Only a limited number of organic or silicone liquid phase thermal exchange fluid types are suitable for operation over a wide range of hot and cold temperatures and these have an undesirable property of degrading at high temperatures. It is accordingly desirable to subject the thermal exchange fluid to high temperature only to the extent and times required by users. The present invention includes intermittent users which occasionally are required to attain maximum working temperatures at which thermal degradation of thermal exchange fluid occurs at a significant rate and a hot reservoir at the minimum working temperature. A stable heat transfer fluid transfers heat from the hot reservoir to the degradable thermal exchange fluid through a common intermediate heat exchanger. The degradable thermal exchange fluid is heated to a temperature just sufficient to satisfy the maximum current setpoint temperature of the intermittent users by controlling circulation of the stable heat transfer fluid. Lifetime of the degradable thermal exchange fluid is thereby extended without compromising effective heating of the intermittent users.