What is instant heater? Tankless water heaters—also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heaters are water heaters that instantly heat water as it flows through the device, and do not retain any water internally except for what is in the heat exchanger coil.What are electric heaters? Electric heating. An electric heater is an electrical device that converts an electric current into heat. The heating element inside every electric heater is an electrical resistor, and works on the principle of Joule heating: an electric current passing through a resistor will convert that electrical energy into heat energy.What is a heater control? Heater Control Valve. The heater core is the heart of the heating system. It uses heated engine coolant to provide heat to the passenger compartment. A heater control valve controls the amount of coolant flowing through the heater core.