if its below 5 degrees the FBH will run. If it runs above 5 degrees, the sensor behind the bumper has gone and needs replacement. you can prove it by disconnecting the sensor and it should stop.
picture of sensor is here:
http://img163.imageshack.us/img163/8284/oct1321fd1.jpg
if its below 5 degrees and the engine is running, the fbh will burn fuel to heat the coolant until it reaches about 70 degrees. once it reaches this, the fbh will still run the coolant pump but not burn fuel.
when you stop and turn the engine off, the fbh will run its purge cycle for 2 mins.