A mixture of magnesium-iron alloy and salt is confined within a nonwoven cellulose covering and forms an elongated cartridge which is wrapped with multiple layers of absorbent paper. The cartridge and paper are enclosed within a pouch of perforated plastic film to form a heating element. One or more heating elements are attached by adhesive tape to the inside of a generally triangular polyethylene film envelope, and peripheral portions of the envelope are heat sealed to form an enclosed chamber. An overlying mating sheet of polyethylene coated nylon film and a pad of thermoplastic foam material are heat sealed to peripheral portions of the envelope, and parallel strips of double-sided adhesive tape, with removal protective strips, are attached to the outer surface of the envelope to form a flexible heater device. The device is used by being wrapped around either a vertical or horizontal pipe joint containing a heat activated epoxy. An upper sealed corner portion of the envelope is cut and removed to form an opening for the chamber, and water is poured through the opening into the chamber to initiate an exothermic reaction with each heating element for heating the pipe joint and epoxy to accelerate curing of the epoxy.