I answered a call in the middle of the night for a customer who had no heat. The filter inside the furnace was completely packed with dirt, and the furnace had overheated, yet dozens of clean filters were scattered throughout the basement. When I told the customer what had happened she said "No way! We change our filters every month! On the 1st day of every month and my husband changes them." I said "Ma'am, have you been in the basement lately?" When I showed her the piles of clean filters... she said something I can't repeat.