It has been proposed that municipal water mains be used as the heat sink or the heat source for air-conditioning or heating, respectively. This paper addresses the extent of thermal impact for such residential applications. A small residential neighborhood in Austin, Texas, was selected for analysis, and typical residential a/c loads and measured water supply rates in the main were used in the assessment. It is shown that no more than about 1 to 2% of residents could adopt such systems before water temperature rises in the mains became significant in the cooling season. The impact for winter heat pump applications is similar, or possibly more stringent. The general conclusion is that, while the benefit to an individual using this concept may be positive, the impact on water temperature is excessive.