Ask Mar­i­lyn

Chattanooga Times Free Press - Parade - - Picks - By Mar­i­lyn vos Sa­vant Send ques­tions to mar­i­lyn @ pa­

Say you have two iden­ti­cal houses, and the out­door tem­per­a­ture is 32°F. The Smith ther­mo­stat is set to 75°F, and the Jones ther­mo­stat is set to 65°F. As each house cools from that tem­per­a­ture, its fur­nace runs and takes the tem­per­a­ture back up to its set­ting. Here's what stumps me: If each fur­nace is per­form­ing ex­actly the same op­er­a­tion (say, run­ning when the in­door tem­per­a­ture drops by two de­grees), why does the house with the higher set­ting use more en­ergy?

—Jim S., Ore­gon, Ohio The Smith house, due to its higher tem­per­a­ture dif­fer­en­tial (from the en­vi­ron­ment), will lose ther­mal en­ergy to the out­doors faster than the Jones house, so the Smith fur­nace will run more— and use more en­ergy.

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