There's a monster in our basement. It eats fistfuls of dollar bills, guzzles No. 2 heating oil, and belches filthy clouds of soot and CO2. We have to kill it before it kills us. Only problem is, we and our tenants are dependent on it -- this being New England, we need something down there to keep us from freezing our butts off when winter rolls around again. Nothing to fear but furnace itself.Ever since my partner Edith and I bought our 100-year-old Boston triple-decker two years ago, we've been plotting the demise of its beastly old, big-as-a-refrigerator, criminally inefficient, …