Abstract: A system of sealed heat pipes enclosing a working fluid. In the heating mode, the working fluid is evaporated and the latent heat of vaporization is transported through the heat pipes to the condenser region. The condensate returns to the evaporator region through a liquid bypass line containing a liquid storage reservoir and a heating control valve. In the cooling mode, a cooling control valve is opened to increase the specific volume of the heat pipe system. During operation, heat is absorbed from the evaporator region and transferred by the working fluid vapor through the heat pipe system to the cold media. Condensate returns to the external evaporator by a liquid return line containing a liquid storage reservoir and a cooling control valve. Electric resistance heating can be incorporated to heat the reservoir. Vapor from the reservoir passes into the heat pipe system through a check valve vapor line. Shutdown and system control are regulated by the reservoir cutoff valve and electric heating.