Heat Surge models built after 2007 have a safety device installed inside the unit to protect from fire and overheating. The thermocoupler is like a circuit breaker that cannot be reset, this wire cuts off power to the units heating element. This thin black wire plugs into the control board, and is easily replaced. 2007 models have it built into the top of the heater element. No longer available for 2007. 2007 units require total update of blower, heater, and control board.