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