41. In 2006 the Gelsenkirchen plant began to produce heat pumps . 42. Air-earth heat pumps circulate the building's air through underground ducts. 43. That's when back-up systems kick in to supplement the heat from the heat pump . 44. Energy-efficient heat pumps are in place for Main Street businesses. 45. The heat pump gained popularity in the 1950s in Japan and the United States. 46. Heat pumps can be used for space heating essentially anywhere.47. An alternative approach to traditional heating and air systems is a geothermal heat pump . 48. Direct exchange geothermal heat pumps are therefore more efficient than air source heat pumps . 49. Direct exchange geothermal heat pumps are therefore more efficient than air source heat pumps. 50. After that, the heat pump starts to work again.
