Auxiliary geothermal systems are passive systems of pre-heating and pre-cooling the air that may lead into substantial efficiency benefits with a relatively low investment. In its simplest form, the system imports outbound air through a large diameter pipe crossing the subsoil around the house or a net of small pipes and a ventilation device. Air that circulated through the subsoil exchanges heat with the subsoil which, in summer stands much cooler, and so we may decrease imported air temperature and decrease the use of air-conditioning. The above combined to a heat exchanging ventilation system, in a more radical form, can increase energy-to-be-lost recovery up to more than 80%, and therefore provide a much lower need for heating and cooling. Better results can be achieved when the house is thickly insulated and completely airtight, such as in passive houses.