A storage building such as a garage of a residential house is heated to a low temperature above freezing but below that suitable for human habitation by connecting a closed loop pipe and a pump so as to pump heat transfer liquid through a portion of the pipe embedded in a cast concrete lid of a septic tank and through another portion of the pipe embedded in a cast concrete floor of the building. The simple system avoids use of a complex heat pump by using the limited heat available from the septic tank to directly heat the garage floor to a low temperature but above freezing.