An apparatus for heating liquid in a pipe to prevent freezing of the liquid and for de-icing of frozen pipes includes an elongated housing constructed of plastic material and having a central bore and fittings for detachably coupling each end of the housing to the open end of a liquid pipe so that the central bore connects a first liquid pipe to a second liquid pipe. The fittings each comprise a sleeve attachable to the pipe end, a sealing gasket between the end of the housing and an annular flange on the sleeve, and a threaded coupling nut cooperating with external threads on the housing and having a flange engaging the sleeve flange. The housing includes a lateral passageway intermediate its end communicating with the central bore and closed by a sealer valve having an opening with a normally closed penetrable seal of elastomeric material. An elongated electric resistance heating cable, including a probe containing an electric heating core at the free end thereof, is slidably insertable through the penetrable seal into the housing bore and then into one of the pipes through the open end thereof. An electric control means including a switch and rheostat is mounted on the housing and connected to the heating cable for controlling the operation and temperature of the heating cable. The cable is sufficiently rigid to allow the cable to be pushed through the sealer valve into the bore and flexible enough to follow the contour of the pipe bends as the cable is pushed into the pipe.