A method of heating a subsurface formation includes introducing a molten salt into a first passageway of a conduit-in-conduit heater at a first location. The molten salt is passed through the conduit-in-conduit heater in the formation to a second location. Heat transfers from the molten salt to a treatment area during passage of the molten salt through the conduit-in-conduit heater. The molten salt is removed from the conduit-in-conduit heater at a second location spaced away from the first location.