What is the primary function of a tankless domestic water system?

The primary function of a tankless domestic water system is best described as providing hot water on demand. Unlike traditional systems that store heated water in a tank, tankless systems heat water directly as it flows through the unit. When a hot water tap is opened, cold water travels through a pipe into the unit, where it is heated by either gas burners or electric elements.

This method eliminates the need for a storage tank and ensures a constant supply of hot water as long as the system is operational. Therefore, the correct answer reflects the mechanism by which tankless systems deliver hot water efficiently and without the limits imposed by a storage tank's capacity. This capability aligns closely with the description of providing hot water from a coil in the boiler, as the heating occurs instantaneously rather than utilizing pre-stored water.