Abstract: Heat conduction along and through the line of a nonambient temperature liquid system is deterred by a thermal barrier having insulative stop and seat components interengaged for thermally isolating the liquid on one side of the barrier from the liquid on the other side of the barrier when the liquid within the line is stagnant. The barrier automatically opens to permit liquid flow through the line and closes to thermally isolate the liquid when the flow is interrupted, i.e. the liquid flow within the line is substantially unimpeded by the thermal barrier. For an application in which the line is subjected to low temperature which could freeze trapped liquid, such as in the outlet line of a water heater located in an uninsulated space, a small bypass passage is provided which is open when the barrier stop is closed so that a limited amount of heat is conducted past the barrier to deter freezing of the water.