A geothermal heat pump system with three ground coils was installed in a residence on Lake Coeur d’Alene in northern Idaho. A portion of the ground heat exchanger in this system is wrapped around the residential septic tank. The primary purpose of this research was to examine the feasibility of using septic tanks as a heat source for geothermal heat pumps using experimental data collected from this site.

Ground temperatures around this septic tank averaged between −3.5°C and 1.0°C during the 1995–96 heating season. This septic tank is located such that there is little soil surrounding it on all sides. Ground temperatures around the septic coil were influenced more by the outside air temperature than were the other two coils. There was no evidence of the septic tank freezing up or failing to properly function.

Utilizing a septic tank as a heat source for GHP systems is feasible design option if the septic tank is used on a full-time basis. However, the tank should be surrounded on all sides by a large amount of soil and/or insulated from the ground surface to ensure that ground temperatures near the tank remain warm, especially during the cold winter months.

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