Water heaters transfer heat to water using either a flame (in gas heaters) or electric heating elements.
Types of Water Heaters and Their Heat Transfer Methods
There are two main types of water heaters: gas and electric. They employ different methods for transferring heat to the water.
Gas Water Heaters
- Flame-based Heating: Gas water heaters typically have a burner located at the bottom of the tank. This burner combusts natural gas or propane, creating a flame.
- Direct Heat Transfer: The flame directly heats the bottom of the tank.
- Conduction: The heat from the flame is transferred through the metal of the tank via conduction. This heated metal then warms the water inside.
- Convection: As the water at the bottom of the tank heats up, it becomes less dense and rises to the top, while cooler water sinks to the bottom to be heated. This creates a convection current, ensuring the entire tank of water is heated.
Electric Water Heaters
- Electric Heating Elements: Electric water heaters use one or more electric heating elements submerged directly in the water.
- Resistance Heating: When electricity passes through these elements, they resist the flow of current, generating heat. This is an example of resistance heating (Joule heating).
- Direct Heat Transfer: The heat from the element is transferred directly to the surrounding water.
- Convection: Similar to gas heaters, convection currents are established as the heated water rises and cooler water sinks, resulting in even heating throughout the tank.
Common Elements in Heat Transfer
Regardless of the type, all water heaters use the following basic principles to transfer heat:
- Conduction: Heat is transferred through the solid materials of the heater (e.g., the tank walls, heating elements).
- Convection: Heat is distributed through the water via convection currents.
Water Circulation
Both gas and electric water heaters are designed with inlets and outlets to facilitate the circulation of water:
- Cold Water Inlet: Cold water enters the tank at the bottom.
- Hot Water Outlet: Hot water is drawn from the top of the tank when needed (e.g., for showers, dishwashers). This ensures the hottest water is always available.
