Abstract: An electromagnetic induction heating apparatus comprises a heating unit which includes a tank having a removable cover and inlet and outlet openings and containing a quantity of liquid. A plurality of electromagnetic induction heaters arranged in series and releasably supported in fixed position within the tank between the inlet and outlet openings are immersed in the liquid and controlled by an external electrical control circuit to heat the liquid to a predetermined temperature. Conduits connected to the heating unit carry liquid to and from the unit and may be arranged in a closed loop to continuously recirculate heated liquid through the heating unit when the apparatus is provided to heat a building or the like or to preheat lubricating oil in a lubrication system. The apparatus may also be arranged to receive fuel oil from a supply source, preheat the oil, and supply it to a burner for firing a boiler or the like.