Abstract: A system of interconnecting a plurality of users of low grade thermal energy exchange has a number of users which are heat sources and a number of users which are heat sinks to balance heat flux. Heat is exchanged between a nontoxic primary heat transfer liquid and secondary heat transfer fluids for serving such users. The users are charged in proportion to the amount of heat transferred. Separate means are provided for selectively exchanging heat with the primary heat transfer liquid for maintaining the average temperature of the liquid within a desired temperature range. A municipal water main is a desirable source or sink of low grade thermal energy and nontoxicity of primary heat transfer liquid maintains the integrity of the water system.