Abstract: Apparatus for exchanging heat between biothermal and geothermal ground heat sources and a heat pump for a building comprises a tank having a chamber for holding septic waste underground. The tank includes a heat conductive inner membrane for transferring heat to and from the waste. A fluid jacket located outside the chamber passes a heat transfer fluid adjacent a portion of the inner surface of the tank. Waste entry and exit piping transfers the waste to and from the chamber. Fluid inlet piping and outlet piping supplies the heat transfer fluid to and from the heat pump and the fluid jacket. When coupled to a heat pump in the heating mode, heat transfer fluid absorbs heat from biothermal sources within the tank and geothermal sources in and around the tank. In the cooling mode, heat transfer fluid conducts heat from a building to the tank and ground.