A tankless water heater, also known as an on-demand water heater, is a type of water heater that heats water only when it is needed, instead of storing hot water in a tank. When a hot water tap is turned on, cold water flows through a heating element or coil in the unit and is quickly heated to the desired temperature before being delivered to the tap. Tankless water heaters can be powered by electricity, natural gas, or propane, and are available in a variety of sizes to meet different household needs. They are known for their energy efficiency and space-saving design, as they do not require a storage tank. Tankless water heaters can provide an endless supply of hot water, making them a popular choice for households with high hot water demands. However, they can be more expensive to install than traditional tank water heaters, and may require upgrades to gas lines or electrical systems.