In which type of heating system does water pass through a heated coil?

The type of heating system in which water passes through a heated coil is an instantaneous water heater. This system operates by allowing cold water to flow through a heating element or coil, which heats the water on demand. This means that water does not need to be stored in large quantities, as it is heated as it flows through the system. This characteristic allows for a continuous supply of hot water without the need for a storage tank, making it efficient for on-demand heating needs.

While other heating systems may involve water, they do so in different ways. For instance, a boiler system generally heats water and distributes it through pipes to radiators or underfloor heating systems rather than just passing it through a coil. An electric radiator typically uses electric resistance to produce heat without circulating water. A heat pump can also use water in some scenarios but primarily functions by transferring heat from one place to another rather than passing water through a coil for direct heating. Thus, the distinctive operation of the instantaneous water heater with direct water flow through a heated coil makes it the correct choice in this scenario.