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TAPPING THE EARTH FOR CHEAP HEAT If you think heat pumps aren't practical up North, it's time to look at closed-loop, earthcoupled heat pumps by John Siegenthaler In 1852 Lord Kelvin, an English physicist and engineer, proposed a novel way to heat houses: Extract heat from the outside air. The technology has come a long way since then, and heat pumps have become a major industry in the U.S. With engineering improvements, the "air-to-air" technology envisioned by Lord Kelvin has become a practical, economically competitive way to heat and cool houses in moderate climates. Heat is absorbed from outside air in winter and delivered to the house via forced air ducting. In summer, the refrigeration cycle is reversed, with heat taken from inside the house and rejected outside. But air-to-air