that there is a pilot light. If so, a valve opens, allowing gas to flow to the burner, igniting a flame. The flame heats the bottom of the tank, causing warmer water to rise while cool water sinks, creating a natural circulation cycle. Additionally, the internal temperature is kept more uniform by heat rising through the central flue as it first rises through the tank and then leaves the home through a ventilation system. When the water temperature reaches the desired setting, the thermostat sends a signal to the gas control valve, instructing it to turn the gas flow off again.
Variations on Water Heaters
Some water heaters and most tankless systems use a hot water recirculating system which keeps hot water moving through the heating system and prevents hot water flow from being interrupted with “cold” spurts of unheated water. Solar water heaters use a similar system as the primary method of heating water. As hot water rises, it expands, pushing cooler water ahead of the hot water and circulating water through the internal pipes of the solar heater. In principle, a solar water heater is really just a circulation system that passes the water continuously through a concentrating device that is exposed to direct sunlight and channels heat into the core of the solar heater.